2016MNRAS.461..647C

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 Civ λλ1548, 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 λ6565 emission line, for 19 luminous (L_Bol=46.5-47.5 ergs^-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.