Correcting C IV-based virial black hole masses. (2017)
Keywords :
galaxies evolution
Abstract:The CIV{lambda}{lambda}1498,1501 broad emission line is visible in optical spectra to redshifts exceeding z~5. CIV has long been known to exhibit significant displacements to the blue and these 'blueshifts' almost certainly signal the presence of strong outflows. As a consequence, single-epoch virial black hole (BH) mass estimates derived from CIV velocity widths are known to be systematically biased compared to masses from the hydrogen Balmer lines. Using a large sample of 230 high-luminosity (L_Bol_=10^45.5^-10^48^erg/s), redshift 1.5<z<4.0 quasars with both CIV and Balmer line spectra, we have quantified the bias in CIV BH masses as a function of the CIV blueshift. CIV BH masses are shown to be a factor of 5 larger than the
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The aim of this work is to measure empirically the systematic bias in
CIV-based virial BH-mass estimates for high-z quasars as a function of
the CIV emission line blueshift. The basis for the CIV blueshift-based
correction is a large sample of quasars, where it is possible to make
a direct comparison of the CIV line width with the line width of the
low-ionization Balmer lines H{alpha} and H{beta}, which are believed
to provide reliable proxies for the virial velocity. Such an approach
has not been possible hitherto as spectra that cover both the
observed-frame optical (where the redshifted CIV appears) and
near-infrared (where H{beta} and H{alpha} lie) are required.
We have compiled a sample of 307 quasars at redshifts 1.5<z<4 with
both optical and near-infrared spectra to enable such a comparison to
be performed. Reliable emission line properties were measured for 230
quasars, with 164 possessing H{alpha} line measurements
and 144 H{beta} line measurements.
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