Nandini Sahu

Local early-type galaxies (ETGs) with directly measured black hole masses, Mbh, have been reported to represent a biased sample relative to the ETG population at large. Such galaxies with Spitzer Space Telescope imaging have been purported to possess velocity dispersions, σ, at least ∼0.1 dex larger for a given galaxy stellar mass, M*,gal, than is typically observed among thousands of ETGs imaged by the Sloan Digital Sky Survey (SDSS). This apparent offset led Shankar et al. to reduce the normalisation of the observed Mbh ∝ σ5 relation by at least ∼0.5 dex to give their ‘intrinsic relations’, including σ-based modifications to the observed Mbh–M*,gal relation. These modifications were based on the untested assumption that the stellar masses had been derived consistently between the two samples. Here, we provide the necessary check using galaxies common to the Spitzer Survey of Stellar Structure in Galaxies (S4G) and the SDSS. We find that the stellar masses of galaxies with and without directly measured black hole masses had appeared offset from each other due to the use of inconsistent stellar mass-to-light ratios, Υ*, for the optical and infrared data. We briefly discuss the ‘intrinsic relations’ and why some of these will at times appear to have had partial success when applied to data based on similarly inconsistent values ofΥ*. Finally, we reiterate the importance of the υ (lower-case Υ) term, which we previously introduced into the Mbh–M* relations to help avoid Υ*-related mismatches.

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.