The extended ROSAT-ESO Flux-Limited X-ray Galaxy Cluster Survey (REFLEX II)

The mass function of galaxy clusters is a sensitive tracer of the gravitational evolution of the cosmic large-scale structure and serves as an important census of the fraction of matter bound in large structures. We obtain the mass function by fitting the observed cluster X-ray luminosity distribution from the REFLEX galaxy cluster survey to models of cosmological structure formation. We marginalise over uncertainties in the cosmological parameters as well as those of the relevant galaxy cluster scaling relations. The mass function is determined with an uncertainty of less than 10% in the mass range 3 × 1012 to 5 × 1014M⊙. For the cumulative mass function we find a slope at the low-mass end consistent with a value of − 1, while the mass-rich end cut-off is milder than a Schechter function with an exponential term exp( − Mδ) with δ smaller than 1. Changing the Hubble parameter in the range H0 = 67 − 73 km s-1 Mpc-1 or allowing the total neutrino mass to have a value in the range 0 − 0.4 eV causes variations less than the uncertainties. We estimate the fraction of mass locked up in galaxy clusters: about 4.4% of the matter in the Universe is bound in clusters (inside r200) with a mass larger than 1014M⊙ and 14% to clusters and groups with a mass larger than 1013M⊙ at the present Universe. We also discuss the evolution of the galaxy cluster population with redshift. Our results imply that there is hardly any clusters with a mass ≥1015M⊙ above a redshift of z = 1