The (black hole)-bulge mass scaling relation at low masses

Abstract

Several recent papers have reported on the occurrence of active galactic nuclei (AGNs) containing undermassive black holes relative to a linear scaling relation between black hole mass (Mbh) and host spheroid stellar mass (Msph,∗). However, dramatic revisions to the Mbh-Msph,∗ and Mbh-Lsph relations, based on samples containing predominantly inactive galaxies, have recently identified a new steeper relation at Mbh ≲ (2-10) × 108 M⊙, roughly corresponding to Msph,∗ ≲ (0.3-1) × 1011 M⊙. We show that this steeper, quadratic-like Mbh -Msph,∗ relation defined by the Sérsic galaxies, i.e., galaxies without partially depleted cores, roughly tracks the apparent offset of the AGN having 105 ≲ Mbh/M⊙ ≲ 0.5 × 108. That is, these AGNs are not randomly offset with low black hole masses, but also follow a steeper (nonlinear) relation. As noted by Busch et al., confirmation or rejection of a possible AGN offset from the steeper Mbh-Msph,∗ relation defined by the Sérsic galaxies will benefit from improved stellar mass-to-light ratios for the spheroids hosting these AGNs. Several implications for formation theories are noted. Furthermore, reasons for possible under- and overmassive black holes, the potential existence of intermediate mass black holes (<105 M⊙), and the new steep (black hole)-(nuclear star cluster) relation, Mbh ∝ Mnc2.7±0.7, are also discussed.