text:
\section{Introduction}

Although M31 is the nearest large spiral galaxy to the Milky Way at a distance of $785 \;\pm\; 25\kpc$ \citep{mcc_etal_05}, we are still not clear of its exact location on the Hubble tuning fork diagram, which is key to understand its formation and evolution. The debates have a quite long history, probably started from \citet{lindbl_56} who first claimed that M31 is a barred galaxy based on the twist of central isophotes. The isophotal twist cannot be reproduced by an axisymmetric stellar distribution \citep{stark_77}. Latter studies argued that unbarred galaxies can also have twisted inner isophotes as they can originate from triaxial bulges, not necessarily bars \citep{stark_77, zar_lo_86, ber_etal_88, ger_etal_89, men_etal_10, cos_etal_18}, leaving puzzles on the true morphology of M31. Besides, the strong enhancement of star formation between $2-4$ Gyr ago \citep{wil_etal_15} could be linked to a recent single merger event \citep{ham_etal_18}. The age-velocity dispersion relation in the stellar disk \citep{bha_etal_19} suggested that a merger occurred $\sim3-4$ Gyr ago with an estimated mass ratio of 1:5. The merger would largely enhance the stellar velocity dispersion in the disk and affect the dynamical evolution of M31. After the merger event, another star formation burst in the whole bulge region happened near $\sim$ 1 Gyr ago, which might be caused by the secular evolution \citep{don_etal_16}.

While it is difficult to identify whether there is a bar based on the photometry of a highly inclined M31 disk, numerical simulations have provided more insights into the properties of the inner stellar structures. \citet{ath_bea_06} first used $N$-body models to reproduce the twist of central isophotes and the boxy shape seen in the near-infrared band \citep{beaton_etal_07}. They concluded that M31 has both a classical bulge and a bar whose major axis deviates from that of the disk by about $20^{\circ}$. The scenario is updated by \citet{bla_etal_17} who constructed $N-$body models for M31 to analyse the boxy-shape and the tilted velocity field \citep{opitsc_16}. The authors conclude that the classical bulge and the box/peanut bulge (BPB) in M31 contribute $\sim1/3$ and $2/3$ of the total stellar mass in the central part, respectively. \citet{blana_etal_18} further constructed made-to-measure (m2m) models for M31, using constraints from $3.6\;{\mu}m$ photometry \citep{bar_etal_06} and stellar kinematics \citep{opitsc_etal_18}. The models evolved from $N-$body buckled bar models. The BPB results from a buckled bar in their best model, with a half-length of $\sim4\kpc$ and a pattern speed of $40 \;\pm\; 5\freq$. They also checked a model with a very low pattern speed and found it does not match the data as nicely as the more rapidly rotating models. Moreover, \citet{sag_etal_18} found that the stellar metallicity is enhanced along the proposed bar structure. Based on the m2m models in \citet{blana_etal_18}, \citet{gaj_etal_21} constrained the 3D distribution of metallicity and $\alpha$-enrichment using observed $[Z/H]$ and $[\alpha/Fe]$ maps \citep{sag_etal_18}, and an X-shaped metallicity distribution is found in the bulge region. These results imply that M31 is a barred galaxy.

On the other hand, evidence from gas kinematics in M31 also hints for a bar. Large non-circular motions of gas have been identified in $\HI$ \citep{bri_bur_84, chemin_etal_09} and CO \citep{loinar_95,loinar_99,nieten_etal_06}. Besides, $\oiii$ emission from ionized gas shows a twisted zero-velocity curve in the central bulge region \citep[]{opitsc_etal_18} , and its morphology suggests a tilted spiral pattern with a lower inclination angle compared to the stellar disk \citep{opitsc_etal_18}. All of these are typical features seen in barred galaxies \citep{jacoby_etal_85, emsell_06, kuzio_etal_09, fat_etal_05}, although there are alternative explanations \citep[e.g. a head-on collision between M31 and M32 proposed by][non-circular motions of gas caused by a triaxial bulge]{block_etal_06}. Nevertheless, the overall shape of position-velocity diagrams (PVDs) of $\oiii$ in \citet{opitsc_etal_18} is similar to what has been observed in barred galaxies \citep{bur_ath_99, mer_kui_99}.

One of the most characteristic gaseous features in typical barred galaxies is a pair of dust-lanes on the leading side of the bar \citep{athana_92}. Dust-lanes are generally associated with shocks, which result in sharp velocity jumps in gas kinematical maps, and thus can be identified by integral-field unit (IFU) data \citep[e.g.][]{opitsc_etal_18}.

The goal of the current paper is to search and identify such shock (velocity jump) features from the observations. It should be noted that the shocks we are searching for are "large-scale" shocks \citep[e.g. of the type proposed by][]{robert_69,rob_etal_79} which are generally extended over a few kiloparsecs, rather than those "small-scale" shocks due to local turbulence of interstellar medium or supernova feedbacks. If the positions and properties of velocity jumps are similar to those expected from bar-driven shocks, this would provide independent, strong evidence for the existence of a bar in M31.

The paper is organized as follows: \S 2 describes the data of $\oiii$ and $\HI$ in M31. \S 3 discusses the criteria we use to identify shock features on PVDs of $\oiii$ and $\HI$. In \S 4 we present the results, and compare the shock features in $\oiii$ and $\HI$. We also present a map showing the positions of identified shocks. In \S 5 we compare the shock features in $\oiii$ with the bar-driven shocks in hydrodynamical simulations. \S 6 mainly discusses the potential physical reasons for the asymmetry in the shock features between the far side and near side of M31. We briefly summarize our findings in \S 7.

\begin{figure*}[ht!]
\includegraphics[width=\textwidth]{optical_image_shock_position_min.png}
\caption{Identified shock positions of $\oiii$ (red circles) and $\HI$ (blue triangles) superposed on the optical image of M31. The background image is taken from \url{https://apod.nasa.gov/apod/ap220119.html}. The image is credited to Subaru (NAOJ), Hubble (NASA/ESA), Mayall (NSF) and processed by R. Gendler and R. Croman. X-axis and Y-axis are taken along the major and minor axes of the stellar disk of M31, respectively. Solid, open, and dashed markers represent the Class I, Class II, and Class III shock features, respectively (see Table.~\ref{tab:criteria_shock_feature}). The black dashed line indicates the major axis of the bar in \citet{blana_etal_18}.}
\label{fig:shock_on_optical_image}
\end{figure*}

\section{Observational Data}

The main results of the current paper are summarized in Fig.~\ref{fig:shock_on_optical_image}, which shows the positions of the possible bar-driven shocks superposed on the optical image of M31. The distance to M31 is adopted to be $785 \;\pm\; 25\kpc$ \citep{mcc_etal_05}, so $1\arcmin$ corresponds to $228\pc$.

We use data from \citet{opitsc_etal_18} and \citet{chemin_etal_09} to study the gas kinematics in M31. \citet{opitsc_etal_18} carried out an IFU survey with the VIRUS-W instrument at the McDonald Observatory, which contains emission lines of $\rm H\beta$, $\oiii$, and $\rm [N\:{\RNum{1}}]$. Their observation covers the inner bulge region of M31 (20$\arcmin \times 10\arcmin$). \citet{chemin_etal_09} observed the 21 cm emission using the Syntheses Telescope at the Dominion Radio Astrophysical Observatory. Their $\HI$ survey covers the whole M31 disk.

Multiple gas components with different velocities are found over half of the total bins in \citet{opitsc_etal_18}, among which $\oiii$ has the strongest flux. The components of $\oiii$ with higher and lower velocities are labeled as $\firstcomponent$ and $\secondcomponent$, respectively. Multiple gas components are expected when the line of sight passes through different gas streams, which could be caused by a bar \citep{kim_etal_12a} or a collision between M31 and its satellite galaxy M32 \citep{block_etal_06}. We use the main (higher-velocity) component of $\firstcomponent$ to present the PVDs. We discuss the possible origin of the two components in \S\ref{sec:twooxy3}.

Similar to $\oiii$, $\HI$ observations by \citet{chemin_etal_09} show multiple velocity components in their spectra. The $\HI$ disk is extended and starts to warp beyond $\sim 20 \kpc$ \citep{new_eme_77,hender_79}. The $\HI$ component with a lower velocity is likely from the warped $\HI$ layer in the outer disk \citep{bri_bur_84,bri_sha_84}. \citet{chemin_etal_09} shows that gas from the warped region dominates the $\HI$ emission in the central $20\arcmin$, producing shallow linear structures on their PVDs. The warp may obscure the inner $\HI$ disk with similar velocities on PVDs. To find the main $\HI$ component that best represents the disk rotation, \citet{chemin_etal_09} selected the component with the largest velocity relative to the galactic systemic velocity while rejecting isolated faint features. This main $\HI$ component excludes the lower-velocity features that originate in the outer warp as well as the isolated faint features that possibly come from extra-planar gas (e.g. high-velocity clouds). We therefore use this main component of $\HI$ to identify $\HI$ shock features in the disk.

\begin{figure*}[t!]
\includegraphics[width=\textwidth]{scheme_dust_lane_new.png}
\caption{Schematic plot of the shocks in a counter-clockwise rotating bar under the viewing angle of M31. The shocks are expected to locate on the leading side of the bar. The major axis of the bar (dash-dotted line) is deviated from the major axis of the disk (dashed line along horizontal direction) by $17.7^{\circ}$. X-axis and Y-axis are taken along the disk major and minor axis, respectively. The vertical solid lines represent the locations of pseudo-slits. The thick and thin vertical lines indicate the slits covering both $\oiii$ and $\HI$ data and those covering only $\HI$ data, respectively. Text on the top labels the pseudo-slits for later reference. S0 is along the disk minor axis. Note that the shaded ellipse represents a projected inner circular gas disk, but not the bar of M31.}

\label{fig:scheme_shock}
\end{figure*}

\section{Construction of Pseudo-slits and Shock Identification}
\label{sec:identification_shocks}

Shocks on the leading side of the bar are commonly found in simulations \citep{rob_etal_79,athana_92} and observations \citep{sandag_61,san_bed_94}. The gas velocity component perpendicular to shocks has an abrupt change, producing sharp velocity jump features on the PVDs. In Fig.~\ref{fig:scheme_shock}, we plot the schematic view of the possible shocks under the viewing angles of M31. The position angle of the projected bar major axis (on the sky frame) from \citet{blana_etal_18} is $55.7^{\circ}$, deviating from $PA_{disk} = 38^{\circ}$ by $17.7^{\circ}$. If M31 is a barred galaxy as suggested by \citet{blana_etal_18}, we expect that shocks appear on the leading side of the bar and extend roughly to the bar ends. Thus, we position pseudo-slits perpendicular to the disk major axis so that the slits cut through the shocks nearly perpendicularly. The standard width of the slit is chosen to be $1.2\arcmin$ which is about twice larger than the spatial resolution of $\HI$ ($43.75\arcsec$). The slits cover the inner $20\arcmin$ where shocks are expected.

