text:
\section{Introduction}
Galaxy encounters have been used to explain the presence of peculiar galaxies \citep[e.g.,][]{arp1966}, and facilitate our understanding of galaxy evolution in a number of ways. Figure \ref{fig:mstages} illustrates the typical merger sequence, from initial approach (far left) to final coalescence (far right). These encounters lead to significant changes in stellar and gas morphology \citep[e.g.,][]{mihos1995a, mihos1995b, malin1997, cote1998, knierman2003, lotz2008, wen2016, tapia2017}, including the production of non-axisymmetric torques which enable gaseous inflows \citep[e.g.,][]{duc2004, blumenthal2018}, which may feed the central black hole, producing heightened activity of the nucleus \citep[e.g.,][]{cutri1985, dahari1985, heck1986a, heck1986b, ellison2011b, hewlett2017, trak2017}. Interacting and merging galaxies have been shown to host heightened rates of star formation \citep[e.g.,][]{joseph1985, kenn1987, whit1995, vig1996, mirabel1998, bridge2007, scudder2012a, moreno2015, rich2015, moreno2019}. In the Local Universe, gas-rich mergers manifest themselves as (ultra)luminous infrared galaxies \citep[e.g.,][]{sanders1988, hopkins2008a}. Until the James Webb Space Telescope begins operations, our knowledge of these objects at high redshifts will remain severely limited. However, indirect measurements such as the observation that early discs are dominated by large clumps of gas and dust \citep[e.g.,][]{lotz2006, ravi2006, whitaker2015} and that many early ellipsoids are very compact \citep[e.g.,][]{buitrago2008, williams2014, barro2013}, have led some to postulate that mergers were much more common in the Early Universe \citep[e.g.,][]{conselice2004, genzel2008, bezanson2009, bour2009, dekel2009}. These findings are consistent with $\Lambda$CDM cosmology, which dictates that the hierarchical structure of the universe arises from sequential mergers throughout cosmic time \citep[e.g.,][]{white1978}.

A fundamental component of galaxy evolution, and by extension hierarchical growth, is the galaxy merger rate. In its simplest form, the galaxy merger rate is calculated by dividing the fraction of galaxies undergoing a merger by the typical time a galaxy interaction will be observable. The merger fraction is often determined by counting the number of morphologically disturbed \citep[both automatically or by visual inspection, e.g.,][]{lotz2008, jogee2009, shi2009, lotz2010, bluck2012}, or the number of galaxies in close pairs \citep[projected or 3D, e.g.,][]{bundy2004, kartaltepe2007, bundy2009, robo2014, mundy2017, snyder2017}. The observability timescale is also variable, and may depend on the orbital parameters and initial conditions \citep[e.g.,][]{conselice2006, lotz2010a, lotz2010}, the observational method used to characterize the merger \citep[e.g.,][]{lotz2008}, and the redshift of the interaction \citep[e.g.,][]{snyder2017}. Due to the breadth of observational methods used to derive these quantities, the calculated merger rate varies widely \citep[e.g.,][]{lotz2011}. However, cosmological simulations are providing insight into the limitations of purely observational studies \citep[e.g.,][]{rod2015}.

In this paper, we set out to answer the following questions that are fundamental to the calculation of the galaxy merger rate across cosmic time:
\begin{enumerate}
	\item Does the stellar morphology of a merging pair reliably indicate its dynamical history?
	\item What makes an interaction ``visible''?
	\item Are merger catalogs derived solely from optical observations biased?
\end{enumerate}

\cite{lotz2011} use small-scale hydrodynamic simulations of binary galaxy pairs to derive a realistic observability timescale. They find that applying this parameter to observational data causes the widely divergent merger rates to converge. \cite{simons2019} use synthetic galaxy images from zoom simulations to determine how frequently galaxies are confused for discs in merger catalogues. These interlopers confuse the disc/spheroid ratio that is often used to define merger rates, a trend the authors found was dependent upon stellar mass. More recently there has been a push to apply deep learning \citep[e.g.,][]{bottrell2019, pearson2019, snyder2019} techniques to synthetic galaxy image catalogues to assess the completeness of observationally derived catalogues.

In this work we utilise the IllustrisTNG simulation with a volume of $\sim$100$^{3}$ cMpc$^{3}$ (hereafter TNG100-1), one of the three main runs of the IllustrisTNG cosmological suite \citep{marinacci2018b, naiman2018, nelson2018, pill2018b, springel2018}. The IllustrisTNG model \citep{wein2017, pill2018a} employs state-of-the-art prescriptions for star formation, chemical evolution, and feedback due to active galactic nuclei. Recent work has shown that the IllustrisTNG model matches important observational benchmarks in the chemical and metallicty evolution of galaxies \citep[e.g.,][]{naiman2018, torrey2019}, the quasar luminosity function and black hole mass relationships \citep[e.g.,][]{wein2018}, and the overall morphologies of galaxies \citep[e.g.,][]{rod2019}. Using a sample of galaxy pairs from TNG100-1, we generate ideal mock SDSS images to identify what fraction of the interacting pairs are ``observable.''

This paper is structured as follows: in Section \ref{sec:methods} we describe the cosmological simulation used, the methods associated with its data products, and the pair sample preparation; in Section \ref{sec:results} we present and discuss our results, which compare the TNG100-1 sample of interacting pairs at two epochs; finally, in Section \ref{sec:conclusions}, we state our conclusions and briefly describe our future and ongoing work.

\section{Methods}
\label{sec:methods}
\subsection{IllustrisTNG}
IllustrisTNG is a set of N-body/magnetohydrodynamic cosmological simulations with dark and baryonic matter. Gravity is solved using a Tree-PM algorithm that implements a particle mesh on large scales and a tree code on small scales. Gas is treated as an ideal fluid on an unstructured mesh \citep[{\sc AREPO};][]{springel2010a} that incorporates an ideal treatment of magnetohydrodynamics \citep{pakmor2011, pakmor2016}. Gas is allowed to cool via metal-lines and radiation, and can also heat radiatively by exposure to a redshift-dependent radiation field \citep[e.g.,][]{katz1996, faucher2009}. High density gas can self-shield \citep[][]{vogel2013}, under the appropriate optical depth conditions. The interstellar medium (ISM) is modeled with an effective two phase model, following \cite{springel2003b}: cool clouds are in pressure equilibrium with the hot diffuse medium. These simulations cannot describe the ISM structure in detail, but do include mass exchange via cooling, star formation, and the evaporation of clouds by supernovae. This acts to harden the equation of state of the star forming gas, and also stabilizes gas against instability. The ISM prescription does not reach low (high) enough temperatures (densities) to properly describe the molecular gas component. These simulations do not include modeling of cosmic rays nor explicit radiative transfer.

Each star particle represents a stellar population, not an individual star, based on empirical models that include stellar evolution, enrichment, mass and metal returns and supernova rates \citep{pill2018a}. Star formation and supernovae drive outflows in galaxies. Gas mass is ejected from star forming regions such that the wind velocity is proportional to the dark matter velocity dispersion. Due to resolution limitations, outflowing material is initially hydrodynamically decoupled, and is re-coupled at a density threshold. The winds carry a sufficient metal content out of the galaxy, to approximately match the mass-metallicity (or, M-Z) relation \citep[for further details, see][]{pill2018a}.

Black holes (BHs) -- and the feedback due to active galactic nuclei (AGN) -- are a key part of this simulation, in particular the production of quiescent galaxies. Given the resolution of the simulation, black hole formation cannot be self-consistently modeled, so once a galactic halo reaches a certain mass, a seed black hole particle is inserted at its centre, which then acts as a sink particle. The black hole is thus tied to the potential minimum and grows by subsequent mass accretion via Eddington-limited Bondi-Hoyle accretion \citep{springel2005b}. The channel of AGN feedback \citep{sijacki2007, wein2017} depends upon the accretion rate. At low accretion rates, the galaxy experiences a wind (or, kinetic mode), wherein kinetic energy is deposited into the gas around the black hole. The duty cycle then ensures star formation remains suppressed. At high accretion rates, the galaxy enters the thermal (or, quasar) mode; the strength of this feedback mode is a function of the black hole mass. Full details of the IllustrisTNG BH feedback model are available in \cite{wein2017}.

\subsubsection{IllustrisTNG vs. Illustris}
The IllustrisTNG model differs from its earlier counterpart, Illustris \citep{vogel2014a, vogel2014b, genel2014, sijacki2015}, in several ways: (1) it includes isotropic winds with velocities that scale according to the halo virial mass; (2) the supernova energy has two components (thermal and kinetic) which are applied to winds; (3) the wind energy is metallicity-dependent; (4) the supernova mass limit has been set to 8 M$_{\odot}$, and the yield tables have been updated \citep{naiman2018}; (5) it includes an ideal treatment of magnetohydrodynamics. Further, the IllustrisTNG model was run at three different volumes to generate a simulation series that spans a wide dynamical range: TNG50 \citep{nelson2019, pill2019}, TNG100 and TNG300. Each of these runs was initialised with three (TNG100 and TNG300) or four (TNG50) sets of initial conditions, often indicated as e.g., TNG100-1. For more information on the simulation series structure, see Appendix \ref{sec:App1}. In this work, we utilise the run TNG100-1 for several reasons: (1) it has the same set of initial conditions as the original Illustris run; (2) it has the largest number of resolution elements for its volume; and (3) the volume is large enough to contain many examples of interacting galaxies (c.f., TNG50), but not too large that these galaxies are poorly resolved (c.f., TNG300, which typically has a baryon mass resolution on the order of $10^{7}$M$_{\odot}$).

Many parameters and model choices of the IllustrisTNG model were calibrated using observational scaling relations and galaxy properties \citep{pill2018a}. Several works outline the successes of this model. \cite{nelson2018} shows that the colour bimodality, which was absent in the original Illustris, possibly due to the previous implementation of black hole feedback, was present in both TNG100 and TNG300. \cite{rod2019} compares synthetic images from TNG100 to an analogous sample from Pan-STARRS. They find TNG100 to be a significant improvement over the original Illustris suite, particularly with respect to the galaxy morphologies. Additionally, chemical evolution \citep{naiman2018}, galaxy mass-metallicity relations \citep{torrey2019}, and the present day quasar luminosity function \citep{wein2018} are broadly consistent with observations. Despite its relative success, there are still areas of contention between the IllustrisTNG model and the observed universe. For example, TNG100 may underproduce bulge-dominated galaxies, and may overproduce red discs and blue spheroids \citep[e.g.,][]{huertasco2019, rod2019}. The high-redshift quasar luminosity function, driven by the feedback mechanisms employed by supermassive black holes, may be in tension with observations \citep{wein2018, hab2019}. Additionally, it has been suggested that there is contention between the observed and simulation H$_{2}$ content in high redshift galaxies \citep{popping2019}.

