The Importance of Broad Emission-Line Widths in Single Epoch Black Hole Mass Estimates

TL;DR

This paper demonstrates that broad emission-line widths are crucial for accurate single-epoch black hole mass estimates in local AGNs, challenging previous claims that line widths can be replaced by a constant.

Contribution

It shows that line widths contain significant information for black hole mass estimates in nearby AGNs, contrasting with earlier studies on distant quasars.

Findings

Line widths significantly improve M_BH estimates.

Replacing line widths with a constant reduces accuracy.

Biases in flux-limited samples affect previous conclusions.

Abstract

Estimates of the mass of super-massive black holes (BHs) in distant active galactic nuclei (AGNs) can be obtained efficiently only through single-epoch spectra, using a combination of their broad emission-line widths and continuum luminosities. Yet the reliability and accuracy of the method, and the resulting mass estimates, M_BH, remain uncertain. A recent study by Croom using a sample of SDSS, 2QZ and 2SLAQ quasars suggests that line widths contribute little information about the BH mass in these single-epoch estimates and can be replaced by a constant value without significant loss of accuracy. In this Letter, we use a sample of nearby reverberation-mapped AGNs to show that this conclusion is not universally applicable. We use the bulge luminosity (L_Bulge) of these local objects to test how well the known M_BH - L_Bulge correlation is recovered when using randomly assigned line widths instead of the measured ones to estimate M_BH. We find that line widths provide significant information about M_BH, and that for this sample, the line width information is just as significant as that provided by the continuum luminosities. We discuss the effects of observational biases upon the analysis of Croom and suggest that the results can probably be explained as a bias of flux-limited, shallow quasar samples.