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G.T. Richards (Penn State), SDSS Collaboration
The Sloan Digital Sky Survey (SDSS), with photometry more accurate than the intrinsic width of the quasar color-redshift relation, allows for a detailed scientific analysis of the quasar color-redshift relation -- for the first time. This talk will discuss the distribution of quasar colors as a function of redshift in the SDSS photometric system. We find that the colors of quasars are extremely similar at a given redshift, but occupy largely non-degenerate regions of 4 dimensional color space at different redshifts. These properties enable the investigation of a number of issues related to quasars that have hitherto not been possible.
In particular, we show that it is possible to determine photometric redshifts for quasars to surprising accuracy and efficiency from z=0 to z=5. This fact alone yields a number of scientific applications including the study of quasar magnification bias, the quasar luminosity function, the spatial correlation of quasars and strong gravitational lensing. The tight color-redshift relation for most quasars has also enabled the discovery of a population of reddened quasars; coupled with photometric redshifts, these objects are identifiable from their colors alone.
Furthermore, we find that the structure of the color redshift relation is such that we are able to study the emission line properties of quasars in the ensemble average. Of particular interest is the possible evolution of the so-called small blue bump as a function of redshift. Finally, we address the question of the dividing line of quasars and Seyfert galaxy nuclei, which is traditionally taken to be MB = -23. Together with the emission line widths and morphologies of low-redshift AGN, the color-redshift relation can be used to study the transition from Seyfert galaxy nuclei to full-fledged quasars.