The large-scale bar-driven shocks generally produce sharp velocity jumps on PVDs, for which we try to search in this work. For each PVD, the goal is to identify the positions and amplitudes of shock features. \citet{canny_86} detected edges in signals by convolving data with a derivative of Gaussian, and it is now commonly used in 2D image edge detection. We modify their algorithm to detect step-function shaped and $\delta-$function shaped velocity jumps. The steps and instructions of our procedures are described as follows.

\emph{Requirement of sharpness.}
The amplitude of velocity jumps $\Delta V$ as well as the spatial extent $\Delta Y$ determines the sharpness of a shock feature. Simulations by \citet{ath_bur_99} and \citet{kim_etal_12a} have shown that typical bar-driven shocks have $\Delta V > 100-150\kms$ within $\sim 100-200 \pc$ (after projection $200 \times \cos 77^{\circ} = 45 \pc$). We expect the spatial extents of observed velocity jumps to be wider than those in simulations for several reasons: 1. Sharpest velocity jumps are expected when the slits cut shocks perpendicularly, which may not be the case shown in Fig.~\ref{fig:scheme_shock}. 2. Resolution of the observation is usually lower than that of simulations. 3. Dust extinction and local turbulence could blur the velocity field and produce less clear shock features. Considering these effects, we aim to find shock features showing $\Delta V > 100\kms$, $\Delta Y < 0.6\arcmin$ on PVDs, which corresponds to a velocity gradient over $100\kms/0.6\arcmin = 167\kmsarcmin$. We use a larger $\Delta Y$ of 2$\arcmin$ to identify $\HI$ shock features, which is around three times the spatial resolution (43.75$\arcsec$) of $\HI$. Compared to those of $\oiii$, the shock features of $\HI$ have a smaller velocity gradient due to lower spatial resolution.

\emph{Smoothing}. We use boxcar smoothing to reduce noises on PVDs. Each observed data point is replaced by the median\footnote{We also tested that using median gives sharper shock features than using mean, and the results of the flux density-weighted mean do not differ much from the non-weighted ones.} of its adjacent 15 points for PVDs of $\oiii$. Our tests show that the number of points of 15 is good enough for showing both strong and weak shock features of $\oiii$. We also tested the number as large as 21, the shock features with $\Delta V > 150\kms$ remain robust, but those with small $\Delta V$ are too weak to identify in this case. For shock features close to the boundaries in a few PVDs of $\oiii$ (especially on the near side), the data coverage might be incomplete to show shock features clearly. Therefore we avoid smoothing the 9 points and 15 points close to the far side and near side boundary of $\oiii$ PVDs, respectively. These numbers are empirically chosen to give a more regular shock position pattern. Black lines in Figs.~\ref{fig:PVDs_component_1_receding_side} and \ref{fig:PVDs_component_1_approaching_side} indicate the smoothed $\oiii$ curve. We find that shock features in $\HI$ are more sensitive to the smoothing parameter than that of $\oiii$. We therefore replace each point of $\HI$ main component by the median of its adjacent 5 points to avoid over-smoothing.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_OIII_receding_side.png}
\caption{PVDs of $\firstcomponent$ color-coded with flux density on the receding side of M31. The data is from \citet{opitsc_etal_18}. Each panel corresponds to one pseudo-slit in Fig.~\ref{fig:scheme_shock}. Coloured points represent the velocities of $\firstcomponent$. Black curves represent the boxcar smoothed result of the coloured points. Thick, thin, and dashed red lines indicate the positions of Class I, Class II, and Class III shock features, respectively. The shock features are clear mostly inside 7.2$\arcmin$ and they are mainly on the far side (negative Y values, below the disk major axis).}
\label{fig:PVDs_component_1_receding_side}
\end{figure}

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_OIII_approaching_side.png}
\caption{Same as in Fig.~\ref{fig:PVDs_component_1_receding_side} but on the approaching side. The shock features are mainly on the near side (positive Y values, above the disk major axis).}
\label{fig:PVDs_component_1_approaching_side}
\end{figure}

\emph{Locating a jump feature.}
Given a curve showing several velocity jumps, we would like to detect jump features automatically. Our procedure first creates a window of 0.6$\arcmin$ (2$\arcmin$ for $\HI$) wide and use it to cover a selected part of the curve. Then it convolves the data within the window using a derivative of Gaussian to highlight the position of the velocity jump, similar to the edge-detection algorithm in \citet{canny_86}. The derivative of the Gaussian operator has the form:

\begin{equation}
y = -\dfrac{x}{s^2}\exp{\dfrac{-x^2}{2\sigma^2}}.
\end{equation}
Here $s = 3$ is a scaling factor and $\sigma = 4$ represents the dispersion of the Gaussian. Vertical lines in Figs.~\ref{fig:PVDs_component_1_receding_side} and \ref{fig:PVDs_component_1_approaching_side} indicate the positions of selected shock features using this method. Tests of the edge-detection algorithm in identifying step-function shaped and $\delta$-function shaped jump features are given in Appendix \ref{appendix:A}.

\emph{Identification.} We calculate the velocity difference within 0.6$\arcmin$ (2$\arcmin$ for $\HI$) for each point on the boxcar smoothed curve. Since we want to find sharp velocity jumps, we focus on regions showing velocity differences over 100$\kms$. Our algorithm targets the region with the largest velocity difference, and returns one shock position using the method above. Then it moves to regions with smaller velocity differences, which departs from the identified shock features by at least 1.5$\arcmin$. We repeat this process several times until all of the velocity jumps over 100$\kms$ are found. The criteria to identify shock features is summarized in Table~\ref{tab:criteria_shock_feature}.

\emph{Classification of shock features.}
We classify the gas velocity jump features into three classes according to the likeliness that a bar-driven shock is present based on $\Delta V$.

$-$ Class I: if the feature shows a velocity jump over $170\kms$, it is classified as a gas feature "very likely" being a shock.

$-$ Class II: if the feature shows a velocity jump between $125\kms$ and $170\kms$, it is classified as a gas feature "likely" being a shock.

$-$ Class III: if the feature shows a velocity jump between $100\kms$ and $125\kms$, it is classified as a feature "possibly" being a shock.

\begin{table}
\caption{Criteria to identify shock (velocity jump) features.}
    \begin{center}
    \begin{tabular}{|m{1.8em}|m{6.7em}|m{5.8em}|m{7.2em}|}
        \hline
        {Class} & {$\Delta V$ within $\Delta Y$ ($\kms$)} & {Possibility of being shocks} & {Cross-comparison} \\
        \hline
        {I} & {$> 170$} & {Very likely} & \multirow{3}{7.5em}{Similar positions on PVDs of $\firstcomponent$ and $\HI$} \\
        \cline{1-3}
        {II} & {$125 \sim 170$} & {Likely} & {} \\
        \cline{1-3}
        {III} & {$100 \sim 125$} & {Possible} & {} \\
        \hline
    \end{tabular}
    \end{center}
\tablenotetext{}{$\Delta Y < 0.6\arcmin$ for $\oiii$ shock features and $\Delta Y < 2\arcmin$ for $\HI$ shock features. Note that the flat part of rotation curve of M31 is around 250 $\kms$.}
\label{tab:criteria_shock_feature}
\end{table}

The $\Delta V$ values in Table 1 are empirically chosen.  Previous observations and simulations of barred galaxies give a rough reference value of shock velocity jump $\Delta V \ga 100\kms$ \citep[e.g.][]{ath_bur_99}, which we choose to be the threshold $\Delta V$ of shock features. We have tested that minor variations of the $\Delta V$ ranges do not change the main result. We expect that positions of Class I and Class II shock features in $\firstcomponent$ are similar to those of $\HI$, except for central regions due to the lack of $\HI$.

\section{Results}
\label{sec:result}

\subsection{Shock features on PVDs in [OIII]}
\label{sec:shock_features_first_component}

Figs.~\ref{fig:PVDs_component_1_receding_side} and \ref{fig:PVDs_component_1_approaching_side} present the PVDs of $\firstcomponent$ and the boxcar smoothed result (black curve) on the receding ($X$ < 0) and approaching ($X$ > 0) side of M31. Negative and positive $Y$ represent the far and near side of M31. Overall, shock features are clearer on the receding side (i.e. Fig.~\ref{fig:PVDs_component_1_receding_side}), showing Class I shock features (thick red lines) in most panels. Class I shock features appear in S0 at $Y \approx -2.7\arcmin$ and $-0.8\arcmin$, together with a Class III shock feature (red dashed line) at $Y \approx 2.9 \arcmin$ (near side). In Fig.~\ref{fig:PVDs_component_1_receding_side}, the clearest shock features are shown in slits (S-1, S-2, S-3, S-4). Positions of Class I shock features shift downwards from $Y \approx -3.1\arcmin$ to $-3.6\arcmin$ as we go from S-1 to S-6. Further out, $\Delta V$ of Class I shock features decreases and the shock features turn into Class II (thin red line) in S-7.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{shock_clearest.png}
\caption{Sharp shock features of $\firstcomponent$ on the receding side of M31. Each curve corresponds to a Class I shock feature within slits S-1 to S-5 of Fig.~\ref{fig:PVDs_component_1_receding_side}. The color indicates the $X$ positions of the slits on the disk major axis.}
\label{fig:identified_shocks_component_1_receding_side}
\end{figure}

On the approaching side, Class I and Class II shock features show up in most panels, mainly between $Y \approx 3.2\arcmin$ and $4.6\arcmin$. Using the method described in \S\ref{sec:identification_shocks}, we extract the clearest shock features of $\firstcomponent$ on the receding side, which is shown in Fig.~\ref{fig:identified_shocks_component_1_receding_side}. Color indicates the $X$ positions of the slits. Each curve corresponds to one panel in Fig.~\ref{fig:PVDs_component_1_receding_side}. We do not show the shock features along S-6 and S-7 slits in Fig.~\ref{fig:identified_shocks_component_1_receding_side} for they are too close to the boundary of data coverage. The shock features are found between $Y \approx -3.1\arcmin$ and $-3.5\arcmin$, and shift upwards by $\approx 150 \kms$ as the slit moves from S-1 to S-5.

\subsection{Potential shock features in the HI data}
\label{sec:shock_features_HI}

We also try to cross-validate the shock features in $\firstcomponent$ with archival $\HI$ data. The coverage of $\firstcomponent$ extends to only $\sim500\arcsec$, which is smaller than the projected bar length of $\sim600\arcsec$ in \citet{blana_etal_18}. Therefore we cannot check the regions near the bar ends using only $\firstcomponent$. However, $\HI$ data have several weaknesses: First, the point spread function (PSF) is very large and thus the signal is more smeared. Secondly, a strong warp of $\HI$ disk exists in M31. Line-of-sight velocities of gas in the inner $\HI$ disk are contaminated by the gas in the outer $\HI$ warp. Thirdly, there is an absence of $\HI$ within the central 5$\arcmin$.