\subsubsection{Friends-of-Friends Groups and Subhaloes}
The Friends-of-Friends (hereafter FoF) algorithm utilizes percolation to construct associated groups of particles \citep{davis1985}. Dark matter particles (or chains of particles) are said to be linked if they are closer than $b\bar{l}$, where $\bar{l}$ is the mean interparticle distance and is related to the simulation's mean number density, $\bar{l} = \bar{n}^{\, -1/3}$. The free parameter $b$ is the ratio between the maximum linking distance and the interparticle separation for a homogeneous system; in IllustrisTNG $b = 0.2$. Taken together, this all represents an approximate density threshold below which particles are not considered associated. The baryonic (gas and stars) material is assigned to a particular FoF group based on the membership of the nearest dark matter particle. Subhaloes, on the other hand, are identified via the \textsc{subfind} algorithm \citep{springel2001}. This iteratively strips away particles that are unbound from the central structure, until a bound system above a certain size remains -- in the case of IllustrisTNG, this is 20 particles. In many cases, as in this work, subhaloes are considered galaxies, while FoF groups may contain pairs or groups of galaxies.

\subsection{Galaxy Pair Samples}
\subsubsection{Parent Sample}
\label{sec:sample}
We select FoF haloes in the most recent snapshot (i.e., $z=0$) with a FoF group total dark matter mass between 10$^{11}$ and 10$^{13.5}$ M$_{\odot}$. Additionally, subhaloes are required to have a total dark matter mass between 10$^{10.5}$ and 10$^{13}$ M$_{\odot}$. These mass cuts ensure that we limit ourselves to well-resolved galaxies and haloes; that we avoid systems in which visual features are driven by environmental, non-merger related processes. Subhaloes with a total dark halo mass less than $10^{10.5}$ M$_{\odot}$ are likely to be poorly resolved in both the dark and baryonic material. To ensure the proper mass resolution of the stars, we place a final limit on the subhalo total stellar mass such that both subhaloes in the pair have a total stellar mass above 10$^{9}$ M$_{\odot}$. We place this restriction on the stellar mass primarily because a preliminary inspection of the images described in Section \ref{sec:sdssims} shows that it is very difficult to identify tidal features in systems with stellar mass below 10$^{9}$ M$_{\odot}$ (see Section \ref{sec:sdssims}.  However, abundance matching \citep[e.g.][]{sawala2015} indicates that this might lead to a sample of galaxies with systematically high stellar masses. No limit is placed on the distance between the subhaloes, although they are required to belong to the same FoF halo. We do not consider pairs that straddle two FoF haloes \citep[as in, e.g.,][]{moreno2012, moreno2013}, and note that these systems are not only relatively rare, but are likely to be unbound (and as such, not orbiting one another). We consider only pairs of galaxies with a stellar mass ratio between unity and 1:4 (``major merger'') at the present day. Lastly, the majority of observations \citep[e.g.,][]{bridge2010, ellison2010, lar2016, ventou2017, mantha2018} and idealised simulations \citep[e.g.,][]{tt1972, barnes1991, barnes1996, dimatteo2008, lotz2008, rupke2010, bour2011, hopkins2013, moreno2015, moreno2019} of galaxy mergers typically assume the system is composed of only two galaxies. In order to approximate this assumption, we required that any tertiary subhalo in the FoF group be at most 1/16 the stellar mass of the primary (or, most massive) halo. It should be noted that this restriction will not exclude all recent minor mergers. There may be systems with strong tidal features at the present day due to low mass ratio interactions in the past. However, observers do not have unlimited knowledge about their targets. By not removing these objects, we remain more closely connected to observational surveys of interacting galaxies. Our final set of galaxies contains 446 binary galaxy pairs at $z=0$.

\subsubsection{Ideal Mock SDSS Images}
\label{sec:sdssims}
We generate ideal mock SDSS images for each of the 446 galaxy pairs in our sample. TNG100 provides magnitudes in eight bands for each star particle, which are calculated using \cite{bc03} (assuming no dust). These include SDSS $g, r, i, z$, Buser $U, B, V$, and Palomar $K$. Here, we generate ideal mock SDSS $g$-band images using all star particles bound to a FoF group. This band was chosen to facilitate future comparisons with wide-field observational surveys. To calculate the luminosity of each star particle, we determine the true SDSS $g$  magnitude
\begin{equation}
M_{\rm true} = M_{\rm obs} - \chi k_{\rm filter}
\end{equation}
where $M_{\rm obs}$ is the TNG100 SDSS $g$-band absolute magnitude, $k_{\rm filter}$ is the filter-dependent first order extinction correction, and $\chi$ is the airmass, assumed to be 1.3 for all SDSS bands in TNG100. The apparent magnitudes are needed to derive the flux:
\begin{equation}
m_{\rm true} = M_{\rm true} + \mu
\end{equation}
The distance modulus, $\mu$, is calculated for each set of galaxy pairs using a set distance of 35 Mpc for every system.
The flux is then
\begin{equation}
f = 10^{0.4 ( m_{\rm true} - m_{\rm zp} )}
\end{equation}
where $m_{\rm zp}$ is the zero-point of the desired filter. For our images, we use $k_{\rm filter} = 0.15$, $\chi = 1.3$, and $m_{\rm zp} = 25.11$ \citep{stoughton2002}. We project the three-dimensional distribution of particles onto a flat two-dimensional plane, and apply a 2D Gaussian smoothing function with FWHM equal to the radius of a sphere enclosing the 32 nearest star particles, following \cite{torrey2015}. For simplicity, we use the $x$ and $y$ coordinates to define this plane, and do not assume a location or viewing angle for an observer. Thus the sample represents a random set of orientations with no preferred observing direction. Further the images include no treatment of dust attenuation nor a convolution with the SDSS resolution. This affords us optimal conditions to ``observe'' any tidal features in the mocks. Figure \ref{fig:mstages} contains five (rgb) examples of our ideal mock observations. For the full postage stamp collection of the interacting pairs, refer to Appendix \ref{sec:appb}.

\subsubsection{Visual Classification Scheme}
\label{sec:viz}
The merger sequence is defined by the presence (or absence) of tidal features. \cite{lar2016} devised a merger stage classification scheme that includes non-interacting single galaxies (s), minor mergers (m) and major mergers, ranging from before first pericentric passage through final coalescence and post-merger remnant (M$1-$M$5$) of Ultra-Luminous Infrared Galaxies (ULIRGs). The major merger sequence is as follows (Figure \ref{fig:mstages}): \\
\textbf{M1} - Galaxies are well separated and appear to be on their initial approach. \\
\textbf{M2} - Tidal features (bridges and tails) are clearly visible, and likely just after the first close passage. \\
\textbf{M3} - Two individual nuclei are visible in highly disturbed overlapping discs. The tidal tails are still well defined. \\
\textbf{M4} - The two nuclei have now coalesced, but the tidal debris are still visible. \\
\textbf{M5} - A post-merger remnant, with a diffuse outer shell, and little-to-no evidence of tidal features. \\
Using this merger stage classification as a guide, three of the authors independently classified the pairs as either interacting (roughly, stage M2-M5) or not interacting (s-M1). Visual cues including the projected distance, tidal features, and stellar debris were used. Following Galaxy Zoo \citep[][]{galzoo}, we adopt the group consensus (2 out of 3) as the morphological classification for any given pair.

Identifying merger stage in this way may be subject to certain pitfalls. For example, the production of tidal features is dependent upon encounter geometry, and can, in some retrograde encounters, be completely absent \citep[e.g.,][]{tt1972, dimatteo2007}. Further, \cite{nevin2019} study the effects of initial conditions on various quantitative morphology measures which are often linked to merger activity. They find that the initial conditions, including viewing angle, may affect the derived quantitative morphologies, thus impacting the perceived merger stage. \cite{dubinski1996, mihos1998, springel1999, barnes2016} perform systematic theoretical studies on the tidal response of interacting galaxies, spanning a wide range of galaxy structures. They find that the visibility  of tidal features additionally depends on the galaxy's internal structure (e.g., the dark halo mass and concentration), and that under certain circumstances, galaxy pairs may not show any obvious signs of interaction.

\cite{lotz2008} use quantitative morphological metrics \citep[e.g., Gini and M$_{20}$ of][]{lotz2004} of simulated galaxy mergers to dive deeper into the idea of time-scales for tidal features. They determine when, over the course of an interaction, the tidal response and subsequent morphological disruption are greatest, and find that galaxies tend to exhibit strong tidal features at first pericentre and near final coalescence. In contrast, the morphologies at intermediate passes are largely consistent with a control sample of isolated galaxies.

In addition to the object's look-back time, internal structure, and encounter geometry, the observer viewing angle can also drastically alter the prominence of tidal features. For example, \cite{pop2018} studied shell stellar debris fields (e.g., their Figure 1, and the last panel of our Figure \ref{fig:mstages}) which they found to be present predominantly in merger remnants. The authors show that whether or not a shell is visible in a particular projection depends on the orbital trajectory of the progenitor system.

Though in this work we do not directly account for the viewing angle, we follow \cite{rod2019} and adopt a fixed projection. This is directly analogous to observational studies; one of the main goals of this paper is to test how well observationally-derived and morphology-based classifications identify interacting systems. Projection will be important for the observability of individual systems' tidal features. However in this work, we look at the properties of the interacting pairs as a whole. In doing so, we sample over a random set of viewing angles, thereby minimizing the biases of individual interacting pairs. Clearly, schema which depend entirely on tidal features in the stellar material might be biased in a variety of ways, thereby affecting the kinds of interactions captured.

Several studies have shown that the internal structure of tidal features is highly dependent upon spatial resolution \citep[e.g.,][]{wetz2007}. However, the broader structure of tidal features is less sensitive to this parameter. For example, \cite{moreno2015} and \cite{moreno2019} perform similar simulations of galaxy interactions: improving the spatial resolution by two orders of magnitude does not affect the presence of visual features. Additionally, we can expect a typical TNG100-1 tidal feature to have about 200 star particles, corresponding to less than 1\% relative error in the feature's resolution.

\begin{table}
\centering
\begin{tabular}[width=4in]{c|c|c|c|}
\cline{3-4}
\multicolumn{2}{c|}{\multirow{2}{*}{}}                                                                                & \multicolumn{2}{c|}{\begin{tabular}[c]{@{}c@{}}Does the pair appear\\ to be interacting?\end{tabular}} \\ \cline{3-4}
\multicolumn{2}{l|}{}                                                                                & Yes & \multicolumn{1}{c|}{No}                                      \\ \hline
\multicolumn{1}{|c|}{\multirow{3}{*}{\begin{tabular}[c]{@{}c@{}}Is the pair \\ actually \\ interacting?\end{tabular}}} & Yes & \textbf{\color[rgb]{0.977,0.488,0.445} 38 } & \multicolumn{1}{c|}{\textbf{\color[rgb]{0.4922,0.0039,0.4922} 47}} \\ \cline{2-4}
\multicolumn{1}{|c|}{}                                                                                & \multirow{2}{*}{No} & \multirow{2}{*}{18} & \multicolumn{1}{c|}{\multirow{2}{*}{214}} \\
\multicolumn{1}{|c|}{} & & & \multicolumn{1}{c|}{}                                        \\ \hline \hline
\multicolumn{3}{|l|}{Recently entered the same group} & 97 \\ \hline
\multicolumn{3}{|l|}{Group} & 22 \\ \hline
\multicolumn{3}{|l|}{Not Orbiting} & 10 \\ \hline \hline
\multicolumn{3}{|l|}{Total Pairs} & 446 \\ \hline
\end{tabular}
\caption{A complete description of the classifications for all pairs in the parent sample. In the top section, we report the results of the combined visual (``Does the pair appear to be interacting?'') and trajectory schema (``Is the pair actually interacting?''), including the VIP and nonVIP which are the subsamples used in this paper. Note that the report from the trajectory is taken to be ``truth.'' The middle section reports the number of pairs which were manually eliminated from the sample.}
\label{table:acnt}
\end{table}

