Towards Cosmological Concordance on Galactic Scales

Abstract

We use the observed abundance and clustering of galaxies from the 2dFGRS to determine the matter density Omega_m and the linear amplitude of mass fluctuations sigma_8. We use a method based on the conditional luminosity function, which allows straightforward computation of the luminosity dependent bias of galaxies. In addition, it allows the inclusion of constraints on the redshift space distortion parameter beta=Omega_m^{0.6}/b, and yields average mass-to-light ratios as function of halo mass. Using only the luminosity function and the correlation lengths as function of luminosity we obtain constraints on Omega_m and sigma_8 that are in good agreement with COBE. Using additional constraints on the mass-to-light ratios of clusters and on the value of beta as determined from the 2dFGRS yields Omega_m = (0.27 +/- 0.06) and sigma_8 = (0.77 +/- 0.06). Adding further constraints from current CMB data, we obtain Omega_m = (0.25 +/- 0.04) and sigma_8 = (0.78 +/- 0.06). Thus, we find strong indications that both Omega_m and sigma_8 are significantly lower than their ``standard'' concordance values of 0.3 and 0.9, respectively. Finally, we show that the cosmologies favored here predict dark matter haloes that are significantly less centrally concentrated than for the standard LCDM concordance model. This may solve both the problem with the rotation curves of dwarf and LSB galaxies, as well as the problem of simultaneously matching the galaxy luminosity function and the Tully-Fisher zero-point. (Abridged)