Cluster surveys 339Figure 11.13. The best determination to date of the cluster x-ray luminosity function
(i.e. the cluster space density) out to z 1 .2. Data points at z < 0. 85
are derived from a complete RDCS sample of 103 clusters over 47 deg^2 , with
FXlim= 3 × 10 −^14 erg s−^1 cm−^2 (Rosatiet al1999). The triangles representalower
limit(due to incomplete optical identification). to the cluster space density obtained from
a fainter and more distant subsample. Long dash curves are Schechter best fits to the XLF
φ(LX,z), plotted atz= 0 .4andz= 0 .6.
mass function, i.e.N(LX,z)→N(T,z)→N(M,z)(e.g. Borganiet al1999).
Such a method can be used to set significant constraints onm(figure 11.14). The
fact that a large fraction of relatively massive clusters is already in place atz1,
indicates that the dynamical evolution of structure has proceeded at a relatively
slow pace sincez1, a scenario which fits naturally in a low density universe
(figure 11.14, see Borganiet al2001, Ekeet al1996).
11.4.3 Determiningmand
Besides the method of the evolution of cluster abundance (which we can call
‘universal dynamics’), galaxy clusters, as the largest collapsed objects in the
universe, also offer two other independent means to estimate the mean density
of matter that participates to gravitational clustering (i.e.m):