\subsubsection{Trajectory Classification}
\label{sec:int}
Using the Sublink merger tree \citep{rod2015}, we extract the 3D orbital motion of the secondary with respect to the primary. A pair is considered ``interacting'' if it has had at least one close passage, the pair is at or nearing an apocentre (i.e., there is an apparent turnover in the relative separation), and there is apparent orbit decay (i.e., sequentially deeper pericentric passages). For interacting pairs which have had only one close passage, we require the pericentric distance be less than $\sim$150 kpc. In considering the full trajectories, we more reliably remove those galaxies which have merely flown past one another. Whilst it could be argued that such systems have \textit{interacted}, they are not \textit{currently interacting}, and are thus not part of our sample.

\begin{figure*}
\centering
\begin{minipage}{\textwidth}
\centering
	\includegraphics[width=0.8\textwidth]{Figure2}
	\caption{Galaxy-pair property distributions for the Visually Identified Pairs (VIP; salmon) and the non-Visually Identified Pairs (nonVIP; purple). The panels show: (a) the current ($z=0$) stellar mass ratio, (b) $z=0$ 3D separations, (c) relative velocity, and (d) FoF group dark matter mass. The vertical coloured lines correspond to the medians of each sample, and colored rectangles indicate the range within $\pm$ one median absolute deviation from the median.}
	\label{fig:dists_bulk}
\end{minipage}%

\begin{minipage}{\textwidth}
\centering
	\includegraphics[width=0.8\textwidth]{Figure3}
	\caption{Individual galaxy distributions for the Visually Identified Pairs (VIP; salmon) and the non-visually identified pairs (nonVIP; purple). The panels show: (a) stellar mass, (b) star formation rate, (c) total gas mass, and (d) the star forming gas. The vertical coloured lines correspond to the medians of each sample, and colored rectangles indicate the range within $\pm$ one median absolute deviation from the median.}
	\label{fig:dists_s}
\end{minipage}
\end{figure*}

\subsubsection{Sample Selection Summary}
In Table \ref{table:acnt}, we provide a full account of the results of our various classification schema. Some pairs were manually removed from the sample. This includes systems with multiple prominent subhaloes, which comprised only $\sim$5 per cent of the parent pairs sample. Pairs were also discounted if they have only been in the same group for less than 1 Gyr ($\sim$22 per cent). These are exclusively subhaloes determined not to be interacting, based on the aforementioned criteria. There are a small number of subhaloes ($\sim$2 per cent) which appear to be interacting based on their morphologies in the mock images and/or their trajectories, but were not orbiting one another. Namely, their orbits appear to be dominated by structures outside the FoF group. The majority ($\sim$48 per cent) of our parent sample are not interacting and are not visually identified as mergers. However, there is a small fraction ($\sim$4 per cent) of the non-interacting sample which were misidentified as mergers. Pairs which were visually identified as mergers (\S \ref{sec:viz}) and were found to be interacting (\S \ref{sec:int}) are hereafter referred to as Visually Identified Pairs, or VIP. The nonVIP, then, are those pairs which are interacting, but were not selected visually. The majority of interacting pairs are nonVIP. This is because many of them lack the prominent tidal features or disrupted morphologies that are typically used to identify mergers (\S \ref{sec:viz}; see also Appendix \ref{sec:appb} for visual examples).

\section{Results and Discussion}
\label{sec:results}
\subsection{The VIP and nonVIP Samples}
\label{sec:vnvsamp}
With the sample of pairs defined, and delineated into VIP and nonVIP, we investigate the bulk and individual properties of the interacting pairs at the present day, and at the time of their last pericentres. Among other properties discussed here, we will show that the VIP are not only more massive, but they have undergone a close passage more recently than the nonVIP.
\subsubsection{Present-day ($z=0$) Properties}
\label{sec:prop_z0}
Panel (a) of Figure \ref{fig:dists_bulk} shows that the distribution in stellar mass ratios, which peak at unity and peter out toward larger mass ratios for both samples, with the VIP having slightly larger median stellar mass ratio. Though we do see this trend with the median values, the VIP and nonVIP stellar mass ratios are not distinct distributions: a two-sided KS test indicates ($p\approx0.3$) that these are drawn from the same sample. Panel (b) shows the present-day ($z=0$) 3D separation, with the VIP having separations shifted to smaller values (here, the two-sided KS test indicates that VIP and nonVIP relative separations are drawn from distinct distributions: $p\approx2\times10^{-3}$). Interacting pairs at wider projected separations may be overlooked in preparing samples of merging galaxies. There has, however, been some work which indicates that interacting galaxies may exhibit heightened rates of star formation, even with separations as large as 150 kpc \citep[e.g.][]{patton2013}. In panel (c), we present the relative velocity distributions (i.e. the difference between the subhalo velocities); the VIP and nonVIP samples attain low velocities, facilitating their interaction and eventual merging. The VIP do appear to be moving faster on average than the nonVIP ($p\approx0.05$), which hints at their local dynamics. That is, the relative velocities of these galaxies may be affected by their environment (\S \ref{sec:env}), despite our efforts to avoid this using our FoF group mass cuts. Finally, panel (d) displays the VIP and nonVIP FoF group dark matter mass distributions; the VIP inhabit slightly more massive FoF haloes ($p\approx4\times10^{-4}$).

In Figure \ref{fig:dists_s}, panel (a) shows that VIP galaxies tend to have higher stellar masses than the nonVIP ($p\approx1.3\times10^{-6}$). Because visual classification is based on tidal disruptions in the stellar material, we might predict that VIP galaxies to have a higher stellar mass (i.e., more stars to disrupt) on average. In fact, we do find that the VIP median stellar mass is about one-half dex greater than the nonVIP, consistent with the findings in panel (d) of Figure \ref{fig:dists_bulk}. Panel (b) shows that the present-day star formation rate (SFR) is relatively consistent for both samples (this is confirmed by a two-sided KS test with $p\approx0.33$). Panels (c) and (d) show the total gas mass and the cool (star forming) gas mass, respectively. The present-day contribution due to cold gas appears to be nearly the same for both samples ($p\approx0.23$). This suggests that the gas reservoir available for star formation is not significantly different for the VIP or nonVIP, consistent with the findings of panel (b).

\begin{figure}
\centering
	\includegraphics[width=\columnwidth]{Figure4}
	\caption{Here we show a (nonVIP) pair at the present day (left) and at its last pericenter (right), which occurred about 1.3 Gyr prior. The stellar mass ratio has stayed at $\sim$3:1 over this period. Bars in the bottom left corner of each image indicate 50 kpc. The inset in the left panel shows an enlarged image of the primary galaxy to highlight its visible structures.}
	\label{fig:z0LP}
\end{figure}

\begin{figure}
\centering
	\includegraphics[trim = 0.6cm 0.57cm 0.3cm 0.35cm, clip, width=\columnwidth]{Figure5}
	\caption{Time since last pericenter (t$_{0}$ - t$_{LP}$) distribution and median values (vertical lines) for the VIP (salmon) and the nonVIP (purple). The VIP have more recently undergone a close passage. Vertical coloured lines show the position of the median, and the coloured rectangles indicate the range within $\pm$ one median absolute deviation from the median.}
	\label{fig:tlast}
\end{figure}

\subsubsection{A Comparative Epoch: Last pericentric (LP) passage}
\label{sec:prop_lp}
We additionally utilise merger trees to study the interacting pairs at the time of their last pericentric passage (LP) -- a local maximum in the strength of their interaction. We note that the interacting pairs do not all reach their respective LP events at the same time, but are at a dynamically similar moment in their histories. In this way, we analyse all interacting pairs at a point in time when the effects of their interaction are at a near a peak. Figure \ref{fig:z0LP} shows a nonVIP galaxy image at the present-day (left), and at its last pericentre (right). At this pair's LP, there is a clear tidal debris field with several star forming regions in the primary galaxy. The LP's span a range of ages relative to the present day of 70 Myr to 5.66 Gyr (Figure \ref{fig:tlast}). The VIP have more recently undergone a close passage than the nonVIP by nearly a factor of two ($p\approx0.01$).

\subsubsection{The Failures of Morphological Identification}
In $\sim$55 per cent of the interacting pairs, it was unclear if an interaction was underway (that is, the nonVIP). There are a number of reasons why morphological identification schema may fail:
\begin{enumerate}
\item Due to the finite resolution of the simulations, star particles have relatively large masses. This may inhibit our ability to resolve the fine-grained structures within tidal interactions \citep[e.g.,][]{wetz2007}.
\item The stellar material may not be the best indicator of a tidal interaction \citep[discussed as the ``internal properties'' in e.g.,][]{darg2010}. The gas disc has been shown to be as large, if not larger than the stellar disc \citep[e.g.,][]{broeils1997}. Thus, gas discs are much more likely to be perturbed by one another, even in the case of wide pericentric distances. Integral Field Unit surveys \citep[e.g.,][]{croom2012, sanchez2012, bundy2015} of interacting galaxies may be necessary to get a realistic measurement of the local merger rate.
\item The present-day separations (Figure \ref{fig:dists_bulk}) are larger than expected from observationally motivated merger catalogues. What observers assume to be the first passage may, in many cases, be the second \citep[e.g.,][]{patton2013}.
\item If encounters are sufficiently wide, tidal forces may not be strong enough to produce visible (i.e. observable) bridges and tails.
\item If an encounter has occurred within the last Gyr, it is more likely to host obvious tidal features. As time passes, material from the bridge and tails settles back into the discs, and is able to phase-mix with the surrounding material \citep[e.g.,][]{lotz2008, lotz2010}.
\end{enumerate}

\begin{figure*}
\centering
	\includegraphics[width=0.75\textwidth]{Figure6-eps-converted-to}
	\caption{The dynamics of the interacting pairs at $z=0$: relative velocity as a function of pair separation (left) and of dynamical mass (bottom) for the VIP (salmon diamonds) and the nonVIP (purple circles) sample. The dashed lines in the right panel indicate the trend expected from a parabolic trajectory, whilst the solid lines are a fit to the data.}
	\label{fig:dyn_z0}
\end{figure*}

\begin{figure*}
\centering
	\includegraphics[width=0.75\textwidth]{Figure7-eps-converted-to}
	\caption{Analogous to Figure \ref{fig:dyn_z0}, but at last pericenter (LP). Values at LP are reported in colour, and in gray points for comparison with $z=0$ values form Figure \ref{fig:dyn_z0}.}
	\label{fig:dyn_LP}
\end{figure*}
\subsection{Galaxy pair dynamics}
\label{sec:dyn_pairs}
To better understand the distinctions between the VIP and nonVIP, we discussed the fundamental physical properties of the pairs in the previous section. Here, we will show that the VIP are closer together and move faster than the nonVIP at both the present day and LP.

