Constraining Cosmological Models with the Brightest Galaxy Clusters in the X-Ray Sky

Abstract.

A new X-ray selected and X-ray flux-limited galaxy cluster sample has been constructed. Based on the ROSAT All-Sky Survey the 63 brightest clusters with galactic latitude $\mid b_{\sc II}\mid \ge 20$deg and flux fx(0.1 - 2.4keV) $\ge 2 \times 10^{-11}$ ergs s^-1 cm^-2 have been compiled. Gravitational masses have been deterrnined utilizing intracluster gas density profiles, derived mainly from ROSAT PSPC pointed observations, and gas temperatures, as published mainly from ASCA observations, assuming hydrostatic equilibrium. Using this sample the galaxy cluster mass function has been determined and used to constrain the mean cosmic matter density and the amplitude of mass fluctuations. Comparison to Press-Schechter type model mass functions in the framework of Cold Dark Matter cosmological models yields the constraints $\Omega_{\rm m} = 0.12^{+0.06}_{-0.04}$ and $\sigma_8 = 0.96^{+0.15}_{-0.12}$ (90% c.l.). Various sources of possible systematic uncertainties have been tested. Adding all identified systematic uncertainties to the statistical uncertainty in a worst case fashion results in an upper limit $\Omega_{\rm m} < 0.31$. For comparison to previous results a relation $\sigma_8 = 0.43 \Omega^{-0.38}_{\rm m}$ has been derived. The results obtained here are compared to results from measurements of distant supernovae and cosmic microwave background (CMB) fluctuations.