Figs.~\ref{fig:PVDs_HI_receding_side} and \ref{fig:PVDs_HI_approaching_side} show the PVDs of $\HI$ on the receding ($X < 0$) and approaching side ($X > 0$) of M31, respectively. We present the integrated $\HI$ emission with contours and the velocity of the main component of $\HI$ with black points. The black curves represent the boxcar smoothed result of the $\HI$ main component. Overall $\HI$ emission is composed of two parts with different origins and velocities. The gas features with smaller velocities distributing in most regions are possibly caused by the warp in the outer gas disk. The gas features with larger velocities showing beyond the warp features originate from the inner disk. Although emission from the $\HI$ warp obscures parts of shock features with small velocities, Class II shock features (thin blue lines) show up clearly in the main component of $\HI$ in most panels of Fig.~\ref{fig:PVDs_HI_receding_side}. Positions of Class II shock features are found between $Y = -2.7\arcmin$ and $Y = -3.9\arcmin$ on the far side of M31. Further out, $\Delta V$ of Class II shock features decreases, and the shock features turn into Class III (blue dashed lines) in S-9, S-10. On the approaching side, a Class I shock feature (thick blue line) and several weak shock features show up in panels (S+3, S+5, S+8) of Fig.~\ref{fig:PVDs_HI_approaching_side}, but in other regions the gas features with large velocities are quite clumpy and do not show clear shock features. Further out, Class II and III shock features appear near the disk major axis ranging from S+9 to S+13.

We make a cross-comparison of the PVDs of $\HI$ and $\firstcomponent$. The identified shocks distribution of these two tracers are quite similar, hinting for a common origin that is probably due to large-scale bar dynamics. PVDs of S-3, S-4, S-5, S-6 in Fig.~\ref{fig:PVDs_component_1_receding_side} and Fig.~\ref{fig:PVDs_HI_receding_side} illustrate that positions of Class I shock features of $\firstcomponent$ and Class II shock features of $\HI$ are similar. Further out ($X < - 7.2\arcmin$), Class II shock features of $\HI$ have profiles similar to that of $\firstcomponent$ though at slightly different $Y$ positions. Class III shock features are mainly on the near side of M31. There are panels where shock features in $\firstcomponent$ are clearer than in $\HI$, as well as panels where the opposite is true. When we overlay PVDs of $\firstcomponent$ and $\HI$ the shock features are usually easier to recognize.

Fig.~\ref{fig:PVDs_oiii_HI_ShockFeature[-11,11]} compares the PVDs of $\HI$ with several clearest shock features of $\firstcomponent$. The first panel is same as Fig.~\ref{fig:identified_shocks_component_1_receding_side} and it shows the overall pattern of $\firstcomponent$ shock features on the receding side. Other panels show the comparison between $\HI$ and $\firstcomponent$ shock features for each slit. In Fig.~\ref{fig:PVDs_oiii_HI_ShockFeature[-5,-0.5]} we show the same comparison but on a smaller spatial scale. Overall shock features of $\HI$ coincide with $\firstcomponent$, especially at $X = -1.2\arcmin$ and $X = -3.6\arcmin$. The $\HI$ features are quite clumpy, therefore determining the exact shock positions of $\HI$ is not easy. It is possible that the shocks in $\HI$ are not shifted from those in $\firstcomponent$, but obscured or hidden by some missing clumps instead. For example, the shock profile appears to be incomplete at $X = -2.4\arcmin$. If there were a clump near $Y \sim -3.5\arcmin$ with a velocity around $-100\kms$, the shock feature would have been clearer to see.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_HI_receding_side.png}
\caption{PVDs of $\HI$ on the receding side of M31. The data is from \citet{chemin_etal_09}. Each panel corresponds to one pseudo-slit in Fig.~\ref{fig:scheme_shock}. Contours indicate the integrated emission of $\HI$. Black points indicate the velocity of the main component of $\HI$. Black curves represent the boxcar smoothed result of the black points. The thin blue solid line and blue dashed line indicate the positions of Class II and Class III shock features of $\HI$, respectively. Shock features are mainly on the far side (negative Y values, below the disk major axis). Low-velocity features lie horizontally on each panel, which likely come from the warped gas disk.}
\label{fig:PVDs_HI_receding_side}
\end{figure}

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_HI_approaching_side.png}
\caption{Same as in Fig.~\ref{fig:PVDs_HI_receding_side} but on the approaching side. Thick, thin, and dashed blue lines indicate the positions of Class I, Class II, and Class III shock features, respectively.}
\label{fig:PVDs_HI_approaching_side}
\end{figure}

\subsection{The map of shock positions}
\label{sec:map_shock_position}

We plot the positions of shock features of $\firstcomponent$ (red circles) and $\HI$ (blue triangles) in Fig.~\ref{fig:position_shock}. We use the solid markers to represent Class I shock features, open markers for Class II shock features, and dashed markers for Class III shock features. The solid line in Fig.~\ref{fig:position_shock} represents the bar in \citet{blana_etal_18} with a projected bar angle of $17.7^{\circ}$ and a projected half-length of $2.3 \kpc$. We also show the fitted $3.6\;\mu m$ isophote (dashed) in \citet{blana_etal_18} that is closest to the bar ends.

The shock features are found mainly on the leading side of the bar and this is consistent with our expectation of bar-driven shocks. In general, shock features are clearer on the far side of M31 than on the near side. The shock positions of $\HI$ and $\oiii$ are very similar for $X$ between -7.2$\arcmin$ and -3.6$\arcmin$. Further out, Class II shock features of $\HI$ extend to the bar ends and turn into Class III shock features with smaller velocity jumps. In the central region of M31, $\oiii$ shows shock features that do not show in $\HI$. Such difference could be due to the lack of $\HI$ in the central regions.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_oiii_HI_ShockFeature-12,12.png}
\caption{\textit{First panel}: same as Fig.~\ref{fig:identified_shocks_component_1_receding_side}. \textit{Other panels}: comparison of shock features of $\firstcomponent$ (curves) and PVDs of $\HI$ (contours) on the receding side. The slit numbers are also labeled.}
\label{fig:PVDs_oiii_HI_ShockFeature[-11,11]}
\end{figure}

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{PVDs_oiii_HI_ShockFeature-5.5,-0.5.png}
\caption{\textit{First panel}: same as Fig.~\ref{fig:identified_shocks_component_1_receding_side}. \textit{Other panels}: same as Fig.~\ref{fig:PVDs_oiii_HI_ShockFeature[-11,11]} but on a smaller spatial scale.}
\label{fig:PVDs_oiii_HI_ShockFeature[-5,-0.5]}
\end{figure}

Shock features are, however, absent near $X\sim5\arcmin$. The observation of $\firstcomponent$ mainly covers the bulge region with $|Y| < 5\arcmin$, but it covers only $|Y| < 4\arcmin$ at $X = 5\arcmin$. Class I and Class II shock features in $\firstcomponent$ are found at larger $Y$ distances on the near side than the far side by $\sim 0.5\arcmin$. It is possible that shock features do exist, but extend beyond the data coverage near $X\sim5\arcmin$.

\section{Simulated gas flow versus the observed shocks}
\subsection{Hydrodynamical simulation}
We also make simple isothermal 2D gas models in a constrained M31 potential to compare with the observed shock features. The simulations here are for illustrative purposes only, but not meant to match all the details of shocks.

We solve Euler equations in the initial frame using the grid-based MHD code {\tt Athena++} \citep{sto_etal_20}. We adopt a uniform Cartesian grid with a resolution of 4096 $\times$ 4096 covering the simulation domain with length $L = 15 \kpc$ in each direction. The setting corresponds to a grid spacing of ${\Delta}x = {\Delta}y = 7.3\pc$.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{shock_spatial_map.png}
\caption{Spatial map of $\firstcomponent$ and $\HI$ shock features. The solid line represents the bar major axis in \citet{blana_etal_18} with a projected half-length of $2.3\kpc$. The dashed line represents the fitted $3.6\;\mu m$ band isophote in \citet{blana_etal_18} that is closest to the end of the bar. Red points and blue triangles indicate the shock positions in $\firstcomponent$ and in $\HI$ data in \S\ref{sec:result}, respectively. Solid, open and dashed markers represent Class I, Class II and Class III shock features.}
\label{fig:position_shock}
\end{figure}

For simplicity, we use the isothermal equation of state $P = \Sigma \;c_s^2$, here $P$, $\Sigma$ denote the gas pressure and gas surface density, respectively. The effective sound speed $c_s$ describes the turbulent properties of the gas. \citet{kim_etal_12a} systematically explores the effects of $c_s$ on gas substructures in barred potentials. They found that models with larger $c_s$ are more perturbed, and produce off-axis shocks closer to the bar major axis and smaller nuclear rings. Previous gas dynamics study suggests that the isothermal assumption can explain many observed gas features. The gas dynamics simulation of \citet{li_etal_16} explained various observed features on the Galactic $l-v$ diagram. \citet{wei_etal_01b,wei_etal_01a} ran hydrodynamical simulations of gas flow in the barred galaxy NGC 4123. By matching the non-circular motions near the dust lane, \citet{wei_etal_01b,wei_etal_01a} suggested a high-mass stellar disk and a fast-rotating bar in NGC 4123.

We set the initial surface density distribution of the gas disk to an exponential profile $\Sigma_{gas}(R) = \Sigma_{0}\;{\rm exp}(-R/R_{d})$, here $\Sigma_{0} = 76 \Msun\pc^{-2}$ and $R_{d} = 6.0\kpc$, which results in a total mass of 1.2$\times 10^{10}\Msun$. The initial azimuthal rotation velocity of the gas disk is set to balance the azimuthally averaged gravitational force. We input linear growth of the non-axisymmetric force and gradually ramp up the barred potential in one bar rotation period. We accomplish this by increasing the fraction of bar potential from 0 to 1.0 and decreasing the fraction of axisymmetrized bar potential from 1.0 to 0 linearly with time in $T_{grow} = 2\pi/{\Omega_{b}}$, similar to previous studies \citep[e.g.]{athana_92,kim_etal_12a}.

\begin{figure}[ht!]
\includegraphics[width=\columnwidth]{Model1_T749_shock_position_HI_OIII.png}
\caption{\textit{Upper panel}: Gas surface density of Model 1 projected with an inclination of 77$^{\circ}$. The snapshot is at $T = 749 \Myr$. Color represents gas surface density in the model. Pink circles and brown triangles indicate the shock positions in $\firstcomponent$ and $\HI$, respectively. The orientation of the bar major axis in the model is shown by the white dashed line. \textit{Lower panel}: Face-on view of Model 1 at $T = 749 \Myr$. The disk major axis (inclination axis) is along the X-axis.}
\label{fig:shock_position_model1}
\end{figure}

