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The Large Scale Distribution of Galaxies in the Shapley Supercluster

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The Large Scale Distribution of Galaxies in the Shapley Supercluster CSIRO PUBLISHING www.publish.csiro.au/journals/pasa Publications of the Astronomical Society of Australia, 2004, 21, 89–96 The Large Scale Distribution of Galaxies in the Shapley Supercluster Michael J. Drinkwater,1 Quentin A. Parker,2,3 Dominique Proust,4 Eric Slezak,5 and Hernán Quintana 6 1 Department of Physics, University of Queensland, QLD 4072, Australia (e-mail: [email protected]) 2 Department of Physics, Macquarie University, NSW 2109, Australia 3 Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia (e-mail: [email protected]) 4 GEPI — Observatoire de Paris–Meudon, 92195 Meudon CEDEX, France (e-mail: [email protected]) 5 Observatoire de Nice, 06304 Nice CEDEX4, France (e-mail: [email protected]) 6 Departamento de Astronomia y Astrofisica, Pontificia Universidad Católica de Chile, Casilla 104, Santiago 22, Chile (e-mail: [email protected]) Received 2003 October 13, accepted 2003 December 10 Abstract: We present new results of our wide-field redshift survey of galaxies in a 182 square degree region of the Shapley Supercluster (SSC) based on observations with the FLAIR-II spectrograph on the UK Schmidt Telescope (UKST). In this paper we present new measurements to give a total sample of redshifts for 710 bright (R ≤ 16.6) galaxies, of which 464 are members of the SSC (8000 <v<18 000 km s−1). Our data reveal that the main plane of the SSC (v ≈ 14 500 km s−1) extends further than previously realised, filling the whole extent of our survey region of 10 degrees by 20 degrees on the sky (35 Mpc by 70 Mpc, for −1 −1 H0 = 75 km s Mpc ). There is also a significant structure associated with the slightly nearer Abell 3571 cluster complex (v ≈ 12 000 km s−1) with a caustic structure evident out to a radius of 6 Mpc. These galax- ies seem to link two previously identified sheets of galaxies and establish a connection with a third one at V = 15 000 km s−1 near RA = 13h. They also tend to fill the gap of galaxies between the foreground Hydra- Centaurus region and the more distant SSC. We calculate galaxy overdensities of 5.0 ± 0.1 over the 182 square degree region surveyed and 3.3 ± 0.1 in a 159 square degree region excluding rich clusters. Over the large region of our survey the inter-cluster galaxies make up 46 per cent of all galaxies in the SSC region and may contribute a similar amount of mass to the cluster galaxies. Keywords: Redshifts of galaxies — clusters of galaxies — subclustering 1 Introduction In the past few decades, large galaxy redshift surveys have redshift range 0.04 < z < 0.055. The SSC lies in the gen- revealed structures on ever-increasing scales. The largest eral direction of the dipole anisotropy of the Cosmic structures found are superclusters, collections of thou- Microwave Background (CMB) (Smoot et al. 1992), and sands of galaxies with sizes as large as 100 Mpc. The mere is located 130 h−1 Mpc beyond the Hydra-Centaurus existence of these structures places important constraints supercluster (itself about 50 h−1 Mpc away from us). on theories of the formation of galaxies and clusters. The Quintana et al. (1995) estimated that for 0 = 0.3 and −1 −1 Shapley supercluster (SSC), the subject of this paper, is H0 = 75 km s Mpc , the gravitational pull of the one of the most massive concentrations of galaxies in supercluster may account for up to 25% of the peculiar the local universe (Scaramella et al. 1989; Raychaudhury velocity of the Local Group required to explain the CMB 1989), so it is also of particular interest to consider its dipole anisotropy, in which case the mass of the super- effect on the dynamics of the Local Group. cluster would be dominated by inter-cluster dark matter. The SSC is a remarkably rich concentration of galax- A major study of the SSC was made by Bardelli et al. ies centred around RA = 13h 25m, Dec =−30◦ which (2000, 2001, and references therein) who also studied has been investigated by numerous authors since its inter-cluster galaxies in the core region of the superclus- discovery in 1930 (see Quintana et al. 1995). It con- ter. They derived a total mean overdensity of the SSC sists of many clusters and groups of galaxies in the of N/N ≈ 11.3 on a scale of 10.1 h−1 Mpc and found © Astronomical Society of Australia 2004 10.1071/AS03057 1323-3580/04/01089 Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.93, on 27 Sep 2021 at 01:50:24, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1071/AS03057 90 M. J. Drinkwater et al. that the central part of the supercluster