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

We present an empirical algorithm for obtaining photometric redshifts ofquasars using 5-band Sloan Digital Sky Survey (SDSS) photometry. Our algorithmgenerates an empirical model of the quasar color-redshift relation, comparesthe colors of a quasar candidate with this model, and calculates possiblephotometric redshifts. Using the 3814 quasars of the SDSS Early Data ReleaseQuasar Catalog to generate a median color-redshift relation as a function ofredshift we find that, for this same sample, 83% of our predicted redshifts arecorrect to within |Delta z|<0.3. The algorithm also determines the probabilitythat the redshift is correct, allowing for even more robust photometricredshift determination for smaller, more restricted samples. We apply thistechnique to a set of 8740 quasar candidates selected by the final version ofthe SDSS quasar-selection algorithm. The photometric redshifts assigned tonon-quasars are restricted to a few well-defined values. In addition, 90% ofthe objects with spectra that have photometric redshifts between 0.8 and 2.2are quasars with accurate (|Delta z|<0.3) photometric redshifts. Many of thesequasars lie in a single region of color space; judicious application ofcolor-cuts can effectively select quasars with accurate photometric redshiftsfrom the SDSS database. When the SDSS is complete, this technique will allowthe determination of photometric redshifts for ~10^6 faint SDSS quasarcandidates, enabling advances in our knowledge of the quasar luminosityfunction, gravitational lensing of quasars, and correlations among quasars andbetween galaxies.