An Empirical Calibration of the Completeness of the SDSS Quasar Survey

Spectra of nearly 20000 point-like objects to a Galactic reddening correctedmagnitude of i=19.1 have been obtained to test the completeness of the SDSSquasar survey. The spatially-unresolved objects were selected from all regionsof color space, sparsely sampled from within a 278 sq. deg. area of sky coveredby this study. Only ten quasars were identified that were not targeted ascandidates by the SDSS quasar survey (including both color and radio sourceselection). The inferred density of unresolved quasars on the sky that aremissed by the SDSS algorithm is 0.44 per sq. deg, compared to 8.28 per sq. deg.for the selected quasar density, giving a completeness of 94.9(+2.6,-3.8) tothe limiting magnitude. Omitting radio selection reduces the color-onlyselection completeness by about 1%. Of the ten newly identified quasars, threehave detected broad absorption line systems, six are significantly redder thanother 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 stellarlocus). The fraction of quasars missed due to image defects and blends isapproximately 4%, but this number varies by a few percent with magnitude.Quasars with extended images comprise about 6% of the SDSS sample, and thecompleteness of the selection algorithm for extended quasars is approximately81%, based on the SDSS galaxy survey. The combined end-to-end completeness forthe SDSS quasar survey is approximately 89%. The total corrected density ofquasars on the sky to i=19.1 is estimated to be 10.2 per sq. deg.Comment: 37 pages, 10 figures, accepted for publication in A