Appreciating mergers for understanding the non-linear Mbh–M*,spheroid and Mbh–M*, galaxy relations, updated herein, and the implications for the (reduced) role of AGN feedback

Abstract

ABSTRACT We present revised (black hole mass)–(spheroid stellar mass) and (black hole mass)–(galaxy stellar mass) scaling relations based on colour-dependent (stellar mass)-to-light ratios. Our 3.6 $\mu$m luminosities were obtained from multicomponent decompositions, which accounted for bulges, discs, bars, ansae, rings, nuclear components, etc. The lenticular galaxy bulges (not associated with recent mergers) follow a steep $M_{\rm {bh}}\propto ~M_{\rm {*,bulge}}^{1.53\pm 0.15}$ relation, offset by roughly an order of magnitude in black hole mass from the $M_{\rm {bh}}\propto ~M_{\rm {*,ellip}}^{1.64\pm 0.17}$ relation defined by the elliptical (E) galaxies which, in Darwinian terms, are shown to have evolved by punctuated equilibrium rather than gradualism. We use the spheroid (i.e. bulge and elliptical) size–mass relation to reveal how disc-galaxy mergers explain this offset and the dramatically lower Mbh/M*,sph ratios in the elliptical galaxies. The deceptive near-linear Mbh–M*,sph ‘red sequence’, followed by neither the bulge population nor the elliptical galaxies, is shown to be an artefact of sample selection, combining bulges and elliptical galaxies from disparate Mbh–M*,sph sequences. Moreover, both small bulges with ‘undermassive’ black holes and big lenticular galaxies (including relic ‘red nuggets’) with ‘overmassive’ black holes – relative to the near-linear Mbh–M*,sph sequence – are no longer viewed as outliers. We confirm a steep $M_{\rm {bh}}\propto ~M_{\rm {*,bulge}}^{2.25\pm 0.39}$ relation for spiral galaxies and discuss numerous implications of this work, including how mergers, rather than (only) feedback from active galactic nuclei, have shaped the high-mass end of the galaxy mass function. We also explain why there may be no useful Mbh–M*,sph–Re,sph plane due to M*,sph scaling nearly linearly with Re,sph.