MaxBCG: Systematic discovery, characterization and calibration of galaxy clusters from large optical surveys.

Abstract

With the discovery of the accelerating expansion of the Universe, cosmology and fundamental physics are in the midst of a revolution. The acceleration has far-reaching observational consequences, empowering a host of diverse techniques that may provide clues to the underlying physics of the acceleration. One approach comes from observations of galaxy clusters. Many studies cite the cluster number density and distribution as sensitive cosmological probes. Despite the insistence on the importance of clusters, there is a deficiency of cluster detection techniques that exploit the full power of exquisite CCD-imaging and next-generation simulations required to build and characterize cosmologically relevant samples. In this thesis, we begin to realize the promise of cluster cosmology by developing an algorithm known as MaxBCG, and applying it to the Sloan Digital Sky Survey (SDSS). MaxBCG is built upon observational cornerstones of galaxy clusters: they contain statistical overdensities of bright red galaxies surrounding a brightest cluster galaxy (BCG). Using mock galaxy catalogs, MaxBCG is characterized at unprecedented levels, and it is demonstrated that it operates at high purity and completeness. The algorithm is then used to discover 13,823 clusters in the SDSS, with 0.1 < <italic>z</italic> < 0.3, at least 10 bright galaxies, and masses &gsim; 1 x 10<super>14</super> M&odot; . The catalog is approximately volume-limited, and the masses and redshifts are well-calibrated. Photometric redshift errors are shown to be small, sigma<italic>_z</italic> < 0.015, and the richness in an effective mass proxy, scaling well with both dynamical and weak lensing mass estimators. As cluster masses are of paramount importance in cosmology, we also present a new background-corrected richness measurement, made possible by the immense area of the SDSS, and show that it contains more mass information than previous mass proxies. With the completion of this thesis, we lay the groundwork for cosmology with optical clusters. The wealth of information contained in optical surveys is a resource we have only just tapped, and as simulations become more realistic, their combined power may allow cluster studies to supply reliable constraints on cosmology.