\subsection{Gravitational potential}

The galactic potential is based on the best fit m2m $N$-body model constructed by \citet{blana_etal_18}. The m2m model of M31 \citep{blana_etal_18} was derived by fitting the IRAC-$3.6\;{\mu}m$ photometry \citep{bar_etal_06}, the IFU stellar kinematics in the bulge \citep{opitsc_etal_18} and the $\HI$ rotation curve \citep{cor_etal_10}. The triaxial bulge in the dynamical model consists of a classical bulge component with mass of $1.18^{+0.06}_{-0.07} \times 10^{10}\Msun$ and a BPB component with mass of $1.91 \;\pm\; 0.06 \times 10^{10}\Msun$. The bar in their model has a pattern speed of $\Omega_b = 40 \;\pm\; 5 \freq$ and a length of semi-major axis of 4 $\kpc$. The major axis of the bar is oriented at a position angle of 54.7$^{\circ}$ (in the galactic plane) with respect to the line of nodes. The dark matter halo in their model follows an Einasto profile and the mass of dark matter within the bulge region ($R < 3.2 \kpc$) is $1.2^{+0.2}_{-0.4} \times 10^{10}\Msun$. The authors obtained similar mass values using models with NFW dark matter profiles. The mass-to-light ratio of stellar component in $3.6\;{\mu}m$ is found to be $\Upsilon_{3.6} = 0.72\;\pm\; 0.02 \Msun\Lsun^{-1}$. We use their best fit models JR804 and KR241 as the basis of our galactic potential. The former includes an Einasto dark matter halo and the latter includes an NFW dark matter halo. We add a Plummer sphere in the center to represent the supermassive black hole with a mass of $\MBH = 2 \times 10^8\Msun$ \citep{ben_kor_05}:

\begin{equation}
\Phi(r) = -G\MBH\dfrac{1}{\sqrt{r^2+a^2}}.
\end{equation}

\noindent Here $a = 10\pc$.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{Model1_T749_shock_velocity_jump.png}
\caption{Comparison between simulated and observed gas PVD for Model 1. Black dots represent the PVDs of the model along 4 pseudo-slits with their slit numbers shown on the top left. Red circles and blue crosses represent the observed PVDs of $\firstcomponent$ and main component of $\HI$, respectively.}
\label{fig:PVDs_model1}
\end{figure}

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{Model1_T749_shock_velocity_jump_zoomin.png}
\caption{Zoom-in version of Figure~\ref{fig:PVDs_model1}. We add black dashed lines to represent PVDs in a non-rotating bar model. The slit width for creating PVDs in this model is 0.04$\arcmin$ while for Model 1 it halves to 0.6$\arcmin$ (compared to the previous figure) for better clarity. The $\HI$ component is not shown in this figure.}
\label{fig:PVDs_model1_zoomin}
\end{figure}

\subsection{Simulated bar-driven shocks}

For simplicity, we only compare simulated bar-driven shocks with the clearest shock features of $\firstcomponent$ on the far side (slits S-1, S-2, S-3, S-4 in Fig.~\ref{fig:PVDs_component_1_receding_side}). We discuss the possible reasons for weaker shocks on the near side in \S\ref{sec:asytwoside}. We start with models using a bar pattern speed around $\Omega_b = 40 \freq$ given in \citet{blana_etal_18}. After projection with an inclination of $77^{\circ}$, the shocks are found to be too close to the bar major axis and cannot extend as far as shock positions in $\firstcomponent$. Other parameters that could be important are the uncertain shape and strength of the thin bar. \citet{ham_etal_18} suggested that a single merger event $\sim 2 \Gyr$ ago could explain the recent active star formation \citep{wil_etal_15}. More recent work studied the age-velocity dispersion relation in M31's stellar disk \citep{bha_etal_19}, which suggests a merger with a mass ratio of around 1:5 3-4 $\Gyr$ ago. The bar could have been weakened by the merger \citep{gho_etal_21} and left its shape and strength uncertain. One way to produce shocks with a larger distance from the bar major axis is to decrease the bar pattern speed \citep{li_etal_15}. We tested
models with lower bar pattern speeds and their shocks have a better match with $\firstcomponent$ data. When the bar rotates with a lower pattern speed around $\Omega_b = 20\freq$, the nuclear ring size becomes very large and the effects of thermal pressure need to be considered to reduce the size of the nuclear ring. It requires a high $c_s$ of around $30 \kms$ to make a reasonably sized nuclear ring. Models with low $\Omega_b$ and high $c_s$ produce less curved shocks, larger velocity jumps, and a nuclear ring with a reasonable size. Using an Einasto or NFW dark matter halo does not affect much the substructures.

We present the shock pattern in two models: (1) Model 1 with a bar pattern speed $\Omega_b = 20\freq$ and sound speed $c_s = 30\kms$ based on JR804 potential; (2) Model 2 with a bar pattern speed $\Omega_b = 33\freq$ and sound speed of $c_s = 15\kms$ based on KR241 potential. The bar rotation period $2\pi/{\Omega_{b}}$ for Model 1 and Model 2 is 307$\Myr$ and 186$\Myr$, respectively. The ratio of co-rotation radius to semi-major axis of the bar is $\mathcal{R}\equiv R_{CR}/a_{bar} \sim 3.2$ and $\mathcal{R} \sim 1.8$ for Model 1 and Model 2, respectively. Although simulations have reached quasi-steady after two bar rotation periods, small transient changes still appear on PVDs. We choose snapshots such that the shock features of the models are most similar to those of $\firstcomponent$.

The $x$-axis of the simulation grid is along the major axis of the disk. The major axis of the bar in Model 1 is positioned at an angle of 54.7$^{\circ}$ to the $x$-axis in the face-on case. The upper panel of Fig.~\ref{fig:shock_position_model1} illustrates the gas surface density of Model 1 at T = 749$\Myr$ projected with an inclination of 77$^{\circ}$. Pink circles and brown triangles represent the shock positions of $\oiii$ and $\HI$, respectively. Large, medium-sized, and small markers represent the Class I, Class II, and Class III shock features. We present the face-on view of Model 1 in the lower panel of Fig.~\ref{fig:shock_position_model1}. In Fig.~\ref{fig:PVDs_model1} we present the PVDs of Model 1 on the receding side. Red circles and blue plus signs represent the PVDs of $\oiii$ and the main component of $\HI$, respectively. The average shock velocity jump $\Delta V$ is around 310$\kms$ in both Model 1 and $\firstcomponent$. Fig.~\ref{fig:PVDs_model1} shows that the profiles on PVDs of Model 1, $\firstcomponent$ and $\HI$ are roughly consistent. In addition, we further show in Fig.~\ref{fig:PVDs_model1_zoomin} that a non-rotating bar (i.e. similar to a triaxial bulge) cannot reproduce the shock features. The black dashed lines in Fig.~\ref{fig:PVDs_model1_zoomin} represent the PVDs of a model that has the exact same setups with Model 1 but with zero bar pattern speed. It is clear that no sharp velocity jumps can be found in this non-rotating bar model.

Adjusting the inclination and the bar angle of models can fine-tune the match with the observed shock positions of $\firstcomponent$. As the inclination decreases, the angle of the bar (after projection) in respect of the line of nodes becomes larger, resulting in shock positions further away from the disk major axis. In the process of decreasing inclination, a smaller bar angle is needed to keep the shape of shocks.
\begin{figure}[t!]
\includegraphics[width=\columnwidth]{Model2_T799_shock_position_HI_OIII.png}
\caption{\textit{Upper panel}: Gas surface density of Model 2 projected with an inclination of 67$^{\circ}$. The snapshot is at $T = 799 \Myr$. Color and markers have the same meaning as in Fig.~\ref{fig:shock_position_model1}. \textit{Lower panel}: Face-on view of Model 2 at $T = 799 \Myr$.}
\label{fig:shock_position_model2}
\end{figure}

We run Model 2 to test the effects of inclination and bar angle. Model 2 produces shocks much closer to the bar major axis than Model 1, which shows a different shock pattern from that in $\firstcomponent$. However, with a smaller inclination of 67$^{\circ}$ and a bar angle of 50$^{\circ}$, shock positions in Model 2 can still be similar to those in $\firstcomponent$, especially on the far side. In the upper panel of Fig.~\ref{fig:shock_position_model2} we present the gas surface density of Model 2 at T = 799$\Myr$ projected with an inclination of 67$^{\circ}$. The lower panel of Fig.~\ref{fig:shock_position_model2} shows the face-on view of Model 2. Fig.~\ref{fig:PVDs_model2} illustrates the PVDs of Model 2 on the receding side and its comparison with $\firstcomponent$ and $\HI$. The average shock velocity jump $\Delta V$ is smaller than $\firstcomponent$ by $\approx 30\%$. These results show that there may be a degeneracy between the inclination and the bar pattern speed. We also present the PVDs in a non-rotating Model 2 that has zero bar pattern speed in Fig.~\ref{fig:PVDs_model2_zoomin}. There are no shocks in this model either. Figs.~\ref{fig:PVDs_model1_zoomin} and \ref{fig:PVDs_model2_zoomin} demonstrate that bar rotation is necessary to produce the observed shock features.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{Model2_T799_shock_velocity_jump.png}
\caption{Similar to Figure~\ref{fig:PVDs_model1} but for Model 2.}
\label{fig:PVDs_model2}
\end{figure}

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{Model2_T799_shock_velocity_jump_zoomin.png}
\caption{Similar to Figure~\ref{fig:PVDs_model1_zoomin} but for Model 2 (black dots) and its corresponding non-rotating bar model (dashed lines)}
\label{fig:PVDs_model2_zoomin}
\end{figure}

Both Model 1 and Model 2 have shock positions roughly similar to $\firstcomponent$ on the far side. The main advantage of Model 1 is the large shock velocity jumps similar to those in $\firstcomponent$, but the bar parameters in Model 2 are closer to those obtained from the m2m models \citep[][]{blana_etal_18}. We also tested different pattern speeds within the range of 25 - 50 $\freq$ and sound speeds within the range of 1 - 50 $\kms$. Shock positions move closer to the $x$-axis with a larger bar pattern speed and/or a larger sound speed. The line-of-sight velocity jump at shocks becomes larger as bar pattern speed decreases and sound speed increases. A detailed comparison including more bar parameters will be presented in a follow-up paper.

\citet{berman_01} and \citet{ber_loi_02} performed hydrodynamical simulations in a M31 potential and found a much higher bar pattern speed of $\Omega_b = 53.7\kms$ by fitting the line-of-sight velocity of CO \citep{loinar_95,loinar_99} along the disk major axis. They adopted a simple analytical bar model, and the spatial resolution of their simulations was relatively low ($\sim 125 \pc$, in contrast to to $7.3 \pc$ in this work). Their model can match the PVDs of CO, which, unfortunately, do not cover the shock regions of $\oiii$ and $\HI$ used in this work. The dust lanes predicted by their models are nearly perpendicular to the disk major axis, and are quite different from the position of shock features in Fig.~\ref{fig:position_shock}.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{velocity_map_OIII.png}
\caption{Positions of shock features of $\firstcomponent$ overlaid on its velocity map. Large, medium-sized and small crosses represent the Class I, Class II, and Class III shock features. The velocity field of $\firstcomponent$ is asymmetric between the far (bottom left) and near (top right) side. A large velocity gradient is present near the zero-velocity line on the far side, which locates mostly within the boundary. Class I shock features closely follow the zero-velocity line. On the near side, the velocity field is smoother and we can only identify weak shock features near the boundary.}
\label{fig:velocity_map_first_component}
\end{figure}

\section{Discussion}

\subsection{Asymmetry of shocks between the near and far side}
\label{sec:asytwoside}

We find that the shock features are generally weaker on the near side than on the far side. In Fig.~\ref{fig:PVDs_component_1_approaching_side} it is clear that shock features are absent near $Y\sim5\arcmin$ in slit S+4. In Fig.~\ref{fig:velocity_map_first_component} we overlay the positions of the $\firstcomponent$ shocks on the observed 2D velocity map. Class I shock features (large crosses) are found mainly at $-7.2\arcmin < X < 0\arcmin$ on the far side (bottom left). On the near side (top right), there are mostly Class II shock features (medium-sized crosses) in regions with small velocity gradients. Overall shock features are closer to the boundary of data coverage on the near side than the far side. This implies the shock features of $\firstcomponent$ defined in the current study might be incomplete due to the limited coverage of observation.

