| The number of massive structures in the universe is determined by a small set of cosmological parameters characterizing its content, geometry, and expansion rate. Survey counts of massive clusters of galaxies can constrain these parameters, but require a statistical model relating total cluster mass to relevant, observable signals, such X-ray luminosity, X-ray temperature, and galaxy count. I present empirical and computational efforts to improve estimates of this statistical relationship, with an emphasis on measures of the hot intracluster gas. First, I present my work calibrating the relationship between galaxy cluster mass and X-ray luminosity. This work compared observed cluster counts from the REFLEX survey to expectations for ΛCDM cosmologies derived from a halo mass function. In this comparison, I obtained the first measurement of the scatter and discuss possible systematic biases in parameter estimates due to the scatter. I extended my work on mass selection functions to a full suite of X-ray and Sunyaev-Zeldovich (SZ) signals in the Millennium Gas Simulations (MGS). The MGS are hydrodynamic simulations in a 500 h -1 Mpc box, with two treatments of the gas physics: a model with only shock-heating and gravity (GO) and a simple preheating model (PH). From the MGS, I present scaling relations among multiple signals, including a covariance matrix, for about ∼4000 massive halos. Finally, I investigate the total halo mass function with two pairs of simulations: the MGS and a pair of high-resolution simulations which include a GO model and a refined treatment including cooling, star formation, and supernova feedback (CSF). The CSF and PH models have baryon fractions which differ from the GO models, and therefore systematic shifts in halo mass at fixed number density. These mass shifts result in a ∼30% deviation in number density at fixed mass from a halo mass function calibrated with only dark matter, significantly higher than the 5% statistical uncertainty halo mass function. |