Figure \ref{fig:dyn_z0} focuses on the dynamical properties of the VIP (salmon diamonds) and nonVIP (purple circles) at the present day. Consistent with panels (b) and (c) of Figure \ref{fig:dists_bulk}, the left panel of Figure \ref{fig:dyn_z0} shows the VIP and nonVIP are offset from one another: the VIP have smaller 3-dimensional separations, and move with slightly faster relative velocities than the nonVIP. Figure \ref{fig:tlast} shows that the VIP have more recently undergone a pericentric passage. Thus, their separations will naturally be smaller at the present day, and as they are closer to a pericentre, the VIP should have higher velocities than the nonVIP, which are typically closer to an apocentre. The right panel of Figure \ref{fig:dyn_z0} shows a moderate linear trend between the dynamical mass and relative velocity. This is expected if the relative velocity traces the virial velocity, and the orbits are parabolic: $V = \sqrt{2\mathrm{G M}_{\rm dyn} / \mathrm{R}}$ \citep[e.g.,][]{moreno2013}, where the dynamical mass M$_{\rm dyn}$ is defined as the sum of the galaxies' dark matter masses. The ideal trend (i.e., parabolic) is shown for both the VIP (dashed salmon) and nonVIP (dashed purple) samples, using their corresponding median separations at the present day ($\overline{\rm R}_{\rm VIP}$ = 88.8 kpc; $\overline{\rm R}_{\rm nonVIP}$ = 141.4 kpc). The solid lines indicate lines of best fit for each subsample ($m_{\rm nonVIP} = 0.36$ and $m_{\rm VIP} = 0.17$, with standard errors $\sigma_{\rm nonVIP} = 0.067$ and $\sigma_{\rm VIP} = 0.068$). Outliers have a variable impact on the fitted slopes for both the VIP and the nonVIP, ranging from 0.08 to 0.47 dex. Note that their slopes differ from the parabolic case. There is a substantial amount of scatter in these samples, particularly at the high mass end, where a subset of the VIP dip to lower relative velocities. That these galaxies have lower relative velocities than what might be expected based on their dynamical mass may be indicative of their visual identification.

Figure \ref{fig:dyn_LP} shows the VIP and nonVIP at last pericentre (coloured points), compared with their positions at $z=0$ (gray points). The interacting pairs at LP are significantly closer together, and are moving much faster (left panel) than they are at $z=0$. This is expected: an interacting pair should reach a local maximum in its relative velocity at each pericentric passage \citep[or conversely, should reach a local minimum in its velocity at each apocentre, e.g., Figure 4 of][]{moreno2019}. Similarly, the right panel of Figure \ref{fig:dyn_LP} shows that the the $M_{\rm dyn}-V_{\rm rel}$ relationship is much tighter at LP than at $z=0$ ($m_{\rm nonVIP} = 0.21$ and $m_{\rm VIP} = 0.33$, with standard errors $\sigma_{\rm nonVIP} = 0.05$ and $\sigma_{\rm VIP} = 0.04$). Contrary to the $z=0$ behaviour, the nonVIP appear to have greater scatter, particularly at the low mass end, where a subset achieve higher velocities than their dynamical mass might suggest. The effects of these outliers on the slope is similar to that of the present day population, with a range of 0.02 to 0.33 dex. Moreover, the best fit lines to the data (solid) and the parabolic fiducial curves (dashed) do not agree at last pericenter. This is expected at this epoch, as the orbital elements of the interaction will change rapidly near a close passage. The parabolic trends are elevated from the $z=0$ case, as the median separation values used are $\overline{\rm R}_{\rm VIP} = 57.63$ kpc and $\overline{\rm R}_{\rm nonVIP} = 76.73$ kpc. These curves would be translated down with larger separations. Although the best-fitting lines are still notably different, the VIP slope is now more consistent with the parabolic slopes.

Deviations from the fiducial parabolic slope hint at the shortcomings of our assumptions regarding galaxy interactions. In particular, this implies that the constant weak gravitational encounters that galaxies experience throughout their evolution impact the orbits in measurable ways. This is made most evident at infall, an epoch discussed in our forthcoming paper (Blumenthal et al. \textit{in prep}.) \\

\subsection{Star formation main sequence}
In Section \ref{sec:vnvsamp}, we presented physical properties such as the stellar mass and star formation rate for the VIP and nonVIP. It is known that these two parameters often trace one another, forming the star formation main sequence.

\begin{figure*}
\centering
\begin{minipage}[t]{0.99\columnwidth}
\vspace{0pt}
\centering
	\includegraphics[trim = 0cm 1.25cm 0cm 0cm, clip,width=\columnwidth]{Figure8_top}
	\includegraphics[trim = 0cm 1.45cm 0cm 0.78cm, clip, width=\columnwidth]{Figure8_bot}
	\caption{\textit{Top: }The SFMS at the present day. The positions of all TNG100-1 galaxies at $z=0$ which meet the same mass criteria as the interacting pairs are shown in the grayscale hexagons, with the fiducial fit to that SFMS indicated by the black dashed line. The VIP (diamonds, outlined in black) and nonVIP (circles) are coloured by the log in the change of their SFR. \textit{Bottom: }Distance from the SFMS fiducial line, $\Delta$log(SFR) for the same samples as above. Despite the fact that the (non)VIP are interacting pairs, there is no apparent offset \textit{above} the star formation main sequence, though they are offset from the median value of the total TNG100-1 sample (black solid line). The VIP have more scatter in $\Delta$log(SFR), perhaps indicating that their interaction has triggered a dramatic change in morphology, toward compact quiescent spheroids.}
	\label{fig:sfms_z0}
\end{minipage} \hspace{0.04\textwidth}
\begin{minipage}[t]{0.99\columnwidth}
\vspace{0pt}
\centering
	\includegraphics[trim = 0cm 1.25cm 0cm 0cm, clip,width=\columnwidth]{Figure9_top}
	\includegraphics[trim = 0cm 1.45cm 0cm 0.78cm, clip, width=\columnwidth]{Figure9_bot}
	\caption{The axes and background galaxies are the same as Figure \ref{fig:sfms_z0}, but shown here are the VIP and nonVIP at their last close passage. Compared to the present day values, the interacting pairs sit slightly higher on the MS, and fewer of them lie in the green valley.}
	\label{fig:sfms_LP}
	\end{minipage}
\end{figure*}

The star formation main sequence \citep[e.g.,][hereafter SFMS]{noeske2007, daddi2007, elbaz2007, salim2007, rodi2011, bluck2016, bluck2019, donnari2019} defines a general trend of all star forming galaxies: the star formation rate is tightly correlated with the stellar mass. That this relationship holds for a wide range of redshifts \citep[e.g., ][]{noeske2007, lee2015}, several orders of magnitude in stellar mass, and a relatively small spread in star formation rate implies that star forming galaxies behave in a self-regulatory manner with a fairly consistent star formation history throughout cosmic time \citep[e.g.,][]{bouche2010, lilly2013}. Outliers above the SFMS (starbursts) are thought to represent an important stage (that is, mergers) in galaxy evolution, though their relative contribution to the star formation density is still debated \citep[e.g.,][]{cox2008, rodi2011, hung2013, brennan2015, willett2015, brennan2017, ellison2018}. Quiescent galaxies lie in a so-called ``red cloud'' below the SFMS, with a ``green valley'' of transitioning galaxies between the two. Merging and interacting galaxies, which themselves are examples of starbursting systems, have been shown to lie above the SFMS \citep[e.g.,][]{puech2014, willett2015}. In particular, \cite{hung2013} show that for $z\sim0.4$ galaxies, distance above the SFMS is correlated with disturbed morphologies. However, other studies \citep[][]{willett2015, brennan2017} are unable to to confirm this morphological dependence.

Figure \ref{fig:sfms_z0} shows the star formation main sequence for all galaxies (that is, each point is an individual galaxy) at $z=0$ which meet the same mass criteria as the interacting pairs (grayscale hexagons; the black dashed line shows our fiducial SFMS fit), the VIP (diamonds, outlined in black), and the nonVIP (circles). The interacting pairs are coloured by the change in the log of their star formation rates from the present day to LP. These colours enable mapping from $z=0$ to LP, and show how the galaxies have evolved since their last close passage. Blue colours indicate that a galaxy has increased its rate of forming stars since LP, whilst red colours indicate a decrease in SFR since LP. The bottom panel of Figure \ref{fig:sfms_z0} shows the VIP and nonVIP distances, defined as $\Delta$log(SFR) = log(SFR) - log(SFR$|_{\rm{MS}}$), from the fiducial SFMS line as a function of stellar mass. First, we note that the VIP and nonVIP are consistent with the entire set of local TNG100-1 galaxies. The nonVIP appear to exhibit a tight scatter around the SFMS fiducial line, whilst the VIP display a larger spread. The stellar mass appears to increase with increasing FoF group mass, as is expected from abundance matching \citep[e.g.,][]{colin1999, krav1999, krav2004, vale2004, conroy2009, beh2010, guo2010, moster2010}. That there are more VIP at higher stellar and halo masses (see also Figures \ref{fig:dists_bulk} and \ref{fig:dists_s}) may indicate the VIP experience a dramatic change in morphology, perhaps toward compact quiescent spheroids \citep[e.g.,][]{ellison2018}.

Regardless of the orbital geometry, the tidal interaction at a pericentre will invariably draw material from the outskirts of each galaxy toward the center \citep[e.g.,][]{barnes1996, mihos1996, rupke2010, moreno2015, blumenthal2018}. In the case of prograde interactions, a significant amount of gas can be funneled toward the galaxy's nucleus, sparking a burst of star formation \citep[e.g.,][]{alons2000, barnes2004, evans2008, chien2010, moreno2015, lar2016}. Thus, ``observing'' the VIP and nonVIP at the time of their most recent pericentres would naturally push the points in Figure \ref{fig:sfms_z0} up to higher star formation rates. Figure \ref{fig:sfms_LP} shows the star formation main sequence (top) for the interacting pairs at their last pericentres, in addition to the distance from the main sequence fiducial line ($\Delta$log(SFR); bottom). Note that the galaxies which have met the stellar mass threshold at $z=0$ might not achieve this limit at LP. There are only a few galaxies whose masses become unreliable; they are removed from this LP analysis. The colours in both of these panels are the same as in Figure \ref{fig:sfms_z0}. Using these colours, we note that there are some galaxies which appear to move out of the bottom right part of the SFMS (the so-called ``red and dead'' galaxies) between LP and the present day. This may imply that membership in the various regions of the SFMS is fluid: galaxies might undergo periods of starbursts and relative quiescence \citep[e.g.,][]{forbes2014a, forbes2014b}.

