Cosmological Constraints From SDSS MaxBCG Cluster Abundances

We perform a maximum likelihood analysis of the cluster abundance measured in the SDSS using the maxBCG cluster finding algorithm. Our analysis is aimed at constraining the power spectrum normalization σ8, and assumes flat cosmologies with a scale invariant spectrum, massless neutrinos, and CMB and supernova priors (Omega)mh2 = 0.128 ± 0.01 and h = 0.72 ± 0.05 respectively. Following the method described in the companion paper Rozo et al. (2007), we derive σ8 = 0.92 ± 0.10 (1σ) after marginalizing over all major systematic uncertainties. We place strong lower limits on the normalization, σ8 > 0.76 (95% CL) (> 0.68 at 99% CL). We also find that our analysis favors relatively low values for the slope of the Halo Occupation Distribution (HOD), α = 0.83 ± 0.06. The uncertainties of these determinations will substantially improve upon completion of an ongoing campaign to estimate dynamical, weak lensing, and X-ray cluster masses in the SDSS maxBCG cluster sample