Dependence of Virial Factors on Optical Spectral Properties of Active Galactic Nuclei

TL;DR

This study calibrates virial factors for SMBH mass estimates in AGNs, revealing their dependence on optical spectral properties and proposing improved single-epoch mass estimators for large samples.

Contribution

It introduces new calibrations of virial factors based on bulge types and spectral properties, enhancing SMBH mass measurement accuracy in AGNs.

Findings

Virial factors show significant anti-correlations with emission-line widths.

Inclination angle influences the virial factor correlations.

New relations enable more precise single-epoch SMBH mass estimates.

Abstract

Reverberation mapping (RM) has long been a powerful tool for measuring the masses of supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGNs), but the precision of these mass measurements depends on the so-called virial factors. It has been demonstrated that the virial factors exhibit significant diversity, spanning approximately 1-2 orders of magnitude across different AGNs. However, the underlying physical drivers for the diversity have not yet been finalized. Here, adopting the SMBH mass -- spheroid luminosity relations of inactive galaxies with different bulge classifications, we calibrate the virial factors corresponding to the AGNs with pseudobulges (PB) and classical bulges (or elliptical hosts, CB) using the latest nearby RM sample. We investigate the correlations between virial factors and the AGN spectral properties, and find that for both PB and CB samples, the FWHM-based virial factors exhibit significant anti-correlations with the emission-line widths and profiles, while the $\sigma_{\rm line}$-based virial factors only show moderate anti-correlations with line widths for PB. We attribute these correlations mainly to the inclination angle or opening angle of the broad-line regions. Moreover, we establish new relations to give more precise virial factors and, in combination with the latest iron-corrected radius-luminosity relation, develop tentatively new single-epoch estimators of SMBH masses, which enable more accurate measurements of SMBH masses in large AGN samples.