Orientation dependency of broad-line widths in quasars and consequences for black hole mass estimation

Abstract

In this paper, we report new evidence that measurements of the broad-line widths in quasars are dependent on the source orientation, consistent with the idea that the broad-line region is flattened or disc like. This reinforces the view derived from radio-selected samples, where the radio-core dominance has been used as a measure of orientation. The results presented here show a highly significant (>99.95 per cent) correlation between radio spectral index (which we use as a proxy for source orientation) and broad-line width derived from the Hβ and Mg ii emission lines. This is the first time that this type of study has used quasars derived from a large optically selected quasar sample, where the radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) have indistinguishable distributions in redshift, bolometric luminosity and colour, and therefore overcomes any biases which may be present in only selecting via radio emission. We find that the mean full width at half-maximum (FWHM) for the flat-spectrum (αrad≲ 0.5) radio-loud quasars (FSQs) to be ⁠, which differs significantly from the mean FWHM of the steep-spectrum (αrad > 0.5) radio-loud quasars (SSQs), where ⁠. We also find that the distribution in FWHM for the FSQs is indistinguishable from that of the RQQs, where ⁠. Considering other observational results in the literature, we interpret this result in the context of a significant fraction of the FSQs being derived from the underlying RQQ population which have their radio flux Doppler boosted above the RLQ/RQQ divide. Under the assumption of a disc-like broad-line region, we find no evidence for a difference in the average line-of-sight angle for RQQs and RLQs, implying that the difference is due to black hole mass. However, we caution against the virial method to estimate black hole masses in small or ill-defined quasar samples due to significant orientation dependencies. Disentangling the relative importance of black hole mass and orientation would require higher resolution radio observations. However, orientation effects could be minimized by obtaining low-frequency (<1 GHz) radio observations of the optically selected Sloan Digital Sky Survey (SDSS) quasars.