An Empirical Algorithm for Broad-band Photometric Redshifts of Quasars from the Sloan Digital Sky Survey

We present an empirical algorithm for obtaining photometric redshifts of quasars using 5-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 |∆z| < 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 non-quasars 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 (|∆z| < 0.3) photometric redshifts. Many of these quasars lie in a single region of color space; Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802. Department of Physics, Astronomy, and Geophysics, Connecticut College, Box 5622, 270 Mohegan Avenue, New London, CT, 06320. Princeton University Observatory, Peyton Hall, Princeton, NJ 08544. Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686. Department of Astronomy and Astrophysics, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637. Enrico Fermi Institute, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637. Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349.