---
abstract: 'We discover alignment of galaxies in clusters by analyzing the distribution of their position angles. We assume that galaxies are aligned, if their number at one $90\deg$ position angle interval is more than twice higher than at another $90\deg$ interval. We selected for study the isolated clusters in order to exclude clusters the distribution of the galaxy orientation in which could possibly be altered by gravitational influence of a nearby cluster. Also, we study the galaxy position angle distribution at the outer regions of clusters with small space density where the variation of the position angles in the result of interactions between galaxies is smaller than at the central dens regions used as a control sample. We found that the alignment of galaxies is more often observed in poor clusters. We conclude that originally galaxies were aligned, but in the result of accretion of field galaxies with arbitrary orientation and mutual interactions of galaxies during the cluster evolution the relative part of aligned galaxies decreases.'
author:
- |
    Hrant M. Tovmassian,$^{1}$[^1] and J.P. Torres-Papaqui$^{2}$[^2]\
    $^{1}$377, W. California 30, Glendale, CA, USA\
    $^{2}$Departamento de Astronomía, Universidad de Guanajuato, Apartado Postal 144, 36000, Guanajuato, Mexico\
date: 'Accepted XXX. Received YYY; in original form ZZZ'
title: On Alignment of Galaxies in Clusters
---

\[firstpage\]

galaxies: clusters – galaxies: alignment – galaxies: large-sale structure

Introduction
============

The alignment of galaxies in clusters could be clue to the origin of clusters. In the pancake scenario (Zel’dovich, 1970; Zel’dovich et al. 1982; Doroshkevich, Shandarin, & Saar 1978) galaxies form after the gas-dust cloud collapse and the position angles (PA) of galaxies will be aligned. In the past some evidence on the alignment of galaxies with the parent cluster were reported by Adams, Strom & Strom (1980), Sastry (1968), Carter & Metcalfe (1980), Binggeli (1982), Struble & Peebles (1985), Rhee & Katgert (1987), Lambas et al. (1988), Flin & Olowin (1991), Fong, Stevenson, & Shanks (1990. More certainly the alignment were found between orientations of the cluster and of the BCG (cD) (Sastry 1968; Binggeli 1982; Carter & Metcalfe 1980; Struble 1990; West 1989; West 1994; Plionis 1994; Fller, West & Bridges 1999; Kim et al. 2001; Chambers, Melott, Miller 2002). Plionis et al. (2003) and Rong, Zhang & Liao (2015) found an evidence that significant galaxy alignments are present in dynamically young clusters. Meanwhile, Dekel (1985), van Kampen & Rhee (1990), Trevese, Cirimele & Flin (1992), Djorgovski 1987 and Cabanela & Aldering (1998) found no galaxy alignment, except the alignment of the BCG with its parent cluster.

Hence, no a certain conclusion was achieved on the galaxy alignment in clusters. In this paper we undertook new search for alignment of galaxies in clusters and showed that galaxy alignment is mostly revealed in poor clusters. It is suggested that at the cluster origin the primordial orientations of member galaxies are ordered and that assembly of field galaxies later on and interactions between galaxies introduce disorder in the the galaxy orientations.

The data
========

We study the possible alignment of galaxies in ACO (Abell, Corwin, & Olowin, 1989) clusters. Many Abell clusters are themselves clustered (Abell 1958; Bogart & Wagoner 1973; Hauser & Peebles 1973). The gravitational influence of the nearby cluster may affect on the orientation of galaxies in the studied cluster. In order to exclude this effect we selected for study isolated clusters. We assume that the cluster is isolated, if the nearest neighbor ACO cluster is located at the projected distance $>10$ Mpc on the sky.

