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

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.