A Breakdown of the Black Hole - Bulge Mass Relation in Local Active Galaxies

TL;DR

This study examines whether low-luminosity active galactic nuclei (AGN) follow the established black hole mass to bulge mass relation, finding they are significantly offset, which impacts gravitational wave predictions and galaxy evolution models.

Contribution

It provides a detailed analysis of the black hole to bulge mass relation in AGNs, revealing a significant offset from the canonical relation for early-type galaxies.

Findings

AGN have smaller black hole masses than expected for their bulge mass

The offset persists even after bulge-disk decomposition and color-based mass estimates

Using standard relations for all galaxy types can lead to biased interpretations

Abstract

We investigate the relation between black hole (BH) mass and bulge stellar mass for a sample of 117 local ($z \sim 0$) galaxies hosting low-luminosity, broad-line active galactic nuclei (AGN). Our sample comes from Reines & Volonteri (2015), who found that, for a given $total$ stellar mass, these AGNs have BH masses more than an order of magnitude smaller than those in early-type galaxies with quiescent BHs. Here we aim to determine whether or not this AGN sample falls on the canonical BH-to-$bulge$ mass relation by utilizing bulge-disk decompositions and determining bulge stellar masses using color-dependent mass-to-light ratios. We find that our AGN sample remains offset by more than an order of magnitude from the $M_\mathrm{BH}-M_\mathrm{bulge}$ relation defined by early-type galaxies with dynamically detected BHs. We caution that using canonical BH-to-bulge mass relations for galaxies other than ellipticals and bulge-dominated systems may lead to highly biased interpretations. This work bears directly to predictions for gravitational wave detections and cosmological simulations that are tied to the local BH-to-bulge mass relations.