Note that any primordial galaxy alignments in clusters could be severely damped by the violent relaxation, by the exchange of angular momentum in galaxy interactions that occur in the dense cluster environment over a Hubble time (Coutts 1996) and also by secondary infall (Quinn & Binney 1992). Therefore, we searched the orientation of galaxies in the outer area of clusters. The member galaxies of the most ACO clusters are located within 2 Mpc of the Abell radius (1958), defined as $R_A$ = 2 Mpc $h_{72}^{-1}$ = 1.7’/z $R_A$ = 2 Mpc $h_1= 1.7$/z 72 (Andernach, Waldhausen, & Wielebinski 1980) where z is the cluster redshift. In the selected clusters we studied the distribution of position angles (PAs) of galaxies in the ring with cluster-centric radii $1\div2$ Mpc. As a control sample we use the central area (further - the control area) with 1 Mpc radius of the same clusters. The clusters at $z<0.1$ with more than 10 galaxies within the the studied region and the control area were studied. The used parameters of clusters are taken from NED. Finally for the alignment search we selected 75 isolated ACO clusters.

The member galaxies in the studied clusters were retrieved from photometric catalog of the Ninth Data Release (DR9) of the Sloan Digital Sky Server (SDSS) project (Ahn et al. 2012). The retrieved galaxies are those with the primary mode (marked by 1) and good quality of observations (marked by 3). Also, the galaxies with velocities within $\pm1500$ km s$^{-1}$ of the cluster velocity (Collins et al. 1995) were retrieved.[^3]

Analysis
========

For search of the alignment of the orientation of galaxies in clusters we divided the range of PAs of galaxies in each cluster into two $90\deg$ sections so that to have high number ($N_h$) of galaxies at one section and small number ($N_s$) of galaxies at the other section. We assume that there is an alignment signal, if the number of galaxies at one $90\deg$ section is by at least 2 times higher than at the other $90\deg$ section. The results of counts are presented in Table \[Table01\].

At corresponding columns of Table \[Table01\] the following information is given: 1st - the cluster designation; 2d - the redshift of the cluster; 3d - the number of galaxies in the searched region, 4d - the range of PAs in $90\deg$ section with high number of galaxies; 5th - the interval of PAs at which the high number of galaxies are distributed; 6th - the number $N_h$ of galaxies at this section; 7th - the number of galaxies at the opposite section; 8th - the ratio $N_h/N_s$ at the searched region; 9th - the ratio $N_h/N_s$ at the control region.

