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

Published version
Peer-reviewed

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Article

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Authors

Coatman, L 
Hewett, PC 
Richards, GT 

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 CIV broad emission line, based on the assumption that the observed velocity-widths arise from virial-induced motions. In many quasars, the CIV-emission line exhibits significant blue asymmetries (`blueshifts') with the line centroid displaced by up to thousands of km/s 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 emission line, for 19 luminous (LBol = 46.5-47.5 erg/s) Sloan Digital Sky Survey quasars, at redshifts 2 < z < 2.7, with CIV emission lines spanning the full-range of blueshifts present in the population. A strong correlation between CIV-velocity width and blueshift is found and, at large blueshifts, > 2000 km/s, the velocity-widths appear to be dominated by non-virial motions. Black-hole masses, based on the full width at half maximum of the CIV-emission line, can be overestimated by a factor of five at large blueshifts. A larger sample of quasar spectra with both CIV and Hβ, or Hα, emission lines will allow quantitative corrections to CIV-based black-hole masses as a function of blueshift to be derived. We find that quasars with large CIV 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 four relative to a sample limited by black hole mass.

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Keywords

galaxies: evolution

Journal Title

Monthly Notices of the Royal Astronomical Society

Conference Name

Journal ISSN

0035-8711
1365-2966

Volume Title

461

Publisher

Oxford University Press (OUP)
Sponsorship
Science and Technology Facilities Council (ST/M005305/1)
Science and Technology Facilities Council (ST/L001381/1)
Science and Technology Facilities Council (ST/N000927/1)
Science and Technology Facilities Council