C iv emission-line properties and systematic trends in quasar black hole mass estimates

Abstract

Black hole masses are crucial to understanding the physics of the connection between quasars and their host galaxies and measuring cosmic black hole-growth. At high redshift, z ≳ 2.1, black hole masses are normally derived using the velocity width of the C iv$\tt \lambda \lambda$1548, 1550 broad emission line, based on the assumption that the observed velocity widths arise from virial-induced motions. In many quasars, the C iv emission line exhibits significant blue asymmetries (‘blueshifts’) with the line centroid displaced by up to thousands of km s−1 to the blue. These blueshifts almost certainly signal the presence of strong outflows, most likely originating in a disc wind. We have obtained near-infrared spectra, including the Hα $\tt \lambda$6565 emission line, for 19 luminous (LBol = 46.5–47.5 erg s−1) Sloan Digital Sky Survey quasars, at redshifts 2 < z < 2.7, with C iv emission lines spanning the full range of blueshifts present in the population. A strong correlation between C iv velocity width and blueshift is found and, at large blueshifts, >2000 km s−1, the velocity widths appear to be dominated by non-virial motions. Black hole masses, based on the full width at half-maximum of the C iv emission line, can be overestimated by a factor of 5 at large blueshifts. A larger sample of quasar spectra with both C iv and H β, or Hα, emission lines will allow quantitative corrections to C iv-based black hole masses as a function of blueshift to be derived. We find that quasars with large C iv blueshifts possess high Eddington luminosity ratios and that the fraction of high-blueshift quasars in a flux-limited sample is enhanced by a factor of approximately 4 relative to a sample limited by black hole mass.