  Cluster   $z$      $N_2$   $90\deg$   $I_{PA}$   $N_h$   $N_s$   $\left(\frac{N_h}{N_s}\right)_{1\div 2}$   $\left(\frac{N_h}{N_s}\right)_1$
  --------- -------- ------- ---------- ---------- ------- ------- ------------------------------------------ ----------------------------------
                             degree     degree                                                                
  A595      0.0666   51      32-122     85         12      3       4.0                                        1.0
  A602      0.0619   62      69-159     89         20      10      2.0                                        1.7
  A634      0.0265   104     60-150     -          20      16      1.25                                       1.7
  A635      0.0925   34      78-165     87         8       2       4.0                                        1.4
  A660      0.0642   26      163-73     88         12      3       4.0                                        3.5
  A671      0.0502   99      79-169     -          25      14      1.5                                        1.5
  A690      0.0788   48      10-101     -          17      9       1.9                                        2.1
  A695      0.0687   27      26-116     90         8       2       4.0                                        2.4
  A699      0.0851   31      68-158     80         12      6       2.0                                        1.6
  A692      0.0894   48      19-109     -          15      10      1.5                                        1.6
  A724      0.0933   46      171-81     83         14      3       4.7                                        1.2
  A727      0.0951   57      148-58     -          24      15      1.6                                        5.0
  A744      0.0729   32      127-25     78         7       3       2.3                                        6.0
  A757      0.0517   49      173-83     -          11      8       1.4                                        1.1
  A779      0.0225   112     15-105     -          23      17      1.4                                        1.1
  A819      0.0759   23      67-157     83         8       4       2.0                                        2.7
  A834      0.0709   35      38-128     86         16      3       5.3                                        3.0
  A858      0.0863   26      46-136     80         8       2       4.0                                        1.3
  A1024     0.0734   49      15-105     -          13      12      1.1                                        1.9
  A1028     0.0908   26      179-89     90         10      3       3.3                                        2.25
  A1035     0.0684   57      69-159     90         10      2       5.0                                        1.6
  A1066     0.0690   81      75-165     -          18      13      1.4                                        1.4
  A1100     0.0463   53      90-180     -          15      8       1.9                                        2.0
  A1126     0.0646   33      18-108     88         11      5       2.2                                        16.1
  A1139     0.0398   50      6-96       80         12      6       2.0                                        1.5
  A1142     0.0349   68      17-107     -          18      12      1.5                                        1.9
  A1168     0.0906   41      3-93       81         12      5       2.4                                        2.7
  A1169     0.0586   78      1-91       77         19      8       2.4                                        1.4
  A1238     0.0733   68      105-15     81         23      7       3.3                                        2.4
  A1270     0.0692   63      69-159     87         17      6       2.8                                        1.3
  A1307     0.0817   67      164-74     88         24      11      2.2                                        1.9
  A1314     0.0335   119     30-120     -          23      15      1.5                                        1.3
  A1371     0.0398   60      32-122     86         17      7       2.4                                        2.6
  A1424     0.0768   72      53-143     -          23      15      1.5                                        1.4
  A1480     0.0734   31      160-70     90         11      5       2.2                                        1.5
  A1507     0.0604   57      0-90       -          19      12      1.6                                        1.9
  A1516     0.0769   59      72-162     87         21      10      2.1                                        1.5
  A1541     0.0893   71      7-97       77         21      9       2.3                                        2.5
  A1552     0.0858   75      151-61     90         24      12      2.0                                        2.1
  A1564     0.0792   54      32-122     -          16      12      1.3                                        2.25
  A1599     0.0855   25      100-10     68         7       3       2.3                                        2.0
  A1616     0.0833   75      0-90       87         19      8       2.4                                        1.6
  A1630     0.0648   36      7-96       -          7       4       1.75                                       2.6
  A1658     0.0850   25      153-63     -          11      5       2.2                                        8.1
  A1684     0.0862   27      66-156     -          9       6       1.5                                        2.0
  A1692     0.0879   50      132-42     90         18      9       2.0                                        2.7
  A1750     0.0852   86      13-103     -          36      23      1.6                                        2.7
  A1781     0.0618   45      66-156     87         19      6       3.2                                        1.2
  A1783     0.0690   50      77-167     77         12      3       4.0                                        1.7
  A1809     0.0791   87      15-105     88         23      10      2.3                                        2.5
  A1812     0.0630   27      32-122     84         11      4       3.0                                        2.0
  A1825     0.0595   30      110-20     90         11      5       2.2                                        2.5
  A1827     0.0654   40      78-168     77         12      6       2.0                                        1.75
  A1849     0.0963   27      73-163     77         9       2       4.5                                        3.0
  A1864     0.0870   51      15-105     -          16      12      1.3                                        1.3
  A1890     0.0574   83      51-141     -          21      14      1.5                                        1.4
  A1939     0.0881   41      63-153     90         12      6       2.0                                        2.3

  Cluster   $z$      $N_2$   $90\deg$   $I_{PA}$   $N_h$   $N_s$   $\left(\frac{N_h}{N_s}\right)_{1\div 2}$   $\left(\frac{N_h}{N_s}\right)_1$
  --------- -------- ------- ---------- ---------- ------- ------- ------------------------------------------ ----------------------------------
                             degree     degree                                                                
  A2018     0.0878   54      44-134     90         23      9       2.6                                        1.0
  A2019     0.0807   24      75-165     81         9       4       2.25                                       2.7
  A2022     0.0578   77      6-96       -          21      15      1.4                                        1.4
  A2048     0.0972   60      0-90       -          19      14      1.4                                        2.0
  A2082     0.0862   24      130-40     87         11      1       11.0                                       5.0
  A2107     0.0411   130     15-105     -          33      22      1.5                                        1.3
  A2108     0.0919   50      32-122     76         13      4       3.25                                       2.3
  A2110     0.0980   27      90-180     88         10      3       3.3                                        1.8
  A2122     0.0661   72      51-141     90         22      11      2.0                                        2.25
  A2142     0.0909   123     88-178     -          36      26      1.4                                        1.25
  A2148     0.0877   29      108-18     90         7       3       2.3                                        1.1
  A2162     0.0322   46      98-180     -          10      6       1.7                                        1.5
  A2178     0.0928   46      171-81     83         14      3       4.7                                        1.2
  A2205     0.0876   39      46-136     -          13      11      1.4                                        1.5
  A2255     0.0806   122     16-106     -          28      26      1.1                                        1.4
  A2366     0.0529   56      89-179     56         9       4       2.25                                       1.7
  A2593     0.0413   138     2-92       -          36      24      1.5                                        1.1
  A2630     0.0667   36      40-130     85         9       2       4.5                                        1.5

