The Lick AGN Monitoring Project: Alternate Routes to a Broad-line Region Radius

TL;DR

This study recalibrates the relationship between broad-line region size and various luminosities in active galaxies, revealing new scaling relations that improve black hole mass estimates especially in obscured or high-redshift objects.

Contribution

The paper introduces alternative radius-luminosity relations based on X-ray and narrow-line luminosities, expanding methods for black hole mass estimation beyond optical continuum measurements.

Findings

Broad-line region radius scales with the square root of X-ray and Hbeta luminosities.

The radius correlates most tightly with Hbeta luminosity.

Relations with X-ray and narrow-line luminosities have comparable scatter of a factor of two.

Abstract

It is now possible to estimate black hole masses across cosmic time, using broad emission lines in active galaxies. This technique informs our views of how galaxies and their central black holes coevolve. Unfortunately, there are many outstanding uncertainties associated with these "virial" mass estimates. One of these comes from using the accretion luminosity to infer a size for the broad-line region. Incorporating the new sample of low-luminosity active galaxies from our recent monitoring campaign at Lick Observatory, we recalibrate the radius-luminosity relation with tracers of the accretion luminosity other than the optical continuum. We find that the radius of the broad-line region scales as the square root of the X-ray and Hbeta luminosities, in agreement with recent optical studies. On the other hand, the scaling appears to be marginally steeper with narrow-line luminosities. This is consistent with a previously observed decrease in the ratio of narrow-line to X-ray luminosity with increasing total luminosity. The radius of the broad-line region correlates most tightly with Hbeta luminosity, while the X-ray and narrow-line relations both have comparable scatter of a factor of two. These correlations provide useful alternative virial BH masses in objects with no detectable optical/UV continuum emission, such as high-redshift galaxies with broad emission lines, radio-loud objects, or local active galaxies with galaxy-dominated continua.