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

TL;DR

This study evaluates three proposed correction methods for CIV-based black hole mass estimates at high redshift, finding they are limited and cannot replace IR spectroscopy of low ionization lines for accurate mass determination.

Contribution

The paper critically assesses existing correction techniques for CIV-based SMBH mass estimates, revealing their limitations and emphasizing the need for IR spectroscopy for accurate measurements.

Findings

Correction methods depend on CIV linewidth correlations driven by CIV itself.

CIV-based mass estimates are unreliable compared to IR spectroscopy.

High-redshift SMBH mass estimates require low-ionization line measurements.

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