Galaxy Groups, CDM/CHDM Cosmologies, and the Value of Ω_{0}
Abstract
We present techniques for identifying and analyzing galaxy groups and apply them to largescale particlemesh (PM) Nbody simulations of structure formation in three Ω_{0} = 1 cosmological models: cold plus hot dark matter (CHDM), with Ω_{cold} = 0.6, Ω_{ν} = 0.3, and Ω_{baryon} = 0.1 at bias b≡σ^{1}_{8}=1.5 and two cold dark matter (CDM) models, at bias b = 1.5 and b = 1.0. Groups are identified with the adaptive friendsoffriends algorithm of Nolthenius. Our most important conclusions follow. The standard group M/L method gives Ω_{0} ~= 0.08 for the CfA1 survey (for redshift link parameter V_{5} = 350), and, applied to our Ω_{0} = 1 simulations, it gives Ω_{0} ~= 0.12 for CHDM (V_{5} = 350) and Ω_{0} ~= 0.35 for CDM (V_{5} = 600). This Ω bias appears to be even stronger at higher resolution. We show quantitatively how three different effects conspire to produce this large discrepancy, and we conclude that low observed Ω values need not argue for a lowΩ universe. Our preferred statistics of groups show promise in becoming powerful discriminators between Gaussian cosmological models, whose Ω_{ν} differ and are robust against several methods for assigning luminosity to dark matter halos, and for merging CfA1 data. However, our latest results at higher resolution show such strong sensitivity to how massive overmergers are broken up that more reliable ways of identifying luminous galaxies within largescale simulations will be necessary before these statistics can provide reliable discrimination. When overmergers are broken up, the median virialtoDM mass M_{vir}/M_{DM} of threedimensionalselected groups is ~1 for all simulations. Groups with M_{DM} > 10^{14} M_{⊙} appear virialized in all simulations. We measure global (not pairwise) velocity biases b_{v}, similar to previous studies. Within threedimensionalselected groups, CHDM and CDM with b = 1.5 show a stronger bias of b_{v} = 0.70.8, while CDM with b = 1.0 shows groups of b_{v} ~= 1.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 1997
 DOI:
 10.1086/303934
 Bibcode:
 1997ApJ...480...43N
 Keywords:

 Cosmology: Theory;
 Cosmology: Dark Matter;
 Cosmology: LargeScale Structure of Universe;
 Methods: Numerical