In the case of a roughly uniform distribution of PAs the numbers of galaxies in two $90\deg$ intervals could occasionally differ from each other by more than 2 times. We found that in the ring with cluster-centric radii $1\div2$ Mpc of 47 out of the studied 75 clusters, i.e $\sim$63% of them have alignment signal. The probability that in 47 clusters out of 75 the ratio $N_h/N_s$ exceeds 2 is 62.66% of success and a 95 percent confidence interval from 50.73% to 73.56% with a p-value = 0.0369. Hence, the probability that the found alignments are not by chance is sufficiently high. Meanwhile at the control region the alignment signal have 32 clusters, $\sim$43%. The probability that this is by chance is 42.66% of success and a 95 percent confidence interval from 31.30% to 54.62% with a p-value = 0.2480. Hence, the probability that the found alignments of galaxies at the cluster central region are caused by random distribution of PAs is high.

We more detailed analyze the distribution of PAs in the outer region of clusters within radii $1\div2$ Mpc. We supposed that the alignment could depend on the richness of clusters and on the limiting magnitude of the observed galaxies. In order to find out whether the alignment signal depends on the cluster richness or distance we split the list of clusters into two parts with poor and rich ones and also with high and low redshifts. The deduced parameters are presented in Table \[Table02\]. In the 1st column of Table \[Table02\] the analyzed parameters are given: the redshift, the average number $N$ of galaxies, the average minimal absolute magnitude in r-band, the ratio of the number of clusters with alignment signal to the total number of galaxies, $N_{as}/N_t$ in the studied region, the average value of the ratio of the high number $N_h$ of galaxies in one $90\deg$ interval to the small number of galaxies at the other $90\deg$ interval for the outer ring. In the 2d and 3d columns the corresponding data related to the nearby and distant clusters are presented.

Table \[Table02\] shows that for nearby clusters at average $<z>=0.0559$ the relative number of clusters with alignment signal is 0.58 with 58.00% of success and 95 percent confidence interval from 47.71% to 67.80% with a p-value = 2.182e-03. Meanwhile the relative number of distant clusters at average $<z>=0.0861$ with alignment signal is higher 0.70 with 70.00% of success and a 95 percent confidence interval from 60.01% to 78.75% with a p-value = 9.818e-05. The relative number of clusters with alignment signal in rich clusters at small average $<z>=0.0667$ is smaller of that of the poor clusters at higher average $<z>=0.0767$. The rich clusters with alignment signal has a 53.00% of success and a 95 percent confidence interval from 42.75% to 63.05% with a p-value = 6.173e-02, meanwhile poor clusters with alignment signal has a 77.00% of success and a 95 percent confidence interval from 67.51% to 84.82% with a p-value = 5.514e-08.

Table \[Table02\] shows that the differences between the total numbers of galaxies, the limiting absolute magnitude, the ratios $N_h/N_s$ are higher for rich and poor clusters in comparison to those of nearby and distant clusters. The relative number of clusters with alignment signal in rich and poor cluster differs from each other by 1.6 times, while this difference between nearby and distant clusters is smaller, 1.2. The difference between average absolute magnitudes $M_r$ between nearby and distant clusters is less than $1^m$ and is higher, $0.30^m$ between rich and poor clusters. The ratios $N_{as}/N_t$ and $N_h/N_s$ are also higher between clusters of different richness, 1.45 and 1.2, and 1.38 and 1.04.

It follows that the defining reason for high relative number of clusters with alignment signal is not the presence of relatively large number of faint galaxies in them, but generally the cluster richness The poorer they are the higher is the probability of preserving in them the primordial alignment of galaxies.

