On the geometry of broad emission region in quasars

TL;DR

This study investigates the geometry of the broad emission line region in quasars, suggesting a disc-like structure based on the analysis of Hbeta and CIV emission lines and their relation to black hole mass estimates.

Contribution

It provides evidence supporting a disc-like geometry for the broad line region in quasars, based on a comparison of virial and host galaxy luminosity methods.

Findings

The geometrical factor f varies widely and depends on line width.

A disc-like model better explains the emission line properties.

Hbeta and CIV lines originate in different geometrical regions.

Abstract

We study the geometry of the Hbeta broad emission region by comparing the M_BH values derived from Hbeta through the virial relation with those obtained from the host galaxy luminosity in a sample of 36 low redshift (z around 0.3) quasars. This comparison lets us infer the geometrical factor f needed to de-project the line-of-sight velocity component of the emitting gas. The wide range of f values we found, together with the strong dependence of f on the observed line width, suggests that a disc-like model for the broad line region is preferable to an isotropic model, both for radio loud and radio quiet quasars. We examined similar observations of the CIV line and found no correlation in the width of the two lines. Our results indicate that an inflated disc broad line region, in which the Carbon line is emitted in a flat disc while Hbeta is produced in a geometrically thick region, can account for the observed differences in the width and shape of the two emission lines.