Galaxy cluster number count data constraints on cosmological parameters
We use data on massive galaxy clusters (Mcluster>8×1014h-1M⊙ within a comoving radius of Rcluster=1. 5h-1 Mpc) in the redshift range 0. 05≲z≲0. 83 to place constraints, simultaneously, on the nonrelativistic matter density parameter Ωm, on the amplitude of mass fluctuations σ8, on the index n of the power-law spectrum of the density perturbations, and on the Hubble constant H0, as well as on the equation-of-state parameters (w0,wa) of a smooth dark energy component. For the first time, we properly take into account the dependence on redshift and cosmology of the quantities related to cluster physics: the critical density contrast, the growth factor, the mass conversion factor, the virial overdensity, the virial radius and, most importantly, the cluster number count derived from the observational temperature data. We show that, contrary to previous analyses, cluster data alone prefer low values of the amplitude of mass fluctuations, σ8≤0. 69 (1σ C. L.), and large amounts of nonrelativistic matter, Ωm≥0. 38 (1σ C. L.), in slight tension with the ΛCDM concordance cosmological model, though the results are compatible with ΛCDM at 2σ. In addition, we derive a σ8 normalization relation, σ8 Ωm 1/3 = 0. 49 ± 0. 06 (2σ C. L.). Combining cluster data with σ8-independent baryon acoustic oscillation observations, cosmic microwave background data, Hubble constant measurements, Hubble parameter determination from passively evolving red galaxies, and magnitude-redshift data of type Ia supernovae, we find Ωm = 0. 28+0. 03 -0.02 and σ8 = 0. 73+0. 03 -0. 03, the former in agreement and the latter being slightly lower than the corresponding values in the concordance cosmological model. We also find H0 = 69. 1+1. 3 -1. 5km/s/Mpc, the fit to the data being almost independent on n in the adopted range [0. 90,1. 05]. Concerning the dark energy equation-of-state parameters, we show that the present data on massive clusters weakly constrain (w0,wa) around the values corresponding to a cosmological constant, i. e. (w0,wa)=(-1,0). The global analysis gives w0 = -1. 14+0. 14 -0. 16 and wa = 0. 85+0. 42 -0. 60 (1σ C. L. errors). Very similar results are found in the case of time-evolving dark energy with a constant equation-of-state parameter w=const (the XCDM parametrization). Finally, we show that the impact of bounds on (w0,wa) is to favor top-down phantom models of evolving dark energy. © 2012 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica.