The Origin of the Intrinsic Scatter in the Relation Between Black Hole Mass and Bulge Luminosity for Nearby Active Galaxies

TL;DR

This study explores the causes of intrinsic scatter in the black hole mass versus bulge luminosity relation in nearby active galaxies, highlighting the role of accretion rates and galaxy interactions in this variability.

Contribution

It identifies the zero point offset in the MBH-L(bulge) relation as linked to accretion rates and proposes that modest black hole growth and galaxy interactions influence this scatter.

Findings

Zero point offset correlates with AGN and host properties.

Higher Eddington ratios correspond to lower MBH at fixed bulge luminosity.

Galaxy mergers impact AGN fueling and black hole growth.

Abstract

We investigate the origin of the intrinsic scatter in the correlation between black hole mass (MBH) and bulge luminosity [L(bulge)] in a sample of 45 massive, local (z < 0.35) type~1 active galactic nuclei (AGNs). We derive MBH from published optical spectra assuming a spherical broad-line region, and L(bulge) from detailed two-dimensional decomposition of archival optical Hubble Space Telescope images. AGNs follow the MBH-L(bulge) relation of inactive galaxies, but the zero point is shifted by an average of \Delta log MBH ~ -0.3 dex. We show that the magnitude of the zero point offset, which is responsible for the intrinsic scatter in the MBH-L(bulge) relation, is correlated with several AGN and host galaxy properties, all of which are ultimately related to, or directly impact, the BH mass accretion rate. At a given bulge luminosity, sources with higher Eddington ratios have lower MBH. The zero point offset can be explained by a change in the normalization of the virial product used to estimate MBH, in conjunction with modest BH growth (~ 10%--40%) during the AGN phase. Galaxy mergers and tidal interactions appear to play an important role in regulating AGN fueling in low-redshift AGNs.