The SDSS Quasar Survey: Quasar Luminosity Function from Data Release Three

Abstract

We determine the number counts and z = 0–5 luminosity function for a well-defined, homogeneous sample of quasars from the Sloan Digital Sky Survey (SDSS). We conser-vatively define the most uniform statistical sample possible, consisting of 15,343 quasars within an effective area of 1622 deg 2 that was derived from a parent sample of 46,420 spectroscopically confirmed broad-line quasars in the 5282 deg 2 of imaging data from SDSS Data Release Three. The sample extends from i = 15 to i = 19.1 at z 3 and to i = 20.2 for z 3. The number counts and luminosity function agree well with the re-sults of the Two-Degree Field QSO Redshift Survey (2QZ) at redshifts and luminosities where the SDSS and 2QZ quasar samples overlap, but the SDSS data probe to much higher redshifts than does the 2QZ sample. The number density of luminous quasars peaks between redshifts 2 and 3, although uncertainties in the selection function in this range do not allow us to determine the peak redshift more precisely. Our best fit model has a flatter bright end slope at high redshift than at low redshift. For z < 2.4 the data are best fit by a redshift-independent slope of β = −3.1 (Φ(L) ∝ L β). Above z = 2.4 the slope flattens with redshift to β −2.37 at z = 5. This slope change, which is significant at the 5-sigma level, must be accounted for in models of the evolution of accretion onto supermassive black holes.