Can we improve CIV-based single-epoch black hole mass estimations?

Abstract

In large optical surveys at high redshifts (z > 2), the C IV λ1549 broad emission line is the most practical alternative to estimate the mass (MBH) of active supermassive black holes. However, mass determinations obtained with this line are known to be highly uncertain. In this work, we use the Sloan Digital Sky Survey Data Release 7 and 12 quasar catalogues to statistically test three alternative methods put forward in the literature to improve CIV-based MBH estimations. These methods are constructed from correlations between the ratio of the C IVλ1549 linewidth to the low-ionization linewidths (H α, H β, and MgIIλ2798) and several other properties of rest-frame ultraviolet emission lines. Our analysis suggests that these correction methods are of limited applicability, mostly because all of them depend on correlations that are driven by the linewidth of the CIV profile itself and not by an interconnection between the linewidth of the C IV line and the linewidth of the low ionization lines. Our results show that optical CIV-based mass estimates at high redshift cannot be a proper replacement for estimates based on infrared spectroscopy of low ionization lines like H α, H β, and MgII.