Clear asymmetry in the position of shocks and gas kinematics has been observed in the barred galaxy NGC 1365 \citep{zan_etal_08}. They suggest that the asymmetry in positions of the dust lane is likely caused by a minor merger event. They also provided an alternative explanation that the ram pressure of a gas stream has moved the shock to offset its original position.

\begin{figure}[t!]
\includegraphics[width=\columnwidth]{scheme_warp.png}
\caption{Schematic view of the asymmetric extinction by the warp in M31's gas disk. Dotted lines indicate three possible lines of sight toward M31. (a) The line of sight crosses both the warp region and the inner disk on the near side. The gas and the dust on the warp become foreground and thus obscure the light from shock features. (b) The line of sight passes through the central regions of M31. Light is dominated by the outer warp for the absence of cold gas near the center. (c) The line of sight first crosses the inner disk, then passes the outer warp region. The warp becomes a background and it does not obscure the light from the inner disk.}
\label{fig:scheme_warp}
\end{figure}

Another possible scenario for the asymmetry in the shock feature is that the warp in the outer $\HI$ disk has a stronger extinction effect towards the near side. The $\HI$ disk is extended and has a large-scale warp in the outer region \citep{new_eme_77,hender_79,bri_bur_84,chemin_etal_09,cor_etal_10}. Fig.~\ref{fig:scheme_warp} presents a schematic diagram of the gas warp structure. The gas warp appears as a foreground on the near side and becomes a background on the far side. The asymmetric extinction by the gas warp could result in shock features different between the near side and the far side. Furthermore, abundant dust in the $\HI$ warp may also help blur the $\firstcomponent$ shock features on the near side. The distribution of dust is found to follow that of $\HI$ in the outer disk, and the strong reddening effect suggests a large amount of dust \citep{cui_etal_01, ber_etal_12}. More recent detection of dust by \citet{ruo_hai_20} found that dust in the disk has an exponential distribution and extends over 2.5 times its optical radius (around 54 kpc).

\begin{figure}[!t]
\includegraphics[width=\columnwidth]{dust_CO_shock_position.png}
\caption{Identified shock positions of $\firstcomponent$ (red circles) and $\HI$ (blue triangles) superposed on the surface density map of dust \citep{dra_etal_14} and contours of CO emission \citep{nieten_etal_06}. The solid, open, and dashed markers represent the Class I, Class II, and Class III shock features, respectively. The contour levels are 1, 4, and 10 $K \kms$. The dashed line indicates the bar major axis in \citet{blana_etal_18}.}
\label{fig:compare_dust_CO}
\end{figure}

\subsection{Effects of changing slit orientations on shock features}

Maximum velocity jumps may be expected when slits are positioned perpendicular to the shock fronts \citep{athana_92}. We checked PVDs of $\firstcomponent$ and $\HI$ in slits at different orientations and compared them to Fig.~\ref{fig:scheme_shock} to see if the velocity jump features can be shown even more clearly. Our findings are roughly consistent with the theoretical expectation. When the slits are positioned parallel to the disk major axis, the shock features still appear, but with a smoother profile. The shock features become sharper as we reposition slits to be nearly perpendicular to the shock fronts, with an increase of $\Delta V$ around $30 \kms$ compared to the more parallel slits. In appendix \ref{appendix:B} we tested repositioning the slit to make it more perpendicular to the shock fronts and found that the shock properties are similar to \S\ref{sec:shock_features_first_component}.

\subsection{Comparison of shock features with dust and CO morphology}

In Fig.~\ref{fig:compare_dust_CO} we present the map of our identified shock features in $\firstcomponent$ (red circles) and $\HI$ (blue triangles) on a background of dust (color-coded with surface density) and CO (white contours) morphology. We use data of dust from \citet{dra_etal_14} and CO from \citet{nieten_etal_06}. The black solid line and dashed line indicate $PA_{disk}$ and the major axis of the bar in \citet{blana_etal_18}, respectively. The thickest dust arms correspond to the 10 $\kpc$ ring in M31. Positions of $\oiii$ and $\HI$ shock features are found near a thin dust lane on the far side. Such coincidence also appears in the CO distribution, which generally matches the dust morphology. The shock features of $\HI$, extending to the bar ends in \citet{blana_etal_18}, connect to a possible spiral structure. For the near side, shock features are weaker as we discussed in \S\ref{sec:asytwoside}. There are not enough Class II shock features to show the relation between shock positions and dust morphology. Another reason might be that the amount of $\HI$, CO, and dust in the bulge is scarce on the near side. Near center there is a lack of $\HI$ shock features and CO emission. This could be due to the low gas density in the central region \citep{bri_sha_84,li_etal_09,don_etal_16,li_etal_19,li_etal_20}.

\subsection{Comparison of shock features in other nearby barred galaxies}

Other barred galaxies also show qualitatively similar bar-induced shock features as in M31. \citet{mun_etal_99a} detected velocity jumps of $\HI$ with $\Delta V \approx 130 \kms$ on the leading side of bar of NGC 4151. $\HI$ in NGC 4151 is not interfered by the gas warp, showing a clearer view of shock features. The $\rm H\alpha$ velocity field of the barred galaxy NGC 4123 illustrates similar shock features with $\Delta V > 100\kms$ \citep{wei_etal_01a}. Velocity jumps in NGC 4151 and NGC 4123 have a smaller amplitude than our Class I shock features, probably due to the lower inclinations and lower masses of galaxies. A large velocity gradient near dust lanes has been observed in other barred galaxies as well, e.g. NGC 1530 \citep{rey_dow_98,zur_etal_04}, NGC 7479 \citep{lai_etal_1999}, NGC 5448 \citep{fat_etal_05}, NGC 1365 \citep{zan_etal_08}. The recent high-resolution PHANGS–ALMA survey presents 2D gas kinematics of CO for nearby spiral galaxies \citep{ler_etal_21}. The velocity gradient near dust lanes can even be visually identified in many barred galaxies from their sample, e.g. NGC 2903, NGC 3627, NGC 4536 and NGC 4945. Apart from bar-driven shocks, spiral arms in non-barred galaxies could also produce velocity jumps, but with a much smaller amplitude \citep[usually around 40 $\kms$ in simulations][]{robert_69,pet_etal_20}.

\subsection{Origin of the two velocity components}
\label{sec:twooxy3}

We use $\firstcomponent$ to present PVDs in the major part of the paper, but there is another component $\secondcomponent$ observed in \citet{opitsc_etal_18}. The gas flow forms streams and ring structures in barred potentials \citep{kim_etal_12a}. Multiple gas components could be found when the line of sight passes through several of such gas substructures. A collision between M31 and its satellite galaxy M32 \citep{block_etal_06} could also produce gas streams and rings, leading to multiple observed gas components along the line of sight. It is also possible that part of $\oiii$ comes from the bulge instead of the disk in M31.

Considering that the $\secondcomponent$ may be a foreground or background, we do not expect the $\secondcomponent$ to show clear shock features. It is interesting that PVDs of $\secondcomponent$ show a few velocity jump features, but the amplitude is generally small and their positions do not show a regular pattern.

Apart from the ionized gas in the optical band, CO and $\HI$ observations also revealed multiple gas spectral lines \citep{mel_com_11,chemin_etal_09}. \citet{mel_com_11} proposed a model of the tilted rings to explain the multiple lines in CO. However, it seems that their scenario cannot explain the shape of $\oiii$ spectra well. \citet{chemin_etal_09} attributed the lower-velocity gas components to the outer $\HI$ warp, but whether the warp may cause a split $\oiii$ remains unclear. Future studies are needed to provide a better explanation for the $\secondcomponent$ component.

\section{Conclusion}

We identify shock features in the central region of M31 ($20\arcmin \times 10\arcmin$) using $\oiii$ data from \citet{opitsc_etal_18} and $\HI$ data from \citet{chemin_etal_09}.  The strongest $\oiii$ shock features show a large velocity gradient (over 1.2 $\kmspc$) with $\Delta V$ over 170 $\kms$ in the bulge. The $\HI$ emission shows similar shock features even beyond the bulge region. Note that several shock features show up in $\oiii$ but not in $\HI$ emission near the center, possibly due to the lack of atomic gas there. The shock features are found mainly on the leading side of the possible bar proposed by \citet{blana_etal_18}.
The spatial location of the shocks and the amplitude of shock velocity jumps are qualitatively consistent with our preliminary simulations of bar-induced gas inflow in M31. This result provides independent, strong evidence that M31 hosts a large bar. Detailed comparison with more hydrodynamical simulations will be presented in a follow-up study to provide a better understanding of the gas features in the center of M31, and hopefully determine better the main bar parameters of M31.

\software{
{\tt Athena++} \citep{sto_etal_20},
NumPy \citep{2020NumPy-Array},
SciPy \citep{2020SciPy-NMeth},
Matplotlib \citep{4160265},
Jupyter Notebook \citep{Kluyver2016jupyter}
}

\begin{acknowledgments}

The authors would like to thank Laurent Chemin for sharing $\HI$ data of M31 and the anonymous referee for an encouraging and constructive report. The research presented here is partially supported by the National Key R\&D Program of China under grant No. 2018YFA0404501; by the National Natural Science Foundation of China under grant Nos. 12103032, 12025302, 11773052, 11761131016; by the ``111'' Project of the Ministry of Education of China under grant No. B20019; and by the China Manned Space Project under grant No. CMS-CSST-2021-B03.  J.S. also acknowledges support from a \textit{Newton Advanced Fellowship} awarded by the Royal Society and the Newton Fund. M.B. acknowledges funding from ANID through the FONDECYT Postdoctorado 2021 Nr 3210592 and the Excellence Cluster ORIGINS founded by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) under Germany's Excellence Strategy – EXC-2094 – 390783311, the supports from BASAL CATA Center for Astrophysics and Associated Technologies, and the Max Planck Computing and Data Facility. This work made use of the Gravity Supercomputer at the Department of Astronomy, Shanghai Jiao Tong University, and the facilities of the Center for High Performance Computing at Shanghai Astronomical Observatory.
\end{acknowledgments}

\bibliographystyle{apj}

redpajama_set_name: RedPajamaArXiv


On a rolling 12 month basis, inbound traffic was up 0.2% compared to the rolling 12 months ending in March. Outbound traffic was up 1.0% compared to the rolling 12 months ending in March.
In general imports have been increasing, and exports are mostly moving sideways recently.


