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

Abstract

In large optical survey at high redshifts ($z>2$), the CIV broad emission line is the most practical alternative to estimate the mass ($M_{\text{BH}}$) of active super-massive black holes (SMBHs). 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 $M_{\text{BH}}$ estimations. These methods are constructed from correlations between the ratio of the CIV line-width to the low ionization line-widths (H$\alpha$, H$\beta$ and MgII) and several other properties of rest-frame UV 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 CIV line with 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 IR spectroscopy of low ionization lines like H$\alpha$, H$\beta$ and MgII