The (Black Hole)-bulge Mass Scaling Relation at Low Masses

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 (M _bh) and host spheroid stellar mass (M _{sph, *}). However, dramatic revisions to the M _bh-M _{sph, *} and M _bh-L _sph relations, based on samples containing predominantly inactive galaxies, have recently identified a new steeper relation at M _bh <~ (2-10) × 10⁸ M _⊙, roughly corresponding to M _{sph, *} <~ (0.3-1) × 10¹¹ M _⊙. We show that this steeper, quadratic-like M _bh-M _{sph, *} relation defined by the Sérsic galaxies, i.e., galaxies without partially depleted cores, roughly tracks the apparent offset of the AGN having 10⁵ <~ M _bh/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 M _bh-M _{sph, *} 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, M_bh \propto M_nc^{2.7+/- 0.7}, are also discussed.