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


Authors: Alister W. Graham & Nicholas Scott

Abstract:

Several recent papers have reported on the occurrence of active galactic nuclei (AGN) containing under-massive black holes relative to a linear scaling relation between black hole mass (Mbh) and host spheroid stellar mass (Msph,*). Dramatic revisions to the MbhMsph,* and MbhLsph relations, based on samples containing predominantly inactive galaxies, have however recently identified a new steeper relation at Mbh < (2–10) x 108 Msun, roughly corresponding to Msph,* < (0.3–1) x 1011 Msun. We show that this steeper, quadratic-like MbhMsph,* relation defined by the Sérsic galaxies, i.e. galaxies without partially depleted cores, roughly tracks the apparent offset of the AGN having 105 < Mbh/ Msun < 0.5x108. That is, these AGN are not randomly offset with low black hole masses, but instead also follow a steeper (non-linear) relation. As noted by Busch et al., confirmation or rejection of a possible AGN offset from the steeper MbhMsph,* relation defined by the Sérsic galaxies will benefit from improved stellar mass-to-light ratios for the spheroids hosting these AGN. Several implications for formation theories are noted. Furthermore, reasons for possible under- and over-massive black holes, the potential existence of intermediate mass black holes (<105 Msun), and the new steep (black hole)--(nuclear star cluster) relation, MbhMnc 2.7±0.7, are also discussed.