An Empirical Algorithm for Broadband Photometric Redshifts of Quasars from the Sloan Digital Sky Survey

Abstract

We present an empirical algorithm for obtaining photometric redshifts of quasars using five-band Sloan Digital Sky Survey (SDSS) photometry. Our algorithm generates an empirical model of the quasar color-redshift relation, compares the colors of a quasar candidate with this model, and calculates possible photometric redshifts. Using the 3814 quasars of the SDSS Early Data Release (EDR) Quasar Catalog to generate a median color-redshift relation as a function of redshift we find that, for this same sample, 83% of our predicted redshifts are correct to within |Δz| < 0.3. The algorithm also determines the probability that the redshift is correct, allowing for even more robust photometric redshift determination for smaller, more restricted samples. We apply this technique to a set of 8740 quasar candidates selected by the final version of the SDSS quasar-selection algorithm. The photometric redshifts assigned to nonquasars are restricted to a few well-defined values. In addition, 90% of the objects with spectra that have photometric redshifts between 0.8 and 2.2 are quasars with accurate (|Δz| < 0.3) photometric redshifts. Many of these quasars lie in a single region of color space; judicious application of color-cuts can effectively select quasars with accurate photometric redshifts from the SDSS database—without reference to the SDSS quasar selection algorithm. When the SDSS is complete, this technique will allow the determination of photometric redshifts for ~106 faint SDSS quasar candidates, enabling advances in our knowledge of the quasar luminosity function, gravitational lensing of quasars, and correlations among quasars and between galaxies.