Cosmological constraints from the virial mass function of nearby galaxy groups and clusters

Abstract

In this thesis, I present a new determination of the cluster mass function in a volume ~107 h-03 70 Mpc3 using the ROSAT-2MASS-FAST Group Survey (R2FGS). R2FGS is an X-ray-selected sample of systems from the ROSAT All-Sky Survey in the region [delta] >/= 0° and 0.01 < z < 0.06, with target galaxies for each system compiled from the Two Micron All Sky Survey (2MASS). The sample is designed to focus on lowmass groups and clusters, so as to break a degeneracy between the cosmological parameters ,~ and as. In addition, R2FGS covers a very large area of sky (", 4.13 ster.), which is necessary given the low redshift limit of the survey. I acquire optical redshifts for the target galaxies in R2FGS from the literature and from new data collected with the FAST spectrograph on Mt. Hopkins. After removing foreground and background galaxies (interlopers) using a dynamical maximum-velocity criterion, I estimate the group and cluster masses using the full virial theorem, and subsequently verify the results using the projected mass estimator. I briefly investigate the [sigma]p - Lx and M - Lx scaling relations, as well as the halo occupation function. Due to interloper issues with some of the systems, I apply a luminosity-dependent correction to the virial masses, and subsequently use these masses to compute the virial mass function of the sample. By comparing this mass function to predictions from various cosmological models, I constrain the parameters , [omega]m and [sigma]s. I find m = 0.26 +0.07 -0.08 and [sigma]8 = 1.02 +0.30 -0.15; the R2FGS value for [omega]m agrees very well with the recent FiveYear WMAP (WMAP5) result, although the R2FGS value for [sigma]s is somewhat larger than that found by WMAP5. Future work will include an expansion of the survey to [delta] > -20° and z < 0.04, which will greatly increase its degeneracy-breaking power.