The Mass Function of an X-Ray Flux-Limited Sample of Galaxy Clusters

TL;DR

This paper constructs the HIFLUGCS X-ray flux-limited sample from the ROSAT All-Sky Survey, deriving robust cluster masses (M200, M500) from hydrostatic, isothermal gas in β-model halos and establishing a tight Lx–Mtot relation with low intrinsic scatter. The resulting mass function, computed via Vmax and corrected for Lx–Mtot scatter, is compared to Press–Schechter predictions for CDM cosmologies, yielding tight constraints: Ωm = 0.12^{+0.06}_{-0.04} and σ8 = 0.96^{+0.15}_{-0.12} (90% c.l.), with a practical upper limit Ωm < 0.31 when accounting for systematics. The analysis finds ΩCluster ≈ 0.012, indicating that the bulk of matter in the universe lies outside virialized clusters. The work demonstrates consistency with CDM predictions and provides a direct route to translate X-ray observables into cosmological parameters, while also showing how deviations in the transfer function or mass modeling affect the results within the quoted uncertainties.

Abstract

A new X-ray selected and X-ray flux-limited galaxy cluster sample is presented. Based on the ROSAT All-Sky Survey the 63 brightest clusters with galactic latitude |bII| >= 20 deg and flux fx(0.1-2.4 keV) >= 2 * 10^{-11} ergs/s/cm^2 have been compiled. Gravitational masses have been determined utilizing intracluster gas density profiles, derived mainly from ROSAT PSPC pointed observations, and gas temperatures, as published mainly from ASCA observations, assuming hydrostatic equilibrium. This sample and an extended sample of 106 galaxy clusters is used to establish the X-ray luminosity--gravitational mass relation. From the complete sample the galaxy cluster mass function is 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 and a Harrison--Zeldovich initial density fluctuation spectrum yields the constraints OmegaM = 0.12^{+0.06}_{-0.04} and sigma8 = 0.96^{+0.15}_{-0.12} (90% c.l.). Various possible systematic uncertainties are quantified. Adding all identified systematic uncertainties to the statistical uncertainty in a worst case fashion results in an upper limit OmegaM < 0.31. For comparison to previous results a relation sigma8 = 0.43 OmegaM^{-0.38} is derived. The mass function is integrated to show that the contribution of mass bound within virialized cluster regions to the total matter density is small, i.e., OmegaCluster = 0.012^{+0.003}_{-0.004} for cluster masses larger than 6.4^{+0.7}_{-0.6} * 10^{13} h_{50}^{-1} Msun.

Figures (18)

• Figure 1: Aitoff projection of the 63 HIFLUGCS galaxy clusters in galactic coordinates (filled circles). Additionally shown are 11 clusters above the flux limit but with $\vert b_{\rm II}\vert < 20.0\,\rm deg$ (open triangles).
• Figure 2: Cumulative source count rate as a function of radius (solid line) for the cluster A2029 (pointed observation). The vertical dashed line indicates the outer significance radius, $r_{\rm X}$. The dashed lines just above and below the source count rate indicate the 1-$\sigma$ Poissonian error bars.
• Figure 3: Cumulative source count rate as a function of radius for the cluster EXO0422, shown as an extreme example ( RASS data). The parabolic dashed line indicates the best fit parabola for count rates larger than $r_{\rm X}$.
• Figure 4: Corrected cumulative source count rate as a function of radius for the cluster EXO0422. The count rate correction is less than 5 %.
• Figure 5: Comparison of measured and corrected source count rates for the extended sample of 106 galaxy clusters. The solid line indicates equality.