Galaxy Clusters as Cosmological Probes

Abstract

Galaxy clusters form the largest gravitationally bound structures in the Universe. Galaxy clusters form at rare peaks in the initial density fluctuation field in the Universe. Therefore their abundance is very sensitive to the cosmological parameters which describe our Universe. The goals of this thesis are to understand how to characterize the abundance of galaxy clusters from observations and constrain cosmological parameters from these observations.

The primary cosmological parameters of our interest are $\Omega_{\rm m}$, the matter-density parameter and $\sigma_8$, which is related to the amplitude of the linear power spectrum of density fluctuations in the early Universe. The abundance of massive dark matter halos in which galaxy clusters form is very sensitive to these cosmological parameters. We develop an analytical and numerical framework to infer the cosmological parameters of interest from the X-ray observations of galaxy clusters after marginalizing over the halo mass-observable scaling relations and their scatter.

Our modeling scheme uses inputs from N-body simulations and follow-up observations of galaxy clusters to fit the abundances of galaxy clusters selected using the ROSAT survey data. We use a Bayesian framework to infer the posterior distribution of cosmological parameters of interest given the observations using Monte Carlo Markov Chain (MCMC) techniques. Although our framework is general, in this thesis, we present results using certain simplistic priors on the halo-mass observable relations of galaxy clusters. We obtain $\Omega_{\rm m} = 0.280^{+0.052}_{-0.046}$ and $\sigma_8 = 0.721^{+0.030}_{-0.033}$. We also observe the well-known degeneracy between $\Omega_{\rm m}$ and $\sigma_8$ in the posterior distributions. In the future, we will improve on this estimate of the cosmological parameters by marginalizing over the scatter of the mass-observable relation, appropriately. Future work will involve using weak lensing data to better calibrate the masses used in the mass-observable relation. Upcoming X-ray surveys like those conducted with the X-ray telescope eROSITA, combined with follow-up studies of these galaxy clusters using the Large Synoptic Survey Telescope (LSST) to constrain the mass-observable relationship can place much stronger constraints on the cosmological parameters.