More than three months later, it's clear that the post-election panic over fake news has backfired. Instead of shattering the alternate realities Americans constructed for themselves online, the very real concern over the spread of misinformation has lent legitimacy to those who prefer to dismiss any information they don't like. Politicians and public figures now have a two-word phrase they can use to instantly nullify any bad press, and it's usually expressed by the president of the United States in all caps: FAKE NEWS.
Even as the term is co-opted, fake news itself remains a real problem for the future of American democracy. To confront it without equipping autocrats with the tools they need to evade the truth, we must be discriminating about what fake news is.
That means acknowledging that there is a difference between biased sources of information and false sources of information. For example, although their reporting is marked by a clear and increasingly drastic rightward tilt, Fox News is not fake news. The same is true of MSNBC, most of whose programs present information from a distinctly left-leaning perspective.
Media organizations can and sometimes do report facts selectively, emphasize some facts while downplaying others and supplement facts with their own spin—all in the service of a political agenda. Viewers, listeners and readers should be aware when this happens. But those organizations are still reporting facts, which distinguishes them from fake news.
That being said, within the realm of real news outlets, the presence or lack of an agenda can be used to discredit some while elevating others. During his last three press conferences, President Donald Trump has only called on outlets that serve his agenda, while shutting out more reputable organizations like CNN, the New York Times and the Washington Post — organizations he has repeatedly decried as "failing" and "fake news." To be clear, these organizations don't have clear agendas in the same way that Fox or parts of MSNBC do, but Trump (along with his supporters) perceives their lack of a conservative agenda as the presence of an agenda against his administration.
Being clear about what counts as fake news means respecting the authority of well-established organizations with long records of reporting the truth. It also means understanding who benefits from the delegitimization of information. In the aforementioned interview with Yahoo News, the Syrian president was denying a report by Amnesty International. When a dictator locked in a bloody civil war with his own people is called out by a watchdog group, we should expect him to try and delegitimize that group, and we should know who to believe.
What, then, is fake news? Fake news comes from sources with clear partisan interests and little to no record of any kind, and it consists of information that is not just selectively or dishonestly presented, but flatly false. If a politician or public figure is calling something fake news that doesn't fit that description, then the real fake news is probably coming from them.


text:
TROZO
Furniture Out of the Box
Everyone furnishing a home knows how difficult it is to find pieces that are as comfortable as they are beautiful, and that fit your taste. Which is why the new furniture line, TROZO, launched by design house Shades, has not come a moment too soon. Launched by sisters, Nada and Nehal Mahmoud, TROZO started out as a line of customised furniture individually tailored to each client. Recently, the sisters also launched their own TROZO signature line designed and manufactured in house. When they showed their signature products at the recent Le Marche furniture fair, their amazing success came as a surprise, even to them. eniGma's Ezz Al-Turkey sat down with the sisters to find out more about their new venture and the secret of their success.
Nehal and Nada Mahmoud joined their father's real estate development company fresh out of graduation from university. Nehal graduated with a degree in architecture from the Faculty of Fine Arts, while Nada earned a degree in finance from the American University in Cairo. Soon after joining their father's company, they started their own interior design department within the company, which they called Shades, in 2015. Shades offered both interior design as well as architectural services. Nehal was the principal designer handling the creative side, while Nada handled the business side. They were a perfect match, working very well together. "Being sisters, we've really worked together all our lives. We are also still working with our father. He put us on the right path and helped us become what we are today. We owe a lot to him," says Nehal, who is two years older than Nada. "When we were starting Shades, we saw how well we complimented each other and how we could use both of our expertises to achieve a common dream," adds Nada.
By 2017, the sisters had realised that they loved producing custom made pieces for their clients. Furniture design and production was a natural extension for them given the fact that their family aready had a factory producing high quality furniture for decades. So, they branched out and started TROZO, a showroom dedicated solely to furniture design and production. "We thought, why not customise and tailor furniture for clients? We used to do that sometimes with Shades, and we loved doing it. Once we had the idea, we immediately got to working on it," says Nada.
As soon as they started TROZO, the sisters also took the decision to apply for Le Marche, the renowned yearly furniture exhibition. While it was a big step for a pair of newcomers, they were fearless and determined. "I think we can both agree that this was the most stressful time ever," Nehal recalls, with a giggle. Her sister adds, "It was super stressful to prepare all the furniture on time, and going into this as beginners in the industry was really scary."
With the possibility of failure and disappointment in the back of their minds, the fear stayed with them until Le Marche's opening day. Their dad made sure to sit them down to reassure them that it was okay if they didn't make any deals at the fair. He stressed that their participation alone was a great marketing opportunity and a great promotion for TROZO. In truth, and to everyone's amazement, their hard work paid off! Their display booth turned out to be the most stacked of the whole exhibition, and people were not only browsing, but lining up to place orders for their furniture. "It was wild. People from other firms were coming over and asking us who we were. They all thought we were a huge brand name," says Nada smiling. "We ended up with 17 contracts on our pieces at Le Marche, which was bizarre and just so rewarding. We definitely left feeling like we owned the world," Nehal exclaims.
What probably attracted Le Marche customers to TROZO was their unique sense of style, which can be described as 'out of the box'. TROZO furniture's particular sense of style is captivating to a lot of people, even though it does not necessarily click with everyone. The use of colours, and the way different styles are morphed together to create something completely different, is TROZO's strongest asset. TROZO has it all, from retro to classic to modern. Subtlety is also key at TROZO, as can be discerned in their retro designs, which remind you of the style of previous eras, in a subtle way. Recalling their experience at Le Marche, Nada recounts, "We had this peacock couch that had people really confused. They loved it and were perplexed by it at the same time!" recalls Nada. "Yes, I remember one girl sat on it for about half an hour and didn't want to leave it; while someone else didn't understand the choice of colours," Nehal adds.
While TROZO has something for everyone, their designs are more likely to click with younger clients. TROZO clearly belongs to 2020, and that is clearly reflected in their designs. The sisters are quick to add that they offer much more than their ready-made designs. Since a good part of what they do is customised furniture, everyone can find what they want. If a client has a particular design in mind, TROZO will make it happen. "People often come to us with a design they saw on the internet and we produce it for them. We do that a lot. This service is definitely available," says Nada.
While each sister has her own duties at Shades and TROZO, having worked together for so long, they sometimes do each other's work. Nada has discovered a new-found talent for designing and Nehal has learned the ins and outs of the finances. Both also give a lot of credit to the rest of their design team, who work tirelessly at creating the best designs possible. Nada explains, "The two of us are really like yin yang at this point, we work together so well. It also helps that we're sisters, because when we argue about work, we go back to talking to each other very normally two minutes later." Nehal adds, "We also have a great team, that is so dedicated and talented. They don't just design furniture, they test out the liveability of the products and optimise them to make them as comfortable as possible for the clients," remarks Nehal.
The two sisters are proud that TROZO is a perfect blend of quality and affordability. They remark that people are often shocked at how reasonable their prices are, given the high quality of their furniture. That fact is deliberate and very important to them. They want their furniture to be of high quality and to be accessible, as well. Nada says in conclusion, "Our vision is for TROZO and Shades to go hand in hand together. We want them to be parallel to each other and not to clash. We want to be a popular brand, while maintaining our identity and core."
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redpajama_set_name: RedPajamaCommonCrawl