The interacting pairs' shift above the main sequence from the present day to LP cannot be explained by the vertical translation of the SFMS with increasing redshift \citep[e.g.,][]{noeske2007, lee2015} alone. If that were true, the VIP should have systematically lower $\Delta$log(SFR) than the nonVIP in Figure \ref{fig:sfms_LP}. That this is not the case implies the difference in the merger-driven starbursts is mediated by the strength of the interaction, which we have shown is significantly boosted in the VIP. This is supported by the observation that the VIP have higher stellar masses than the nonVIP (Figure \ref{fig:dists_s}), and inhabit FoF groups with more massive dark matter haloes (Figure \ref{fig:dists_bulk}). Studies have shown \citep[e.g.,][]{sobral2011} a connection between the stellar mass, star formation rate, and density of environment. In later sections (\S \ref{sec:env}), we will show that the VIP environment -- partially as measured by the total FoF group mass -- is marginally more dense than that of the nonVIP.

\subsection{Environment}
In the previous sections, we have detailed the intra- and inter-galaxy properties of the interacting pairs. In this section, we describe the external forces acting on these systems through three environmental metrics: the nearest neighbour, the interaction strength, and the FoF group mass.
\label{sec:env}
\begin{figure*}
	\centering
	\begin{minipage}[t]{0.3\textwidth}
		\vspace{0pt}
		\centering
		\includegraphics[width=1.05\columnwidth]{Figure10_top_left}
		\includegraphics[trim = 0.6cm 0.57cm 0.3cm 0.35cm, clip, width=\columnwidth]{Figure10_bottom_left}
	\end{minipage} \hspace{0.01\textwidth}
	\begin{minipage}[t]{0.3\textwidth}
		\vspace{0pt}
		\centering
		\includegraphics[width=1.05\columnwidth]{Figure10_top_middle}
		\includegraphics[trim = 0.6cm 0.57cm 0.3cm 0.35cm, clip, width=\columnwidth]{Figure10_bottom_middle}
	\end{minipage} \hspace{0.01\textwidth}
	\begin{minipage}[t]{0.3\textwidth}
		\vspace{0pt}
		\centering
		\includegraphics[width=1.05\columnwidth]{Figure10_top_right}
		\includegraphics[trim = 0.6cm 0.57cm 0.3cm 0.35cm, clip, width=\columnwidth]{Figure10_bottom_right}
	\end{minipage}
	\caption{\textit{Top: }Placement of the nonVIP (purple circles) and VIP (salmon diamonds) in the TNG100-1 cosmic web. The point sizes are scaled by the total group mass (M$_{\rm FoF \, group}$, left), nearest neighbour statistic ($\Sigma_{\rm n}$, centre), and the interaction strength ($Q_{\rm int}$, right). \textit{Bottom: }We also show the distributions of the three environmental measures for the VIP (salmon) and nonVIP (purple), as before. Vertical coloured lines show the median values, and the corresponding coloured rectangles indicate the range within $\pm$ one median absolute deviation of the median. The VIP sit in significantly more massive haloes, in denser environments and are more affected by their surroundings than the nonVIP.}
	\label{fig:web}
\end{figure*}

There is no universal definition of galactic environment \cite[e.g.,][and references therein]{muldrew2012}. Many studies attempt to compare the various definitions of this fundamental property \citep[e.g.,][]{cooper2005, gallazzi2009, wilman2010, haas2012, shattow2013, fossati2017}. Parameterisations used to characterise the environment include the local number density \citep[e.g.,][]{dressler1980, lewis2002, cooper2005, shattow2013}, measurements of galaxy clustering \citep[e.g.,][]{skibba2013, skibba2015, gun2018}, and placement within cosmic structures \citep[e.g.,][]{yang2007, darvish2014, kuutma2017, liao2019}.

The top panels of Figure \ref{fig:web} show the placement of the VIP (salmon diamonds) and nonVIP (purple circles) samples in the TNG100-1 cosmic web, as traced by all subhaloes in the $z=0$ slice. Qualitatively, the VIP typically lie in denser regions than the nonVIP (which seem to mostly occupy voids). The marker sizes in each of the three panels scale linearly with the log of three independent measures of environment: the total FoF group mass (left, \S\ref{sec:fofm}), the n$^{\rm th}$ nearest neighbour, $\Sigma_{\rm n}$ \citep[e.g.,][middle, \S\ref{sec:sigman}]{dressler1980, lewis2002}, and the interaction strength, $Q_{\rm int}$ \citep[e.g.,][right, \S\ref{sec:qint}]{verley2007}. The structure of the cosmic web is defined by the location of haloes containing individual, groups and clusters of galaxies. Thus, the mass of a halo is indicative of its placement within this structure. The nearest neighbour statistic measures environment based on the number density of nearby galaxies, regardless of mass. On the other hand, the interaction strength measures the balance of external tidal forces from all galaxies within an aperture with the binding force of the galaxy. Whilst it can be difficult to discern any trends from the cosmic web panels, the bottom panels of Figure \ref{fig:web} show the subsequent distributions for each of these environmental metrics.

\subsubsection{FoF Group Mass}
\label{sec:fofm}
The bottom right panel of Figure \ref{fig:web} shows the distribution of FoF group total masses for the VIP (salmon) and nonVIP (purple). This indicates that the VIP sit in preferentially more massive haloes ($p\approx4\times10^{-4}$), surpassing the nonVIP by nearly half an order of magnitude. This is consistent with the fact that most massive haloes are likely to sit in nodes or at intersections of filamentary structures \citep[e.g.,][and sources therein]{bond1996, joachimi2015}.

\subsubsection{n$^{th}$ Nearest Neighbour}
\label{sec:sigman}
The n$^{\rm th}$ nearest neighbour statistic is a number density measurement that uses the distance to the n$^{\rm th}$ nearest neighbor, r$_{\rm n}$, to define the volume. That is,
\begin{equation}
	\Sigma_{\rm n} = \frac{\rm n - 1}{\frac{4}{3} \pi \mathrm{r}_{\rm n}^{3}}
\end{equation}
where the numerator n$-1$ is used to discount the central or, primary galaxy. (Note that here we employ a three-dimensional version of what is typically used by observers.) Thus, centrals with larger $\Sigma_{\rm n}$ sit in denser environments. For the purposes of this paper, we adopt n$=5$. The bottom left panel of Figure \ref{fig:web} shows that the VIP lie in preferentially denser environments than the nonVIP ($p\approx0.07$).

\subsubsection{Interaction Strength}
\label{sec:qint}
One major drawback of the $\Sigma_{\rm n}$ measure is that it does not account for the mass of neighbouring galaxies. The interaction strength, $Q_{\rm int}$, thus serves a useful counterpoint to  $\Sigma_{\rm n}$ in its careful accounting of the tidal effect of nearby galaxies. \cite{verley2007} defined the interaction strength as the ratio of the cumulative tidal forces tugging on the galaxy from all neighbouring galaxies within a set aperture, and the binding force keeping the central together:
\begin{equation}
	Q_{\rm int} \equiv \frac{F_{\rm tidal}}{F_{\rm bind}} \>\> , \>\>
	F_{\rm tidal} = \frac{M_{\rm n} D_{\rm c}}{R_{\rm nc}^{3}} \>\> , \>\>
	F_{\rm bind} = \frac{M_{\rm c}}{D_{\rm c}^2}
\end{equation}
where $M_{\rm n}$ is the mass of the neighbor, $R_{\rm nc}$ is the distance from the central galaxy to that neighbor, $M_{\rm c}$ is the mass of the central, and $D_{\rm c}$ is the diameter of the central. Following observational studies (for which this metric was developed), we take all masses to be the total mass within twice the stellar half-mass radius, and the diameter of the central galaxy which corresponds to that mass (that is, four times the stellar half-mass radius). This value is calculated in a number of different ways in \cite{verley2007}, including using a fixed and infinite aperture (that is, all galaxies within a fixed volume). They find that there was very little difference between the two, as distant galaxies will contribute only a small amount to the tidal field of the central. To accommodate the large present-day separations of some of our interacting pairs, we use an aperture of 5 Mpc (Figure \ref{fig:web}, bottom right). The VIP are affected by the tidal effects of their neighbours nearly ten times as much as the nonVIP ($p\approx7\times10^{-4}$).

\subsubsection{The Effects of Environment}
In the previous subsections, we demonstrated that the VIP belong to more massive FoF haloes, sit in denser environments, and are more affected by interactions with their neighbours than the nonVIP. Here we disentangle the effects of mass and environment and show that although the (more massive) VIP are in systematically more dense environments, there is no statistically significant difference between the $\Delta$log(SFR) of the nonVIP and VIP, when controlling for stellar mass.

Figure \ref{fig:envsfr} shows the $\Delta$log(SFR) as a function of stellar mass. The stellar mass distribution is split into three bins: 9.0 $\leq$ log(M$_{\star}$) $<$ 9.75, 9.75 $\leq$ log(M$_{\star}$) $<$ 10.5, and log(M$_{\star}$) $\geq$ 10.5. These mass increments were chosen to separately analyse ``normal'' star forming galaxies (log(M$_{\star}$) $<$ 10.5) from those which have begun to dip below the main sequence (log(M$_{\star}$) $\geq$ 10.5). Environment is considered independently within each of these bins. Different colours indicate the galaxies which sit in a relatively low (light colours) or high (dark colours) density environment. The median environmental measure in each mass bin is used to delineate between low and high densities. We present this in Figure \ref{fig:envsfr} for the M$_{\rm FoF \, group}$ (top), $\Sigma_{\rm n}$ (middle) and Q$_{\rm int}$ (bottom). As before, the VIP and nonVIP are distinguished by diamond and circle markers, respectively. Centrals and satellites are indicated by the marker size (large and small, respectively). The median $\Delta$log(SFR) values for both environment bins within each mass bin are displayed as stars with error bars indicating the  first and third quartiles (that is, the width of the distribution). For comparison, the median value of $\Delta$log(SFR) of the underlying TNG100-1 distribution is displayed by the X's.

The first two mass bins of all three environmental metrics shown in Figure \ref{fig:envsfr} show no clear trend with environment. That is, not only do they show no distinction between high and low density environments, but they are consistent with the background distribution of all TNG100-1 galaxies (coloured X's). Only in the largest mass bin do we see any significant difference between the low and high density environments across all samples. In the largest mass bin of the top panel (M$_{\rm FoF \, group}$), the less massive FoF groups have systematically higher $\Delta$log(SFR) than the high mass FoF groups, as these are likely quenched or are in the process of quenching. It should be noted however that in this panel, the interacting pair sample (and its individual components) are consistent with the background.

The middle and bottom panels of Figure \ref{fig:envsfr} indicate that denser environments foster higher star formation rates only within the highest mass bin. Whereas before, the interacting pairs behaved similarly to the background TNG100-1 galaxies, in the $\Sigma_{\rm n}$ and Q$_{\rm int}$ panels, the interacting pairs diverge significantly from the background TNG100-1 galaxies. Further, that the environmental dependence of $\Delta$log(SFR) only becomes appreciable at higher masses -- when AGN activity and quenching begin to dominate a galaxy's evolution -- implies environment plays a larger role in suppressing quenching than it does in boosting star formation.

