The Statistical Approach to Quantifying Galaxy Evolution

Abstract

Studies of the distribution and evolution of galaxies are of fundamental importance to modern cosmology; these studies, however, are hampered by the complexity of the competing effects of spectral and density evolution. Constructing a spectroscopic sample that is able to unambiguously disentangle these processes is currently excessively prohibitive due to the observational requirements. This paper extends and applies an alternative approach that relies on statistical estimates for both distance (z) and spectral type to a deep multiband data set that was obtained for this exact purpose. These statistical estimates are extracted directly from the photometric data by capitalizing on the inherent relationships between flux, redshift, and spectral type. These relationships are encapsulated in the empirical photometric-redshift relation that we extend to z≈1.2, with an intrinsic dispersion of δz~0.06. We also develop realistic estimates for the photometric-redshift error for individual objects and introduce the use of the galaxy ensemble as a tool for quantifying both a cosmological parameter and its measured error. We present deep, multiband, optical number counts as a demonstration of the integrity of our sample. Using the photometric redshift and the corresponding redshift error, we can divide our data into different redshift intervals and spectral types. As an example application, we present the number-redshift distribution as a function of spectral type.