text:
iPhone 8, iPhone X Specs: Apple iPhone X A11 chip to have six CPU cores
Sep 11, 2017 by Parvinder Singh
We still have to wait a bit to see the next Apple iPhone. However, the wait has not stopped fans fervent looking for rumors about the iPhone X, the name that is supposed to have the next Apple cell phone. Every time we know more details about the new terminals that Apple will present on September 12. After months of confusion over the name the next new iPhone rumors declare that the so-called iPhone 7s and iPhone 7s Plus will actually be iPhone 8 and iPhone 8 Plus and the special version for the tenth anniversary will receive the name of iPhone X Edition . This will be the most exclusive smartphone and will therefore introduce the major new features that Apple wants to beat the recent Samsung Galaxy Note 8 and remain a benchmark in the sector.
a reference to iPhone X, which acts as fresh evidence that Apple intends to unveil a high-end model alongside more modest updates to its handset line
images of a new Apple Watch and AirPod headphones
a set-up process for Face ID – an alternative to the Touch ID system fingerprint system – that says it can be used to unlock handsets and make online purchases from Apple, among other uses
Things can be a bit different this year as it's the tenth anniversary of the iPhone. So the rumors suggest that we could see up to three models – the iPhone 7S, the 7S Plus, and the iPhone X (or iPhone 8) -. The latter is expected to be a bit costly, considering that it will be released as a celebration model.
If you prefer you want to know more about the iPhone 7S and 7S Plus, you can find out here .
But if you're more interested in the new iPhone X or iPhone 8, this is what we know so far about the new Apple phone.
IOS 11 Filtration
A huge leak called iOS 11GM has provided numerous details about the Apple event that will take place this Tuesday, September 12. The most important information of this filtration, is the confirmation of the names that will have the new cells of Apple. Whoever released this information was developer Steve Troughton-Smith.
That's right, instead of being called iPhone 7S and 7S Plus, the improved versions of the iPhone 7 will be called iPhone 8 and iPhone 8 Plus. The flag cell phone would be called iPhone X, and would confirm the rumor about the name of the device that has been hovering for months.
But other details were also released on what to expect. For example, Animoji in iMessage will be a feature of iOS 11 that are basically animated emojis, in 3D.
There will also be a lot of wallpapers that according to 9to5Mac , you can see with OLED technology, especially those that are darker.
Presentation, price and launch
Finally the date of the Apple event is confirmed . On the morning of August 31, 2017 the apple bite company sent invitations to the event where they are expected to be released the new iPhone X and possibly two other special models of the iPhone 7. The event will be held in the new Apple's amphitheater, located in front of its new campus, dubbed "Spaceship." It will undoubtedly be an unforgettable event for the company, as they will not only launch an iPhone anniversary edition, but will also show the world their new offices.
The Wall Street Journal reported on the morning of August 28 that the iPhone X would be announced on September 12, although the invitations had not been sent yet. The report said Apple would release three models, including a new flag cell phone.
A previous rumor of Mac4Ever , a French news site, also revealed that this cell phone would be released on September 12. And according to the same publication, the cell phone would hit the stores on September 22, The report cites industry sources and mentions three cell phones: an iPhone X, iPhone 7S and iPhone 7 S Plus as well as a new smartphone clock brand.
Some reports indicate that the launch may be delayed. Ming-Chi Kuo and JPMorganindicate that although the iPhone 7 S is already in production, the availability of the iPhone 8 could be limited the first month. Unfortunately, this will not improve, according to the Wall Street Journal .
According to the Journal, the Cupertino giant claims that because of failing to secure OLED screens in the early stages of manufacturing, problems to incorporate the Touch ID on the screen. Therefore, the company would have decided instead to use facial recognition.
Analyst Ming-Chi Kuo indicated that Apple will introduce three new iPhone in September, the iPhone 8 (or iPhone X), as well as the iPhone 7S and iPhone 7S Plus. Kuo says the iPhone 8 will be tight when it's released and the production line for the iPhone will produce between 2 and 4 million units this quarter. However, it claims that production will increase to 45 and 50 million units during the year.
For its part, Apple will produce between 35 and 28 million iPhone 7 S that would have 4.7-inch screen and between 18 and 20 million iPhone 7S Plus that would have the screen 5.5 inches. According to the analyst. these are already in production.
There are constant rumors about a possible delay in the launch of the iPhone 8 or iPhone X, more than anything, due to challenges to find supplies. In mid-July a note from JPMorgan indicated that although the cell phone would be introduced in September, cellphone supplies would be very limited and production would not level until November.
This would confirm other rumors, such as the one that came out in April from the firm Drexel Hamilton speculating that the iPhone X will be delayed several weeks until the date is revealed in September due to challenges that have occurred in the development of 3D technology, but however, the cell phone would be available for the holiday season. According to MacRumors , the cell phone would be available in pre-orders after it is announced but delivery will be made several weeks later.
Earlier, an Economic Daily News report stated that the iPhone X would not be delivered until October or November and it was possible Apple would announce it this year to be shipped in 2018 . However, analyst Ming-Chi Kuo retracted regarding a statement made by saying that a series of "state-of-the-art components" would delay the launch.
According to a Digitimes report , Apple has a shortage of 3D NAND storage chips for the iPhone X. The company relies on Toshiba and SK Hynix, who supply the iPhone's flash memory. However, the 3D NAND technology – which was first used on the iPhone 7 – has vertical and horizontal memory cells that make them more difficult to produce. It is not clear how this could lead to delays.
Apparently, Apple is receiving 30% fewer chips than they need and as a result, the apple company has resorted to Samsung to replace the remaining percentage. Four years ago, Samsung was the first manufacturer of 3D NAND.
Apparently, the next iPhone could be called "Apple Decade Edition" and could be released in October. This rumor is based on a filtered image of an accessory for what would be the next Apple cell phone.
Despite being referred to as the iPhone 8 in many rumors, the cell phone could be released along with the iPhone 7S and 7S Plus.
And while some media suggest that the new features of the iPhone X or iPhone 8 could delay its launch, as Apple is expected to launch a completely redesigned phone in terms of design and technological innovations, others point in a very different direction.
Thus, as recorded by BGR Digitimes portal now ensures that the presentation and launch of the new iPhone will be carried out following the scheduled schedule, ie we could know the new models next September.
But how does Digitimes know? Well, this site has obtained this information from the Economic Daily News, which emphasizes that there will be no delay related to the new iPhones that will see the light this 2017. In fact, it qualifies that the new models could reveal in September, when Apple usually makes its biggest annual presentation, and be available as early as October.
And it is worth remembering that Apple is also expected to release the 7S and 7S Plusversions as well as a completely different new model that could mean a before and after for the star product of the apple bite signature if, finally, all the rumors about its new interior and exterior appearance are confirmed.
According to the latest published information, several Apple vendors would be prepared to increase production of the new iPhone as of this summer. "TSMC will begin production of A11 chips in June and will deliver them in large volumes in the second fortnight of July ", publishes BGR.
And not only that, but some manufacturers like Foxconn would be increasing their templates in China and these new hires would be focused on the new iPhone X or iPhone 8.
A rumor indicates that the iPhone that would be announced by Apple in September, would cost more than $ 1,200. Speculation was started by podcaster Joh Gruber who once stated that the price of an Apple smart watch made with gold would cost $ 9,999 or more and his speculations were true at the time. Gruber says the starting price could be as high as 1,200 and the more expensive models could cost up to $ 1,500.
An earlier report on the price of the iPhone X would confirm that the phone could cost more than $ 1,000 dollars. On Wednesday July 5, market research firm Nomura Instinet says there will be a high demand for the phone that would be released this summer. The expectation has led to a decline in Apple sales.
Instinet indicates that the expectation along with the great advances that are believed to have the phone, could raise its price above $ 1,000 dollars. For this reason, it is most likely that mobile operators will offer payment plans.
Despite numerous references to the iPhone 8 in many rumors, the phone could come along with the iPhone 7S and 7S Plus in late 2017.
Although this might confuse things a bit towards 2018, when an iPhone 8 would be more logical based on the usual names of Apple phones.
However, recent rumors indicate that while Apple may have been considering a launch in 2017, the company could wait until 2018.
But why? Well, according to a note, several supply chains have pointed out technical problems in the manufacture of the new device.
What's more, Deutsche Bank also says Foxconn only expects Apple to order parts for the iPhone 7S and 7S Plus, according to a Business Insider report .
So it is possible that instead of waiting until 2018 to announce the new phone, Apple could announce it this year, but the first shipments would be made as early as 2018.
And an alternative name that is allegedly being considered is the iPhone X. In addition, some anonymous sources who spoke to Fast Company also claim that the device will have a 5.8-inch OLED screen, with no bezels and a body made of stainless steel.
The back may change and the start button may disappear below the screen. Even the volume and mute keys could be replaced by touch-sensitive panels. Of course, all these new features could increase the price of the phone above $ 1,000 dollars.
Apple usually launches a new iPhone every year in September, but recent reports indicate things could be a bit different this year. Bloomberg says supply restrictions could delay the launch of the iPhone X until "one or two months after" the usual presentation in the fall.
According to a Drexel Hamilton analyst and according to MacRumors , the release of the iPhone X could be delayed several weeks after its September revelation due to the challenges around its 3D detection technology – although it will be available in time for the December holidays – .
But "it's April and this situation could improve," says the analyst, according to MacRumors. "In essence, our contact believes customers will be able to pre-book the new 5.8-inch iPhone X along with the new 4.7-inch and 5.5-inch iPhones in September; However, the iPhone X of 5.8 inches would not be available for delivery until several weeks later, "he adds.
This information is in line with previous Economic Daily New s reports that the new iPhone X would not be delivered until October or November.
And now, there's even more reason to believe we'll have to wait a little longer than usual to see the new iPhone. According to a report by Apple analyst KGI Ming-Chi Kuo, the restricted supply of a number of "cutting-edge components" needed for the long-awaited phone will delay the launch of the iPhone until October or November.
The company claims that the "significant hardware improvements" on the iPhone 8, including its custom OLED display panel, the new 3D touch module and 3D cameras, are contributing to this delay.
And according to KGI, a delay of up to a couple of months could hurt Apple's sales, especially if competitors launch their products ahead of time.
There has also been some disagreement over whether to finally launch three iPhone models this year. According to a new rumor released on April 20 by Chinese tech blog Micgadget.com , "rumors say that Apple will release two models of iPhone 7S and an iPhone 8 (or iPhone Edition)," the publication says.
But "today we can tell you that it is not true. This fall, we can see just two models with new technology, "he adds.
However, Micgadget.com apparently has a rather inconsistent track record when it comes to Apple and its plans.
That said, there is another report suggesting that three iPhones will not be seen this year, though this one claims that it will be the iPhone 8 that will later join the party.
According to a Bloomberg report, the iPhone 8 will have a more vibrant screen that will occupy most of the space of the next iPhone. In general, the bezels would be much thinner and the start button will be replaced by gesture recognition. At the bottom of the phone, we can see a software bar that will be thinner and that users will be able to drag to the middle of the screen to enter the cell phone. Apple also plans to have a larger screen, with curved edges, next to a stainless steel band that curves with glass. Some previous reports indicated that the iPhone 8 will have a small bezel area and surrounded by a screen that in English is called "notch" and that will have easy recognition sensor, horn and a camera for selfie, as shown in the image below.
Categories Apple Tags apple, iphone, iphone 8, iphone x
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Cloud-based tools like Egnyte continue to transform the way we work. And as more tools and services are created, it's become clear that the key to unlocking the true power of the cloud is to assemble a customized toolkit that not only complements your workflow, but also improves and enhances it.
As I keep a close eye on the evolution of cloud platforms, one of the more popular combinations that's emerged is Egnyte synced with Basecamp, Evernote, and Microsoft's SharePoint, thanks in large part to Egnyte's position as a leader for a cloud-based central storage point, especially at the enterprise level.
Yet as companies integrate cloud-based services like Basecamp or SharePoint, they often run into a problem of multiple installations. When that happens, a company might have different versions of SharePoint that are located on-premises and are separate from what's located in the cloud. As a result, the enhanced accessibility and productivity of the cloud is a moot point since most employees can't be assured they're accessing the most up-to-date documents they need—and that's why it's increasingly important to integrate Egnyte (and Basecamp, Evernote, and SharePoint) with other cloud tools.
That sort of confusion among different versions of documents isn't the only issue to emerge amid increased cloud adoption. Data gravity is also proving to be more problematic, especially for enterprise-level companies that deal with high volumes of information. As you've probably read on the Egnyte blog, data gravity occurs once data has grown exponentially and becomes difficult to move to the cloud because of its sheer magnitude or due to privacy or regulatory concerns.
Since Evernote, Basecamp, and SharePoint have a wide range of enterprise functionality, it's safe to assume that, at some point, users might run into data gravity issues, which aren't just problematic to individual users—they're proving to be a hindrance to enterprise IT teams, too.
But by adding Egnyte to the mix, enterprise companies can enhance other cloud apps' capabilities with services like cloud file sharing, private file sharing, local file access and cross-office collaboration, all of which enable seamless access and file sharing from any device. Isn't that sort of instant (and constant) access what the cloud is all about?
For companies that already use SharePoint, adding Egnyte to the mix is a move that makes sense. Yet to add even more functionality—not to mention the peace of mind that comes from data security—it's important to complete the cloud platform trifecta with cloudHQ.
cloudHQ continuously synchronizes and replicates data between multiple cloud services in real-time, which means you have access to your information any time you need it—and regardless of how many cloud tools you use.
When you add cloudHQ to your cloud toolkit, you know your data is accessible and protected in a secure, centralized location like Egnyte, even if you're dealing with large amounts of data and information. At cloudHQ, we currently sync more than 10 million files each day, which means we're no strangers to dealing with a high volume of data.
Interested in adding Egnyte and cloudHQ to your existing SharePoint system? Try a free cloudHQ trial paired with Egntye to see how you can make your SharePoint workflow even more robust and efficient. The cloud is as powerful as you make it—and by using this particular suite of tools, you'll achieve an optimal level of efficiency, productivity, access and collaboration that won't just impact your employee's to-do lists—it'll have a noticeable impact on your company's bottom line, too.


text:
package io.debezium.config;

import static org.junit.Assert.fail;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.junit.Test;