contributes about the 6dF survey of the southern sky currently in progress 26 km s−1 to the peculiar velocity of the Local Group. (Watson et al. 2000). The early studies of the SSC were limited (primarily by The observed galaxy sample and observations are observational constraints) to the various rich Abell galaxy described in Section 2, the results are presented in clusters in the region, but this might give a very biased Section 3 and a brief discussion of the significance of view of the overall supercluster as the clusters represent these new measurements is given in Section 4. A full and only the most concentrated peaks in the luminous matter detailed analysis and interpretation of the new redshifts distribution. We have commenced an investigation into the from D99, this work and a compilation of all literature red- larger scale distributions of galaxies throughout the entire shifts in the SSC is the subject of a separate paper (Proust SSC region and close environs using data from wide-field et al., in preparation). Unless otherwise noted in this paper −1 −1 multi-fibre spectrographs such as the FLAIR-II system we use a Hubble constant of H0 = 75 km s Mpc , on the UK Schmidt Telescope (UKST) of the Anglo- giving a distance of 200 Mpc to the main supercluster Australian Observatory (Parker & Watson 1995; Parker (cz = 15 000 km s−1) so 1 degree subtends a distance of 1997). With such multiplexed facilities we are able to mea- 3.5 Mpc. sure many more galaxy redshifts over large angular extents and obtain a more complete picture of the composition and disposition of galaxies in the entire supercluster. 2 Observations and Data Reductions A preliminary analysis based on 301 new, bright In 1997 we began a concerted campaign to obtain complete (R ≤ 16) galaxy redshifts obtained with FLAIR-II in a samples of galaxies down to the same magnitude below L∗ 77 deg2 region of the supercluster has already been for constituent clusters and inter-cluster regions of the SSC published by our group (Drinkwater et al. 1999, here- using the FLAIR-II system at the UKST. During the period after D99). The measured galaxies were uniformly dis- 1997–2000 we subsequently observed more than 700 tributed over the selected inter-cluster regions, and most selected galaxies over 7 UKST standard fields in the SSC (230 75%) were found to be members of the superclus- region, namely in fields 382–384 and 443–446. The main ter. The D99 sample traced out two previously unknown SSC core centred on A3558 at 13h 25m, −31◦ is in field sheets of galaxies which appeared to link various Abell 444. As each selected Abell cluster has a projected angu- clusters in the supercluster. We also found that in a 44 deg2 lar diameter of 2.5 to 3.0 degrees, the wide-field UKST sub-area of the supercluster excluding the Abell clusters, FLAIR-II system was an ideal facility for this project these sheets alone represent an overdensity of a factor whilst additionally permitting us to probe the regions of 2.0 ± 0.2 compared to a uniform galaxy distribution. between the dominant clusters neglected in previous stud- Within the initial survey area of D99, the new galaxies ies. These combined fields covered an area of 182 deg2 contribute an additional 50 per cent to the known con- which allows us to investigate the outer limits of the SSC tents of the SSC in that region, implying a corresponding out to radii of more than 10 deg (35 Mpc) from A3558. increase in contribution to the motion of the Local Group. A much larger galaxy sample over a similar area of sky was 2.1 Input Galaxy Samples presented by Quintana, Carrasco & Reisenegger (2000). All target galaxies were originally obtained from the red They measured nearly 3000 galaxy redshifts in an area of ESO/SRC sky survey plates of the above fields scanned by about 12 deg by 6 deg. They estimated the upper bound on the Paris Observatory MAMA plate-measuring machine the mass of the central region and found the overdensity (Guibert & Moreau 1991; Moreau 1992). The images were to be substantial, but still only able to contribute to a small classified using standard star/galaxy separation techniques fraction (3−0.4%) of the observed motion of the Local m (e.g. Dickey et al. 1987; Heydon-Dumbleton, Collins & Group (Reisenegger et al. 2000). MacGillivray 1989). We defined a galaxy sample to a In this paper we present radial velocities for an addi- photometric limit of R ≤ 16, corresponding (assuming tional 409 bright (R ≤ 16.5) galaxies spread over an a mean B−R = 1.5) to B<17.5, the nominal galaxy extended region to the East, West and South of the main limiting magnitude of the FLAIR-II system (Parker & SSC concentration.
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