Though there is no clear distinction between the VIP and nonVIP at any mass bin (except for the highest mass bin of Q$_{\rm int}$), the satellites and centrals appear to have divergent evolutionary pathways. The centrals dip low in $\Delta$log(SFR) at high masses whereas the satellites are only moderately affected. This implies that centrals are likely to quench before their satellites. It may be that the evolution of satellites is more sensitive to environment, whilst the evolution of centrals is depends more strongly upon mass; perhaps an example of the interplay between ``environment quenching'' and ``mass quenching'' \citep[e.g.,][]{peng2010, bluck2016, bluck2019}. Thus, the relative importance of environment and stellar mass  depends upon which component of the interaction is the subject of inquiry.

\begin{figure}
\includegraphics[trim = 0.35cm 0.5cm 0.5cm 0.5cm, clip, width=\columnwidth]{Figure11}%
	\caption{We separate the stellar mass into three bins, and further split the subsample into two environmental bins based on the median environmental measure in that mass bin. Stars indicate the median value of $\Delta$log(SFR) for all interacting pairs within each mass bin with error bars that correspond to the median absolute deviation. Markers represent the the median $\Delta$log(SFR) for the galaxies within that mass bin. This is shown for the FoF group mass (top), $\Sigma_{\rm n}$ (middle), and Q$_{\rm int}$ (bottom). X's represent the background distribution of all TNG100-1 galaxies. Stars indicate the sample of interacting pairs (that is, the VIP and nonVIP together). As before, the nonVIP and VIP are indicated by circles and outlined diamonds, respectively. Satellites and centrals are also shown, and are distinguished by symbol size: small and large, respectively.}
	\label{fig:envsfr}
\end{figure}

\section{Conclusions}
\label{sec:conclusions}
In this paper, we identify a set of paired galaxies from the $z=0$ snapshot of the TNG100-1 simulation of IllustrisTNG \citep{marinacci2018b, naiman2018, nelson2018, pill2018b, springel2018}. We generate ideal mock SDSS $g$-band images of all pairs and visually classify each as interacting or not interacting. We then confirm using the information from the Sublink merger tree, and find that of the interacting pairs, we correctly identify 38 (the Visually Identified Pairs, or VIP) and miss 47 (the Non-Visually Identified Pairs, or nonVIP). Our analysis includes a detailed study of the interacting pairs' present day properties, as well as their properties at their respective last pericentres. \\

Our primary findings are as follows:
\begin{enumerate}
	\item Stellar morphologies are not ideal for identifying interactions as the visibility of stellar tidal features, which is in part dependent upon the environment and the time since the last close passage.
	\item Using the merger trees, we trace the interacting pairs back to their time of last pericentric (LP) passage.
	\item The VIP have more recently undergone a close passage than the nonVIP by about a factor of two. As a result, their tidal features are easier to observe. Merger classifications are thus biased toward recent interactions.
	\item Compared with the nonVIP, the VIP sit in very different environments. The VIP are: in groups which are nearly 2.5 times as massive; in nearly twice as dense surroundings; and are affected by interactions with their neighbours by nearly an order of magnitude more than the nonVIP. Classification schema based on stellar morphologies are biased toward dense environments.
	\item Though the VIP sit in distinct environments from the nonVIP, the visibility of a pair does not depend strongly on environment, when correcting for stellar mass.
\end{enumerate}

Care should be taken when interpreting these results within the context of large observational catalogues of galaxy pairs \citep[e.g.,][]{ellison2008, ellison2010, patton2016}, as the mass range covered in this work is relatively limited. The roughly 45\% merger recovery rate that we present here should \textit{not} be taken as a completeness correction. This work can only offer a critique of the observational surveys used to derive merger rates \citep[e.g.][]{kartaltepe2007, kitzbichler2008, robo2014}. To fully answer this question would require a realistic mock (that is, not ideal mock as described above) survey of all galaxy pairs, though this is beyond the scope of this work. However, we have illuminated distinct biases inherent in observational galaxy pair catalogs. If these are used to determine the merger rate, the result is likely to be biased toward close pairs (\S\ref{sec:dyn_pairs}), high stellar masses (\S\ref{sec:prop_z0}) that may result in particularly prominent tidal features, and recent pericentric passages (\S\ref{sec:prop_lp}).

$\>$

In addition to the intrinsic and dynamic properties of galaxies, the production of tidal features depends on the interaction geometry. In a forthcoming paper (Blumenthal et al. \textit{in prep}), we investigate the orbital characteristics of the interacting pairs sample. This work will provide a realistic set of parameters from which to produce the initial conditions of future idealized simulations, including the eccentricities, inclinations, and first pericentric separations. Additionally, we will assess orbital stability and the validity of the Keplerian approximation.

\section*{Acknowledgements}
KB acknowledges partial support from the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1329626. Support for JM is provided by the NSF (AST Award Number 1516374), by the Australian Research Council (ARC), and by the Harvard Institute for Theory and Computation, through their Visiting Scholars Program. FM acknowledges support through the Program ``Rita Levi Montalcini'' of the Italian MIUR. MV acknowledges support through an MIT RSC award, a Kavli Research Investment Fund, NASA ATP grant NNX17AG29G, and NSF grants AST-1814053 and AST-1814259. The authors would like to thank the anonymous reviewer, and the Scientific Editor Prof. Joop Schaye, both of whom provided invaluable feedback on the manuscript. In addition, we thank Dylan Nelson and Annalisa Pillepich for their thoughtful comments. The simulation used in this work, TNG100-1, is one of the flagship runs of the IllustrisTNG project, and was run on the HazelHen Cray XC40-system at the High Performance Computing Center Stuttgart as part of project GCS-ILLU of the Gauss Centre for Supercomputing.

\bibliographystyle{mnras}

redpajama_set_name: RedPajamaArXiv


Helen Castor, Joan of Arc: A History (London: Faber & Faber, 2014).
Winner of the Franco-British Society Book Prize, 2014.
Statues of Joan of Arc, canonised by the Pope in 1920, can be found in nearly every church in France. Churches in Rennes, Rouen, Nice, Orléans, Le Touquet, Versailles and many other places are dedicated to this Saint. Yet, as Helen Castor informs us, Joan of Arc was a name Joan never used. With so much mystery and myth surrounding the subject, Castor succeeds in the difficult task of unravelling the gossip and revealing facts based on archival research.
She commences this tripartite book by setting the scene at the bloody battle of Azincourt (Agincourt to the English) in 1415, the victory of the English and their King's claim to the throne of France, 'heaven's mandate in action' (19). Far from a peaceful outcome, the cauldron bubbles fiercely as rival forces gain power and strength and are more and more determined to win ground. There is conflict between England and France and between the Burgundian and Armagnac claims. This is a story of treachery, destruction, starvation, epidemics and endless warmongering. Castor depicts the lawlessness of France, against a backcloth of a belief in the divine right of Kings. Her style is engaging such as her vivid description of the assassination of John the Fearless, Duke of Burgundy on the bridge over the river Yonne at Montereau: 'As the duke knelt, doffing his black velvet hat in obeisance to his prince, he could hear the water moving softly all around […]. The moment was fleeting. Then the bullet struck: the steel blade of a war axe, driven deep into his skull. There was blood, pooling around the falling body of John of Burgundy, dripping in great gouts from the axe in the hands of Tanguy du Châtel' (36).
No wonder a saviour is needed in the shape of Joan the Maid whose prized virginity is ascertained by several examinations. We marvel at the illiterate Maid's linguistic ability to speak and persuade her followers of her special Divine powers and right (98), and that she has come at God's command to fight the English (106). Even the foreskin of Christ, a sacred relic that offered special protection in childbirth, plays a part in this story. To what extent one can believe in its entirety the story of Joan of Arc is left to the reader.
This complicated story is facilitated by clearly presented family trees, a cast of characters and some delightful illustrations in colour.
Honorary Research Fellow, University of Exeter.
ISBN 978-0-572-28462-7. Hardback, 328 pages. £20.00.


text:
package org.apache.spark.sql

/**
 * A container for a [[Dataset]], used for implicit conversions in Scala.
 *
 * To use this, import implicit conversions in SQL:
 * {{{
 *   import sqlContext.implicits._
 * }}}
 *
 * @since 1.6.0
 */
case class DatasetHolder[T] private[sql](private val ds: Dataset[T]) {

  // This is declared with parentheses to prevent the Scala compiler from treating
  // `rdd.toDS("1")` as invoking this toDS and then apply on the returned Dataset.
  def toDS(): Dataset[T] = ds

  // This is declared with parentheses to prevent the Scala compiler from treating
  // `rdd.toDF("1")` as invoking this toDF and then apply on the returned DataFrame.
  def toDF(): DataFrame = ds.toDF()