/**
 * @author Randall Hauch
 *
 */
public abstract class AbstractFieldTest {

    private Field.Set allConnectorFields;

    public void setAllConnectorFields(Field.Set allConnectorFields) {
        this.allConnectorFields = allConnectorFields;
    }

    @Test
    public void shouldNotAllowDuplicateGroupAssignment() {
        Map<Field.Group, Map<Integer, String>> groups = new HashMap<>();

        List<String> errors = new ArrayList<>();

        allConnectorFields.forEach(field -> {
            Field.GroupEntry currentGroupEntry = field.group();
            if (!groups.containsKey(currentGroupEntry.getGroup())) {
                groups.put(currentGroupEntry.getGroup(), new HashMap<>());
            }
            if (groups.get(currentGroupEntry.getGroup()).containsKey(currentGroupEntry.getPositionInGroup())) {
                if (currentGroupEntry.getPositionInGroup() < 9999) {
                    errors.add("\"" + field.name() + "\" uses an occupied position in group \"" + currentGroupEntry.getGroup() + "\". Position no. "
                            + currentGroupEntry.getPositionInGroup() + " is already taken by field: \""
                            + groups.get(currentGroupEntry.getGroup()).get(currentGroupEntry.getPositionInGroup()));
                }
            }
            else {
                groups.get(currentGroupEntry.getGroup()).put(currentGroupEntry.getPositionInGroup(), field.name());
            }
        });
        if (!errors.isEmpty()) {
            fail("Duplicate field group assingnments found: " + String.join("\n", errors));
        }
    }
}

redpajama_set_name: RedPajamaGithub


1969 STRAT - SOLD !!!!!!
Welcome to Blue Sky Guitars. I am an avid guitar & amp collector. My background in Vintage Guitars go's way back to when a "59" Slab-Board Tele was around 500 bucks, those were the good old day's. I buy & sell vintage stuff as a hobby, I'm not trying to get rich. I just LOVE the old stuff !!! If you have an old Fender Telecaster - Esquire - Stratocaster - Gibson SG - Les Paul - Martin Flat Top or any vintage amps, Call or e-mail me & I WILL MAKE YOU A FAIR OFFER ------- Thank's : CALL ME or Text Me !!!!!
May 3-5, at the Dallas Market Hall, 2200 N Stemmons Fwy, Dallas, TX. For more info visit www.guitarshow.com.
June 8 & 9, at the Infinite Energy Forum, 64000 Sugar Loaf Pkwy, Duluth, GA. Hours 10-5. For more info contact Lee at (678) 557-5641, lee@grinningelk.com, Ray at (404) 895-3459, ray@grinningelk.com or visit www.grinningelkguitarshow.com.
Check out the RECENTLY SOLD GUITAR & AMP PAGE's -- This is the kind of Quality Guitars & Amps I deal in !!!!!!


6 For Lyttelton, alas! is dead.
12 For Lyttelton, alas! is dead.
18 Alas! your honour'd muse is dead.
24 For Lyttelton, alas! is dead.
30 For Lyttelton your muse is dead.
36 For Lyttelton, alas! is dead.
42 For generous Lyttelton is dead.
48 Your kindred influence ever spread.
Title (in Source Edition): On the DEATH of LORD GEORGE LYTTELTON.


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Voyages Ayers Rock Resort lies at the gateway to Uluru-Kata Tjuta National Park, in Australia's Northern Territory.
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Voyages Home Valley Station is located on a sprawling 3.5 million acres on the famous Gibb River Road in the east Kimberley region of Western Australia.
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Use promo code CLUB for 10% off guided tours.
*Conditions and restrictions apply. Available for new bookings only. Not available on all rates and offers. Discounts do not apply to breakfast and tour products or any charges at the hotel / resort. Voyages General Terms & Conditions also apply.


text:
(function () {

  var requestAnimationFrame = window.requestAnimationFrame ||
                              window.webkitRequestAnimationFrame ||
                              function (fn) { setTimeout(fn, 16); };

  function delegate(selector, handler) {
    return function(e) {
      var target = e.target;
      var delegateEl = e.currentTarget;
      var matches = delegateEl.querySelectorAll(selector);
      for (var el = target; el.parentNode && el !== delegateEl; el = el.parentNode) {
        for (var i = 0; i < matches.length; i++) {
          if (matches[i] === el) {
            handler.call(el, e);
            return;
          }
        }
      }
    };
  }

  var skipFrame = function(fn){
    requestAnimationFrame(function(){ requestAnimationFrame(fn); });
  };

  var sides = {
        next: ['nextElementSibling', 'firstElementChild'],
        previous: ['previousElementSibling', 'lastElementChild']
      };

  function indexOfCard(deck, card){
    return Array.prototype.indexOf.call(deck.children, card);
  }

  function getCard(deck, item){
    if (item && item.nodeName) {
      return item;
    } else {
      if (isNaN(item)) {
        return deck.querySelector(item);
      } else {
        return deck.children[item];
      }
    }
  }

  // check if a card is a card in a deck
  function checkCard(deck, card){
    return card &&
           deck === card.parentNode &&
           card.nodeName.toLowerCase() === 'brick-card';
  }

  function shuffle(deck, side, direction){
    var getters = sides[side];
    var selected = deck.selectedCard && deck.selectedCard[getters[0]];

    if (selected) {
      deck.showCard(selected, {'direction': direction});
    } else if (deck.loop || deck.selectedIndex === -1) {
      deck.showCard(deck[getters[1]], {'direction': direction});
    }
  }

  var BrickDeckElementPrototype = Object.create(HTMLElement.prototype);

  BrickDeckElementPrototype.createdCallback = function() {
    this.ns = {};
  };

  BrickDeckElementPrototype.attachedCallback = function() {
    this.revealHandler = delegate('brick-card', function(e) {
      e.currentTarget.showCard(this);
    });
    this.addEventListener('reveal', this.revealHandler);
  };

  BrickDeckElementPrototype.detachedCallback = function() {
    this.removeEventListener('reveal', this.revealHandler);
  };

  BrickDeckElementPrototype.attributeChangedCallback = function (attr, oldVal, newVal) {
    if (attr in attrs) {
      attrs[attr].call(this, oldVal, newVal);
    }
  };

  var attrs = {
    'selected-index': function (oldVal, newVal) {
      var index = parseInt(newVal);
      if (!isNaN(index)  && this.cards[index] !== this.selectedCard) {
        this.showCard(index);
      }
    }
  };

  BrickDeckElementPrototype.nextCard = function(direction){
    shuffle(this, 'next', direction);
  };

  BrickDeckElementPrototype.previousCard = function(direction){
    shuffle(this, 'previous', direction);
  };

  BrickDeckElementPrototype.showCard = function(item, options){
    options = options || {};
    var direction = options.direction;
    var skipTransition = options.skipTransition;
    var card = getCard(this, item);
    if (!checkCard(this, card) || (card === this.selectedCard)) {
      return;
    }
    var selectedCard = this.ns.selectedCard;
    var currentIndex = indexOfCard(this, selectedCard);
    var nextIndex = indexOfCard(this, card);
    if (!direction) {
      direction = nextIndex > currentIndex ? 'forward' : 'reverse';
      // if looping is turned on, check if the other way round is shorter
      if (this.loop) {
        // the distance between two cards
        var dist = nextIndex - currentIndex;
        // the distance between two cards when skipping over the end of the deck
        var distLooped = this.cards.length - Math.max(nextIndex,currentIndex) + Math.min(nextIndex,currentIndex);
        // set the direction if the looped way is shorter
        if (Math.abs(distLooped) < Math.abs(dist)) {
          direction = nextIndex < currentIndex ? 'forward' : 'reverse';
        }
      }
    }
    // hide the old card
    if (selectedCard) { this.hideCard(selectedCard, direction); }
    this.ns.selectedCard = card;
    this.ns.selectedIndex = nextIndex;
    this.setAttribute("selected-index", nextIndex);
    if (!card.selected) { card.selected = true; }
    var hasTransition = card.hasAttribute('transition-type') || this.hasAttribute('transition-type');
    if (!skipTransition && hasTransition) {
      // set attributes, set transitionend listener, skip a frame set transition attribute
      card.setAttribute('transition-direction', direction);
      var transitionendHandler = function() {
        card.dispatchEvent(new CustomEvent('show',{'bubbles': true}));
        card.removeEventListener('transitionend', transitionendHandler);
      };
      card.addEventListener('transitionend', transitionendHandler);
      skipFrame(function(){ card.setAttribute('transition', ''); });
    } else {
      card.dispatchEvent(new CustomEvent('show',{'bubbles': true}));
    }
  };

  BrickDeckElementPrototype.hideCard = function(item, direction){
    var card = getCard(this, item);
    if (!checkCard(this, card) || (card !== this.selectedCard)) {
      return;
    }
    this.ns.selectedCard = null;
    if (card.selected) { card.selected = false; }
    var hasTransition = card.hasAttribute('transition-type') || this.hasAttribute('transition-type');
    if (hasTransition) {
      // set attributes, set transitionend listener, skip a frame set transition attribute
      card.setAttribute('hide', '');
      card.setAttribute('transition-direction', direction || 'reverse');
      var transitionendHandler = function() {
        card.removeAttribute('hide');
        card.removeAttribute('transition');
        card.removeAttribute('transition-direction');
        card.dispatchEvent(new CustomEvent('hide',{'bubbles': true}));
        card.removeEventListener('transitionend', transitionendHandler);
      };
      card.addEventListener('transitionend', transitionendHandler);
      skipFrame(function(){ card.setAttribute('transition', ''); });
    } else {
      card.dispatchEvent(new CustomEvent('hide',{'bubbles': true}));
    }
  };

  // Property handlers
  Object.defineProperties(BrickDeckElementPrototype, {
    'loop': {
      get: function() {
        return this.hasAttribute('loop');
      },
      set: function(newVal) {
        if (newVal) {
          this.setAttribute('loop', newVal);
        } else {
          this.removeAttribute('loop');
        }
      }
    },
    'cards': {
      get: function () {
        var cardList = this.querySelectorAll("brick-card");
        return Array.prototype.slice.call(cardList);
      }
    },
    'selectedCard': {
      get: function() {
        return this.ns.selectedCard || null;
      }
    },
    'selectedIndex': {
      get: function() {
        return this.hasAttribute('selected-index') ? Number(this.getAttribute('selected-index')) : -1;
      },
      set: function(value) {
        var index = Number(value);
        var card = this.cards[index];
        if (card) {
          if (card !== this.ns.selectedCard) {
            this.showCard(card);
          }
        } else {
          this.removeAttribute('selected-index');
          if (this.ns.selectedCard) {
            this.hideCard(this.ns.selectedCard);
          }
        }
      }
    },
    'transitionType': {
      get: function() {
        return this.getAttribute('transition-type');
      },
      set: function(newVal) {
        this.setAttribute('transition-type', newVal);
      }
    }
  });

  // Register the element
  window.BrickDeckElement = document.registerElement('brick-deck', {
    prototype: BrickDeckElementPrototype
  });

})();