EFFICIENT PHOTOMETRIC SELECTION OF QUASARS FROM THE SLOAN DIGITAL SKY SURVEY. II. ∼1,000,000 QUASARS FROM DATA RELEASE 6

We present a catalog of 1,172,157 quasar candidates selected from the photometric imaging data of the Sloan Digital Sky Survey (SDSS). The objects are all point sources to a limiting magnitude of i = 21.3 from 8417 deg(2) of imaging from SDSS Data Release 6 (DR6). This sample extends our previous catalog by using the latest SDSS public release data and probing both ultraviolet (UV)-excess and high-redshift quasars. While the addition of high-redshift candidates reduces the overall efficiency (quasars: quasar candidates) of the catalog to similar to 80%, it is expected to contain no fewer than 850,000 bona fide quasars, which is similar to 8 times the number of our previous sample and similar to 10 times the size of the largest spectroscopic quasar catalog. Cross-matching between our photometric catalog and spectroscopic quasar catalogs from both the SDSS and 2dF survey yields 88,879 spectroscopically confirmed quasars. For judicious selection of the most robust UV-excess sources (similar to 500,000 objects in all), the efficiency is nearly 97%-more than sufficient for detailed statistical analyses. The catalog's completeness to type 1 (broad-line) quasars is expected to be no worse than 70%, with most missing objects occurring at z < 0.7 and 2.5 < z < 3.0. In addition to classification information, we provide photometric redshift estimates (typically good to Delta z +/- 0.3 [2 sigma]) and cross-matching with radio, X-ray, and proper-motion catalogs. Finally, we consider the catalog's utility for determining the optical luminosity function of quasars and are able to confirm the flattening of the bright-end slope of the quasar luminosity function at z similar to 4 as compared to z similar to 2.