                   Nearby clusters   Distant clusters
  -------------- ----------------- ------------------
           $<z>$            0.0559             0.0861
           $<N>$                62                 50
         $<M_r>$            -19.11             -20.04
    $N_{as}/N_t$              0.58               0.70
       $N_h/N_s$               2.4                2.5
                     Rich clusters      Poor clusters
             $z$            0.0666             0.0774
           $<N>$                75                 34
         $<M_r>$            -19.49             -19.81
    $N_{as}/N_t$              0.53               0.77
       $N_h/N_s$               2.1                2.9

  : The parameters of nearby, distant, poor and rich clusters.[]{data-label="Table02"}

The PAs of the large axes of 19 clusters of our list with alignment signal were determined by Plionis (1994). In Figure \[Figure01\] the distribution of PAs of galaxies in 14 clusters and the PAs of the cluster large axis are shown. In 14 clusters the PAs of clusters are within interval of PAs of aligned galaxies. The PAs of large axes of only 5 clusters, A1126, A1139, A1541, A1783 and A1812 are out of the interval of PAs of galaxies with alignment signal.The probability of 14 chance coincidences out of 19 is sufficiently small, 0.02. This proves that the applied method for searching the alignment is reliable.

![The comparison of the distribution of PAs of galaxies in clusters with alignment signal with the cluster PA from Plionis (1994) shown by dotted line. []{data-label="Figure01"}](f01new.png){width="\columnwidth"}

Discussion and Conclusions
==========================

By the study of the distribution of PAs of galaxies in the ring with radii $1\div2$ Mps of 75 isolated clusters the alignment signal is found in 47 clusters. Separate analysis of clusters of different richness showed that alignment depends on the cluster richness. The percentage of clusters with alignment is the highest, 78%, in poor clusters containing on average 35 galaxies. In rich clusters with on average 76 galaxies the percentage of clusters with alignment signal is smaller, 50%. The alignment signal is found also for galaxies of the control sample in the central area of clusters with 1 Mpc. However, the percentages of clusters with alignment signal are smaller, 49% for poor clusters and 36% for rich ones. Hence, the poorer is the cluster, the higher is the chance to reveal the alignment.

The alignment of galaxies is found mostly in poor clusters. It evidences in favor of pancake scenario (Zel’dovich, 1970; Zel’dovich et al. 1982; Doroshkevich, Shandarin, & Saar 1978) of the cluster formation. According to Miller & Smith (1982), Salvador-Sole & Solanes (1993), Usami & Fujimoto (1997), in the hierarchical scenario the galaxies could be aligned due to the tidal field of the cluster. However, the tidal field of the cluster would apparently be more effective in rich clusters with higher mass.

During the cluster evolution the primordial alignment of galaxies will be altered. First, the alignment rate will decrease in the result of mutual interactions between galaxies. Apparently the rate of interactions will be higher in rich clusters and especially at the cluster dense central regions. The gravitational influence would have smaller effect on the orientation for massive galaxies. Indeed the alignment of very massive BCGs (cDs) with the cluster orientation have been found by Sastry (1968), Binggeli (1982), Carter & Metcalfe (1980), Struble (1990), West (1989, 1994), Plionis (1994), Fuller, West & Bridges (1999), Kim et al. (2001), Chambers, Melott, Miller (2002).

The inclusion to the cluster content of the faint field galaxies by the secondary infall (Quinn & Binney 1992) (Blanton et al., 2001) with arbitrary orientations will certainly decrease the relative number of aligned galaxies. The poorer is the cluster, i.e. the less massive it is, the smaller amount of field galaxies would be assembled. Thus, the primordial alignment is better preserved in poor clusters, in which both reasons for altering it, interactions between galaxies and the assembly of field galaxies are less effective.

Acknowledgements
================

T-P acknowledges for support through grant DAIP-UGto (0173/19). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

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[^1]: E-mail: htovmas@gmail.com (HMT)

[^2]: E-mail: jp.torrespapaqui@ugto.mx (JPTP)

[^3]: The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
