An Empirical Calibration of the Completeness of the SDSS Quasar Survey

Abstract

Spectra of nearly 20,000 pointlike objects to a Galactic reddening-corrected magnitude of i = 19.1 have been obtained to test the completeness of the Sloan Digital Sky Survey (SDSS) quasar survey. We focus on spatially unresolved quasars, which comprise 94% of all SDSS quasars to the main survey magnitude limit. The objects were selected from all regions of color space, sparsely sampled from within a 278 deg 2 area (effective area 233 deg 2) of sky covered by this study. Only 10 quasars were identified that were not targeted as candidates by the SDSS quasar survey (including both color and radio source selection). The inferred density of unresolved quasars on the sky that are missed by the SDSS algorithm is 0.44 deg -2, compared with 8.28 deg -2 for the selected quasar density, giving a completeness of 94.9 -3.8+2.6% to the limiting magnitude. Omitting radio selection reduces the color-only selection completeness by about 1%. Of the 10 newly identified quasars, three have detected broad absorption line systems, six are significantly redder than other quasars at the same redshift, and four have redshifts between 2.7 and 3.0 (the redshift range where the SDSS colors of quasars intersect the stellar locus). The fraction of quasars (and other unresolved sources) missed because of image defects and blends, independent of the selection algorithm, is ?4%, but this number varies by a few percent with magnitude. Quasars with extended images comprise about 6% of the SDSS sample, and the completeness of the selection algorithm for extended quasars is approximately 81%, based on the SDSS galaxy survey. The combined end-to-end completeness for the SDSS quasar survey is ?89%. The total corrected density of quasars on the sky to i = 19.1 is estimated to be 10.2 deg -2. The SDSS completeness restricted to z ? 3 quasars is expected to be considerably lower and is a strong function of redshift. The determination of the global completeness is required for the statistical properties of quasars to i = 19.1 derived from the SDSS data set, such as the luminosity function and correlation function, to be accurately determined. ? 2005. The American Astronomical Society. All rights reserved.