  def toDF(colNames: String*): DataFrame = ds.toDF(colNames : _*)
}

redpajama_set_name: RedPajamaGithub


text:
City life is comfortable, but we did not move to the other side of the world to dwell in another concrete jungle. We spent a while in Melbourne, one of the most livable cities in the world, yet a more adventurous approach to the journey was calling. Time to hit the road had come, and we picked a very special one to move on.
The Great Ocean Road is a scenic drive along the southern coast of the Australian state of Victoria, stretching from the town of Torquay, a hundred kilometers far from Melbourne, to Allansford. It is considered one of the most scenic drives in the world, and it was built by returning soldiers, as a permanent memorial to those who died while fighting in the First World War. Therefore, it can also be considered the largest world's war memorial. Winding through stunning beaches, steep cliffs and lush swathes of rainforest, it genuinely recalls the landscape of a Conrad's or Hemingway's book. The breeze coming from the Southern Ocean is cool and pristine, and looking at the horizon you come to understand that there's nothing more southern than Antartica in front of your eyes. Which makes you feel like in a quite remote corner of the Earth.
Before leaving Melbourne, we bought an old Mazda 626. We quit our jobs and left our rooms on rent, acquired some camping equipment, and last but not least, we went to pick our mother up at the airport. At the age of 68, she decided to fly all the way from Italy to join us in this two-week drive.
3, 2, 1. Ready to go.
Since we had already visited Torquay, we headed directly towards a much more exclusive place: Bells Beach. With its waves perfectly crafted in a magnificent bay, it is considered one of the world's surf meccas. It is not a case that the famous Australian brand Rip Curl organizes here the renowned international surf contest Rip Curl Pro Bells Beach, this edition, the 56th, taking place on April 12-24, 2017.
Now, there's to be said that the Great Ocean Road has become, through the years, a massive tourism industry. Although the nature and the landscapes are perfectly conserved and managed, the average accommodation and food along the way are quite expensive. The gorgeous towns became resort for wealthy owners, and the lifestyle tends to be leaning much more to "exclusive" than "budget". Nevertheless, if you just want to camp out and enjoy the wilderness at its fullest, alternative and quite smart solutions can be easily found through WikiCamps Australia, a brilliant app which is well worth all the $7 it costs.
This way, instead of finding our first overpriced accommodation in Anglesea, we ended up camping at the Whinbury Hill Equestrian Centre, a lovely spot just a few kilometers inland. Plunged in the Victorian countryside, it well served as a base to explore the surrounding points of interest, such as the several wineries scattered in the area and the beautiful Erskine Waterfalls, surrounded by a verdant rainforest rich in both wildlife and flora which especially our mother fell in love with.
The direct and constant contact with the Earth is the trademark of this route, and camping out is not only cheaper than any other solution, but by far the most authentic way of enjoying the majestic Australian open spaces.
Moreover, Parks Victoria should be praised for the smart way of managing its abundant natural resources. Abusive camping is forbidden, with a $303 fine applying, but there is plenty of campgrounds, many of them free of any charge. Where fees incur, you can easily book your spot online on the Parks Victoria website.
Among the others, is well worth mentioning the Allenvale Mill campsite, set right on the outskirts of Lorne, the second major town on the way. Here we spent two nights, pitching our tent right in the middle of the forest. The facilities are essentials, but the sounds and colors of nature just overwhelming. We fell asleep on the notes played by an orchestra of birds and koalas, and woke up with kangaroos grazing right in front of our tent. Simply breathtaking.
Lorne would probably like to be the most high-end spot along the way, with several boutiques and restaurants which tourists and second-property owners splash their money out with. We rather focused, though, on enjoying a beautiful sunset at Teddy's Lookout, and swimming at the scenic beach which we noticed right below the aforementioned panoramic spot.
Although the wild way of life is quite aesthetic, after four nights out the pleasure of a hot shower and a proper bed is equally appealing. Thankfully, there was a perfect location on the way to meet our needs: the Seacroft Estate. Nestled right along the Great Ocean Road in between Lorne and Apollo Bay, it was the best solution to restore our energies and cook some food in a proper kitchen, while enjoying a full view of the Southern Ocean.
Laying just a stones's throw away, the awe-inspiring Great Otway National Park was waiting for us in all its majesty. Stretching from the Otway Ranges to the relevant coastline, it represents the habitat of a rich biodiversity, a sample of which can be admired at the Maits Rest Rainforest Trail, a 800mt loop in one of the most ancient rainforest on the planet.
Apollo Bay is a much quieter town than Lorne, relying on fishing and, only lately, on the stream of tourism generated by the Great Ocean Road. Every year, during the Southern winter, these shores host many different species of migrating whales, including Southern Right Whales, Humpback Whales, Blue Whales and Killer Whales (the "Orcas"), which come here to breed, enjoying warmer waters, before heading off to feed in sub-Antarctic waters.
Past Cape Otway, we entered the coastline called "The Shipwreck Coast", where over 500 vessels are thought to be sunk, until eventually reaching the main attraction of the Great Ocean Road: the Twelve Apostles. This is actually only one of the many limestone formations in the surroundings, all contributing to the beauty, yet hellish reputation, of this shoreline. Huge waves and a salty wind relentlessly hit this littoral, but this did not prevent us from having a close look. Every beach is a beach, and we love to live each and every the way we like: beach umbrella and football.
Port Campbell is the closest city to these stunning rocky formations, and it is usually identified with them. The surroundings, though, are well worth exploring too. Especially if you bump into a bad weather day, you can definitely indulge yourself with the 12 Apostles Gourmet Trail, which we combined with some fishing at the Bullen Merri Lake. This time, we found a cheap, comfortable and green refuge at the Port Campbell Recreation Reserve Camping, located right in the middle of the town.
On the very last day of the trip, we explored the last stretch of the Great Ocean Road, visiting one of the most surprising spots we found en route, the Tower Hill Wildlife Reserve. This astounding hill of volcanic origin is completely surrounded by water, except for the narrow strip of land which lets visitors in and out, and it is home of a manifold ecosystem, ranging from kangaroos, koalas and emus to every sort of bird and plant. Of the many walks which take you exploring this piece of Eden, we chose the one taking atop the hill, where a delightful sunset left our eyes brimmed with beauty.
Our last stop was the lovely hamlet of Port Fairy. Our last dance with the Southern Ocean was staged on the idyllic Griffitts Island, were a lonely penguin and a couple of huge stingrays greeted us, before our departure towards other enchanting destinations throughout the endless, wild Australian land.
Photography: Enrico and Matteo Fabi
The Pristine South-West of Australia
A new beginning: Pemberton
Boranup Karri Forest

redpajama_set_name: RedPajamaCommonCrawl


text:
November 20, 2013 November 19, 2013 Anaea Lay Books, Craft Crucible, Good Stuff, Writing
CC: Seven Items in Jason Reynolds' Jacket Pocket, Two Days After His Suicide, As Found by his Eight-Year-Old Brother, Grady
A story that illustrates the heavy lifting you can make prose do just with the title, this week we're analyzing Robert Swartwood's Seven Items in Jason Reynolds' Jacket Pocket, Two Days After His Suicide, As Found by his Eight-Year-Old Brother, Grady published in Pank Magazine in April of 2010. This story was a runner-up for the Micro Award, has been cited in numerous places as an example of quality lit fic gone speculative, and, perhaps most notably, fills in as my best friend when I'm claiming I am not a flash-fiction-bigot.
(Flash fiction, for those of you not hip with the terms, refers to super short fiction, generally 1000 words or less, though there are internecine wars over the true cut-off for flash, complete with heretics, purges, and plagues of locusts. Also, I am totally a flash-fiction bigot. Sorry.)
This story is brilliant and beautiful and full of things I adore in my fiction (Sibling love/protectiveness! Suicide! Creepiness!) but the thing that stands out about it and makes it worth pulling apart is its sheer, relentless efficiency. Genuine efficiency, though, not the pruned and constrained soullessness or lack of development you get with inferior flash. This story chooses its moments carefully, but having chosen them, gives them lots of space to breathe and grow.
Take a careful look at this story. There is no plot. None. It's a list. There's no character development – the entire story functions on Grady's ignorance as he looks at the items and he is just as ignorant at the end of the list as he was at the beginning. There's no lesson – at no point does the story instruct, moralize, or exposit on theme. In short, there no arc contained in the text. The arc, dear reader, happens in your head. It's your ignorance that sets the stage for the development with the climax of your epiphany at the end. Swartwood has, in short, turned the audience into an active participant in the story because the thing that the story does is change the reader. At the beginning, you were just reading some hopped up flash piece with a clunky title. At the end, you need to go hug your sister. (This precise experience may not be universal)
This one's so short I'm not going to quote from it much. Fortunately, the paragraphs are numbered, so it'll be easy to keep track of where I am as I ramble.
The sneaking a story in through talking about objects technique is not new, and gets done badly all the time because the writer forgets that they're supposed to be talking about the object, and wanders off into the story instead. Swartwood doesn't do that. That first paragraph? Relentless. Every sentence is about the compass. Even the last sentence, which is where we get the details of Jason's suicide, is about how the compass failed him. It's a nice trick, too, because we're reading the prose describing the compass failing, but we're clever readers so we know that's Grady projecting on the compass, which means we know it's Grady feeling like he failed his brother. Not just any failure, either, but one he'd thought was a success. It was "the best gift he'd ever received." But that wasn't enough to keep him alive, which is so much more tragic than if it had simply been awesome, or cute, or "gee, thanks, kid."
That doesn't even get into how this compass manages to convey not only that Grady looks up to and idolizes his older brother, which is easy because we expect that given the context, but that he feels protective of him. Jason didn't get that compass just because it was what happened to be in the cereal box on his birthday, but because he needed it. Grady isn't a passive member of this sibling relationship. His contributions aren't effective, and he knows it, but he does contribute. Which, you know, does a fine bit of setting up the gut-wrench in the finale.
Let's move on to take a look at item four. This item doesn't give us any details about the suicide, the murders, the brothers' relationship, or their home life. This paragraph is entirely about Grady's innocence and the contrast of it Jason. Grady doesn't know what a condom is. He's so ignorant that he thinks moist towelette? Candy? Nope, weird balloon. Meanwhile, Jason's carrying a condom around in his pocket, and there's nothing saying anything about that in the earlier paragraphs, but you have to wonder whether maybe the girls weren't just slashed. Exactly how dark is Jason, the reader must wonder, while Swartwood presents you with endearingly cute confuddlement.
Grady's ignorance on this point is also telling just in terms of his relationship with his brother – Jason is apparently not the kind of older brother who does sibling bonding through corrupting innocence, else Grady would know all about condoms, and a whole host of other things besides.
And we end the story with, what? An adorable picture of Grady? Jason kept a memento of his brother right up to the end? A memento so endearing that everybody likes it, so much that mom noticed it missing? Aw, maybe that gave Jason some comfort in his last moments. Or maybe worry about his brother delayed him, convincing him to stay strong and try to pull himself together.
On the back, scrawled like the rest of the pictures: FOURTH.
Syke! Now go cry in a corner, you manipulated little sap, because Grady has no clue what happened but you do and that means you have to have all the appropriate feelings for him. And don't think about it too hard, or you'll start picturing Grady's reactions as he gets older and puts the pieces together, and then you start wondering whether it being midnight EST means your sister's going to be at a spot in her sleep cycle where she won't mind if you call her for an inexplicably maudlin, "You know I love you and will definitely commit suicide rather than let my mental illness/demonic possession force me into cutting you into pieces, right?" Or whatever your personal equivalent of that might be.
And on that cheerful note, I turn it over to the comments.
Next week, Comes the Huntsman by Rachel Acks.
The week after that, The Three Feats of Agani by Christie Yant.
And then, by popular demand, The Things by Peter Watts.
One thought on "CC: Seven Items in Jason Reynolds' Jacket Pocket, Two Days After His Suicide, As Found by his Eight-Year-Old Brother, Grady"
Rebecca Schwarz says:
Good flash is hard to find! And this one is really better than good. As you say, there is no plot here for the characters. In lieu of a plot there is epiphany, not an epiphany for Grady, not here at least. It's all for the reader. This is quite a trick to pull off and perfectly suited to flash fiction.
It's well done all the way through, but what brings it to the next level for me, is Grady's age and innocence. Reading the title, at first it seems like his age is just a ploy to make the story more poignant, but by the end it signals Grady's inevitable loss of innocence. Swartwood essentially wrote the first half of the story and invites the reader to become the storytellers and imagine Grady growing older and coming to understand the terrible solution to the puzzle pieces that these objects in Jason's jacket represent.
A solution that Grady alone will have once he figures out what happened, since apparently the authorities do not suspect Jason. Grady alone is in possession of his big brother's jacket and all the things in his pockets.
Things I also love:
The way that the smell of the gum reminds Grady of the time Jason sacrificed himself to protect his brother, setting up the ending.
The way Swartwood seamlessly slides in more and more gruesome details about the murders, more even than outsiders, especially neighborhood kids would likely hear on the playground.
How the Tweety Pez dispenser seems to be a convenient cover to explain the "voices" Grady hears in his brothers room, but it's in Jason's pocket at the time of his suicide.
When Grady sees the ticket stubs, he remembers a series of details, which he "barely noticed." He doesn't detail the movie he was so excited to see; instead he lists his brother's actions. Actions that perhaps signal the first chink in Grady's image of his brother.
Normally, I'm not a fan of serial killer stories as the usually they deny the complexity of mental illness, and/or the sheer difficulty of trying to relate to a character who lacks empathy. Telling this story through the eyes of Jason's little brother and showing Jason's love and protectiveness made it work for me.
Podcast: A Secret Map of Shanghai
Podcast Reader Auditions


