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 the same redshift interval. These systems contain three to five
spectroscopic galaxies that lie within 500kpc in projected distance (in
physical space) and within 700km/s in velocity. Based on extensive analysis of
mock catalogues 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 eventually bring them all into the same halo, and
almost all (93%) will have at least part of the member galaxies in the same
halo by the present epoch. Most of the candidate groups can therefore be
described as "proto-groups". An estimate of the overdensities is also
consistent with the idea that these systems are being seen at the start of the
assembly process. We also examine present-day haloes and ask whether their
progenitors would have been seen amongst our candidate groups. For present-day
haloes between 10^14-10^15Msun/h, 35% should have appeared amongst 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 10^10Msun 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 colour 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.
