Alexander Gray

First Measurement of the Clustering Evolution of Photometrically Classified Quasars

We present new measurements of the quasar angular autocorrelation function from a sample of ~80,000 photometrically classified quasars taken from the First Data Release of the Sloan Digital Sky Survey. We find a best-fit model of ω(θ) = (0.066)θ-(0.98±0.15) for the angular correlation function, consistent with estimates of the slope from spectroscopic quasar surveys. We show that only models with little or no evolution in the clustering of quasars in comoving coordinates since a median redshift of z ~ 1.4 can recover a scale length consistent with local galaxies and active galactic nuclei (AGNs). A model with little evolution of quasar clustering in comoving coordinates is best explained in the current cosmological paradigm by rapid evolution in quasar bias. We show that quasar biasing must have changed from bQ ~ 3 at a (photometric) redshift of phot = 2.2 to bQ ~ 1.2-1.3 by phot = 0.75. Such a rapid increase with redshift in biasing implies that quasars at z ~ 2 cannot be the progenitors of modern L* objects; rather they must now reside in dense environments, such as clusters. Similarly, the duration of the UVX (ultraviolet-excess) quasar phase must be short enough to explain why local UVX quasars reside in essentially unbiased structures. Our estimates of bQ are in good agreement with recent spectroscopic results (Croom et al. 2005), which demonstrate that the implied evolution in bQ is consistent with quasars inhabiting halos of similar mass at every redshift. Treating quasar clustering as a bivariate function of both redshift and luminosity, we find no evidence for luminosity dependence in quasar clustering, and that redshift evolution thus affects quasar clustering more than changes in quasars luminosity. Our results are robust against a range of systematic uncertainties. We provide a new method for quantifying stellar contamination in photometrically classified quasar catalogs via the correlation function.