text:
2 structures 493 species 0 interactions 1076 sequences 307 architectures
Family: Glucodextran_B (PF09136)
Summary: Glucodextranase, domain B
Glucodextranase, domain B Provide feedback
Members of this family adopt a structure consisting of seven/eight-strand antiparallel beta-sheets, in a Greek-key topology, similar to the immunoglobulin beta-sandwich fold. They act as cell wall anchors, where they interact with the S-layer present in the cell wall of Gram-positive bacteria by hydrophobic interactions. In glucodextranase, Domain B is buried in the S-layer, and a flexible linker located between domain B and the catalytic unit confers motion to the catalytic unit, which is capable of efficient hydrolysis of the substrates located close to the cell surface [1].
Mizuno M, Tonozuka T, Suzuki S, Uotsu-Tomita R, Kamitori S, Nishikawa A, Sakano Y; , J Biol Chem. 2004;279:10575-10583.: Structural insights into substrate specificity and function of glucodextranase. PUBMED:14660574 EPMC:14660574
SCOOP: Big_13 Big_6 Big_7 Cadherin-like
Similarity to PfamA using HHSearch: Cadherin-like Big_7 Big_13
SCOP: 1ug9
The glucodextranase B domain adopts a structure consisting of seven/eight-strand antiparallel beta-sheets, in a Greek-key topology, similar to the immunoglobulin beta-sandwich fold. It acts as a cell wall anchor, interacting with the S-layer present in the cell wall of Gram-positive bacteria by hydrophobic interactions. In glucodextranase, domain B is buried in the S-layer, and a flexible linker located between domain B and the catalytic unit confers motion to the catalytic unit, which is capable of efficient hydrolysis of the substrates located close to the cell surface [ PUBMED:14660574 ].
Seed source: pdb_1ug9 & Pfam-N
Author: Sammut SJ
Number in seed: 8
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Glucodextran_B domain has been found. There are 2 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein sequence.


Strength training should be the foundation for improvement in performance and the reduction of injures in the athletic patient. This course will present the current clinical evidenced based effects of eccentric loading in the treatment of tendinopathy. Eccentric loading has been demonstrated to provide the best form of strength training in the rehabilitation of tendinopathy. The beneficial effects of eccentric training on muscle and the tendon unit along with a review of the treatment guidelines for clinical application of eccentric exercises in rehabilitation programs will be reviewed. Eccentric exercises and plyometric training techniques will be outlined and demonstrated with videos and pictures in conjunction with various case studies. Upon the conclusion of this course the participant will have a better understanding of the role of eccentric exercises and the use of plyometric training techniques for use in the rehabilitation setting.
Dr. Donatelli is a clinician, researcher, author, and educator and considered one of the leading experts in the treatment and prevention of upper and lower extremity injuries in the world. Dr. Donatelli has an extensive list of accomplishments and titles within his profession, including national director of Sports Rehabilitation Physiotherapy Associates; personal physical therapist for Andy Roddick, ATP tour; physical therapist for the PGA Tour; consultant for the Philadelphia Phillies and Milwaukee Brewers; and clinic director of an outpatient facility providing sports and orthopedic rehabilitation. Dr. Donatelli has rehabilitated numerous professional athletes and was the physical therapist for the Champions Tour of Tennis, which included tennis legends Jimmy Conners and John McEnroe. He is the editor of Physical Therapy of the Shoulder 4th Edition, Orthopedic Physical Therapy 3rd Edition, and Sports Specific Rehabilitation. Dr. Donatelli has taught orthopedic topics throughout the United States, and in Canada, England, Romania, Australia, Sweden, China, and more.


text:
timminchin.com is updated by Tim
and the wonderful Shell.
Matilda & Me is now available to download in Australia and on DVD! Plus seven exclusive extra clips on ABC iView
by Shell 5th Apr 2016 | 4 comments
Last Sunday Matilda and Me aired on Australian TV. Narrated by his sister Nel and with the rehearsals for the launch of the show in Sydney forming the backdrop, the documentary charts Tim's long-term fascination with Roald Dahl, his childhood and early career and the creative process behind the smash hit musical. You'll find more details and a teaser clip here.
Matilda & Me is now available on DVD from MADMAN and JB HI-FI across Australia. Both ship internationally but please note it is a Region 4 DVD, so aimed primarily at the Australian and New Zealand market. Outside Region 4, you'll need a multi-region DVD player or PC/Laptop with a DVD drive to play it. (A handy guide to DVD compatibility can be found here courtesy of eBay.)
Australians can also download it from iTunes and it will soon be available on many other digital platforms. (You'll find them listed to the right of the DVD here).
Matilda and Me will be available on ABC iView until 8:40pm on 18th April 2016, where you can now also watch SEVEN exclusive behind the scenes clips.
Sadly iView is geo-blocked for those outside Australia but wheels are in motion for this documentary to be available more widely. We'll let you know as soon as anything is confirmed but it's also worth keeping an eye on the Matilda & Me Facebook Page for updates and news.
Benedikt on 11th of August 2016
What's the point in region-coding the disc? This is a serious question… Why get shows region-coded that are just not available in other parts of the world? Doesn't this just invite piracy for which many people would otherwise be willing to pay?
Mary B. Egger on 18th of May 2016
Germane to none of the above, but in a response to the outstanding nine-point UWA commencement speech, may I remark that Mr. Michin's hairstyle borders on affectation and a sort of "Who does he think he is with that Tiny Tim flowing mop that was getting in the way?' It totally detracted from his message, which was great, but the flowing locks definitely were an unnecessary 'sideshow,' considering that everyone else was well-groomed.
hanna grande on 8th of April 2016
Yeah I want it <3. Thanks for post
Ros on 7th of April 2016
Would love to get this but seems to be a bit complicated to be able to play it in the UK. Hope it's possible!


Mohanlal-Ranjith team's Drama out of Onam race?
Mohanlal-Ranjith team's Drama out of Onam race?
After an eight month long break, Mohanlal is set to make a comeback with four back to back releases. The actor has Neerali, Kayamkulam Kochunni, Drama and Odiyan in gearing up for release. While Neerali is slated to hit screens this Friday (July 13), Kayamkulam Kochunni is scheduled for release by the third week of August. Drama was supposed to hit screens on August 24 during the Onam holidays.
However, latest reports suggest that the movie's release has been postponed to a further date in September. The makers are yet to confirm it officially but if Drama has indeed pulled out of the Onam race, the battle for glory at the Kerala box office will be between Kayamkulam Kochunni and Mammootty's Oru Kuttanadan Blog.
Drama is eagerly awaited by the Malayali audience as Mohanlal is reuniting with ace filmmaker Ranjith after a three year long gap. Unlike their earlier films, Drama will not have any superstar elements like massive action scenes or powerful dialogues. Ranjith has scripted it as a feel-good movie with good dose of humour, sentiments and family drama. The recently released teaser gave hints of a fun entertainer, something that Mohanlal has not done in the recent past.
Mohanlal has pinned high hopes on the movie. According to him, Drama is a kind of movie that has not been attempted before in Malayalam cinema. The story, set entirely in UK, revolves around a middle-aged couple. Asha Sarath plays Mohanlal's pair in the movie. It also has Arundhathi Nag, Niranj Maniyan Pilla Raju, Siddique, Johny Antony, Renji Panicker, Shalin Zoya, Tini Tom and Baiju as part of the cast.
MK Nassar and Maha Subair are jointly producing the movie under the banners of Lilypad Motion Pictures and Varnachithra Good Line Productions.
Celebrities excited about Mohanlal turning a director!!
Nivin Pauly's next with Vysakh titled as Gouri?


The Bear River Board of Trade was established by an act of Parliament in 1905. Our membership of volunteers is drawn from local businesses, community volunteer groups, and individual community members.
The village is not incorporated. The Bear river is the borderline for the counties of Digby and Annapolis. Our volunteers represent the village to our municipal, provincial, and federal governments.
Our general meetings are held quarterly and we have an annual general meeting in the spring.
Goal(s): To create an attractive and functional entryway to the village. To make features accessible for public leisure and parking.
Meet with stakeholders to communicate vision and gather input: make recommendations to the BOT on projects, priorities, and initiatives regarding green space and waterfront development.
Work with local artists, landscapers, iron works to develop signage, artwork, landscaping and parking area.
This website is maintained by the Bear River Board of Trade.


SM entertainment's new female solo singer J-Min will release her debut album in Korea.
According to SM, the singer will release her first ever mini album in Korea in early July.
J-Min debuted as a singer in Japan in 2007 and has taken part in OSTs for various Korean dramas such as 'Warrior K', 'Wild romance', 'The king 2 hearts' and 'To the beautiful you'. She also achieved recognition as a talented singer by appearing musicals 'Jack the ripper' and 'Three musketeers'.
Prior to the album release, on June 23, J-Min will release the song 'After' which will be included in her debut album.


Make the most of your banner with these extremely durable banner frames.
These banner frames are an amazing marketing tool for use at events, exhibitions and conferences, as well as simply outside of your premises to promote your business. Ideal for outdoor or indoor use, these frames are made to last constructed from 35mm diameter steel tubes, and built to impress.


Big Brother Spoilers: First Team America Houseguest Revealed!
We have the Big Brother spoilers for you on the first Team America vote results! In one of the biggest twists of the season, the viewers voted this week to determine the first member of a secret, three-person alliance that will be partially controlled by the fans. As if all the other Big Brother 16 twists with the double Heads of Household, four nominees, new 'Battle of the Block' competition, etc. weren't enough to deal with.
We will be honest, we were pretty much convinced that the person who would get voted for the most this week would be YouTube personality Frankie Grande. Not only does he claim to have over a million followers himself, but he also has the support of his pop singer sister Ariana Grande and access to her millions of fans. It seemed like almost a guaranteed win for Frankie to get the most votes this week.
Imagine our surprise when it turned out that Frankie Grande did NOT win the first #TeamAmerica vote! Instead, it was Seattle native and makeup artist Joey Van Pelt. She seemed utterly thrilled at having been chosen. Especially when she found out that for ever secret mission the Team America alliance completes, each of them will earn $5,000.
We really did not see that coming. In fact, we probably would have chosen quite a few other players instead of her if it wasn't going to be Frankie. She's not a superfan, she's not one of the super hunks or model girls, and she is a self-declared radical left-leaning environmentalist activist. Not really the person we'd think America would vote to the top right off the bat.
But there you go, your first secret alliance member, Joey Van Pelt. Now that you know who the first person is, head on over and get to voting for the second person to join her on Team America.