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Published March 10, 2013 | Published
Journal Article Open

Proto-groups at 1.8 < z < 3 in the zCOSMOS-deep Sample


We identify 42 "candidate groups" lying between 1.8 < z < 3.0 from a sample of 3502 galaxies with spectroscopic redshifts in the zCOSMOS-deep redshift survey within this same redshift interval. These systems contain three to five spectroscopic galaxies that lie within 500 kpc in projected distance (in physical space) and within 700 km s^(–1) in velocity. Based on extensive analysis of mock catalogs that have been generated from the Millennium simulation, we examine the likely nature of these systems at the time of observation, and what they will evolve into down to the present epoch. Although few of the "member" galaxies are likely to reside in the same halo at the epoch we observe them, 50% of the systems will have, by the present epoch, all of the member galaxies in the same halo, and almost all (93%) will have at least some of the potential members in the same halo. Most of the candidate groups can therefore be described as "proto-groups." A crude estimate of the overdensities of these structures is also consistent with the idea that these systems are being seen as they assemble. We also examine present-day halos and ask whether their progenitors would have been seen among our candidate groups. For present-day halos between 10^(14) and 10^(15) M_☉ h^(–1), 35% should have appeared among our candidate groups, and this would have risen to 70% if our survey had been fully sampled, so we can conclude that our sample can be taken as representative of a large fraction of such systems. There is a clear excess of massive galaxies above 10^(10) M_☉ around the locations of the candidate groups in a large independent COSMOS photo-z sample, but we see no evidence in this latter data for any color differentiation with respect to the field. This is, however, consistent with the idea that such differentiation arises in satellite galaxies, as indicated at z < 1, if the candidate groups are indeed only starting to be assembled.

Additional Information

© 2013 American Astronomical Society. Received 2012 October 9; accepted 2012 December 21; published 2013 February 25. This research is based on observations undertaken at the European Southern Observatory (ESO) Very Large Telescope (VLT) under the Large Program 175.A-0839 and has been supported by the Swiss National Science Foundation (SNF). G. Lemson is supported by Advanced grant 246797 GALFORMOD from the European Research Council. The Millennium simulation databases used in this paper and the web application providing online access to them were constructed as part of the activities of the German Astrophysical Virtual Observatory.

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