DO QUASAR BROAD-LINE VELOCITY WIDTHS ADD ANY INFORMATION TO VIRIAL BLACK HOLE MASS ESTIMATES?

We examine how much information measured broad-line widths add to virial black hole (BH) mass estimates for flux-limited samples of quasars. We do this by comparing the BH mass estimates to those derived by randomly reassigning the quasar broad-line widths to different objects and re-calculating the BH mass. For 9000 BH masses derived from the H beta line we find that the distributions of original and randomized BH masses in the M-BH-redshift plane and the M-BH-luminosity plane are formally identical. A two-dimensional Kolmogorov-Smirnov test does not find a difference at >90% confidence. For the MgII line (32,000 quasars) we do find very significant differences between the randomized and original BH masses, but the amplitude of the difference is still small. The difference for the CIV line (14,000 quasars) is 2 sigma-3 sigma and again the amplitude of the difference is small. Subdividing the data into redshift and luminosity bins we find that the median absolute difference in BH mass between the original and randomized data is 0.025, 0.01, and 0.04 dex for H beta, MgII, and CIV, respectively. The maximum absolute difference is always <= 0.1 dex. We investigate whether our results are sensitive to corrections to MgII virial masses, such as those suggested by Onken & Kollmeier. These corrections do not influence our results, other than to reduce the significance of the difference between original and randomized BH masses to only 1 sigma-2 sigma for MgII. Moreover, we demonstrate that the correlation between mass residuals and Eddington ratio discussed by Onken & Kollmeier is more directly attributable to the slope of the relation between H beta and MgII line width. The implication is that the measured quasar broad-line velocity widths provide little extra information, after allowing for the mean velocity width. In this case virial estimates are equivalent to M-BH alpha L-alpha, with L/L-Edd alpha L1-alpha (with alpha similar or equal to 0.5). This leaves an unanswered question of why the accretion efficiency changes with luminosity in just the right way to keep the mean broad-line widths fixed as a function of luminosity.