MASS FUNCTIONS OF THE ACTIVE BLACK HOLES IN DISTANT QUASARS FROM THE LARGE BRIGHT QUASAR SURVEY, THE BRIGHT QUASAR SURVEY, AND THE COLOR-SELECTED SAMPLE OF THE SDSS FALL EQUATORIAL STRIPE

We present mass functions of distant actively accreting supermassive black holes residing in luminous quasars discovered in the Large Bright Quasar Survey (LBQS), the Bright Quasar Survey (BQS), and the Fall Equatorial Stripe of the Sloan Digital Sky Survey (SDSS). The quasars cover a wide range of redshifts from the local universe to z = 5 and were subject to different selection criteria and flux density limits. This makes these samples complementary and can help us gain additional insight on the true underlying black hole mass distribution free from selection effects and mass estimation errors through future studies. By comparing these quasar samples, we see evidence that the active black hole population at redshift four is somewhat different than that at lower redshifts, including that in the nearby universe. In particular, there is a sharp increase in the space density of the detected active black holes (M BH ∼>108 M sun) between redshifts ∼4 and ∼2.5. Also, the mass function of the SDSS quasars at 3.6 ≤ z ≤ 5 has a somewhat flatter high-mass-end slope of β = -1.75 ± 0.56, compared to the mass functions based on quasars below z of 3 (BQS and LBQS quasars), which display typical slopes of β ∼ -3.3; the latter are consistent with the mass functions at similar redshifts based on the SDSS Data Release 3 quasar catalog presented by Vestergaard et al. We see clear evidence of cosmic downsizing in the comoving space density distribution of active black holes in the LBQS sample alone. In forthcoming papers, further analysis, comparison, and discussion of these mass functions will be made with other existing black hole mass functions, notably that based on the SDSS DR3 quasar catalog. We present the relationships used to estimate the black hole mass based on the Mg II emission line; the relations are calibrated to the Hβ and C IV relations by means of several thousand high-quality SDSS spectra. Mass estimates of the individual black holes of these samples are also presented.