Evolution of the clustering of photometrically selected SDSS galaxies

We measure the angular auto-correlation functions, omega(theta), of Sloan Digital Sky Survey (SDSS) galaxies selected to have photometric redshifts 0.1 < z < 0.4 and absolute r-band magnitudes M(r) < -21.2. We split these galaxies into five overlapping redshift shells of width 0.1 and measure omega(theta) in each subsample in order to investigate the evolution of SDSS galaxies. We find that the bias increases substantially with redshift - much more so than one would expect for a passively evolving sample. We use halo-model analysis to determine the best-fitting halo-occupation-distribution (HOD) for each subsample, and the best-fitting models allow us to interpret the change in bias physically. In order to properly interpret our best-fitting HODs, we convert each halo mass to its z = 0 passively evolved bias (b(o)), enabling a direct comparison of the best-fitting HODs at different redshifts. We find that the minimum halo b(o) required to host a galaxy decreases as the redshift decreases, suggesting that galaxies with M(r) < -21.2 are forming in haloes at the low-mass end of the HODs over our redshift range. We use the best-fitting HODs to determine the change in occupation number divided by the change in mass of haloes with constant b(o), delta N/delta M (b(o)), and we find a sharp peak at b(o) similar to 0.9 - corresponding to an average halo mass of similar to 1012 h-1 M(circle dot). We thus present the following scenario: the bias of galaxies with M(r) < -21.2 decreases as the Universe evolves because these galaxies form in haloes of mass similar to 1012 h-1 M(circle dot) (independent of redshift), and the bias of these haloes naturally decreases as the Universe evolves.