Thembh-Lspheroidrelation at High and Low Masses, the Quadratic Growth of Black Holes, and Intermediate-Mass Black Hole Candidates

From a sample of 72 galaxies with reliable supermassive black hole masses M bh , we derive the M bh –(host spheroid luminosity, L ) relation for (1) the subsample of 24 core-Sérsic galaxies with partially depleted cores, and (2) the remaining subsample of 48 Sérsic galaxies. Using K s -band Two Micron All Sky Survey data, we find the near-linear relation M bh ∝ L 1.10 ± 0.20 K s for the core-Sérsic spheroids thought to be built in additive dry merger events, while we find the relation M bh ∝ L 2.73 ± 0.55 K s for the Sérsic spheroids built from gas-rich processes. After converting literature B -band disk galaxy magnitudes into inclination- and dust-corrected bulge magnitudes, via a useful new equation presented herein, we obtain a similar result. Unlike with the M bh –(velocity dispersion) diagram, which is also updated here using the same galaxy sample, it remains unknown whether barred and non-barred Sérsic galaxies are offset from each other in the M bh – L diagram. While black hole feedback has typically been invoked to explain what was previously thought to be a nearly constant M bh / M Spheroid mass ratio of ∼0.2%, we advocate that the near-linear M bh – L and M bh – M Spheroid relations observed at high masses may have instead arisen largely from the additive dry merging of galaxies. We argue that feedback results in a dramatically different scaling relation, such that black hole mass scales roughly quadratically with the spheroid mass in Sérsic galaxies. We therefore introduce a revised cold-gas "quasar" mode feeding equation for semi-analytical models to reflect what we dub the quadratic growth of black holes in Sérsic galaxies built amidst gas-rich processes. Finally, we use our new Sérsic M bh – L equations to predict the masses of candidate intermediate mass black holes in almost 50 low-luminosity spheroids containing active galactic nuclei, finding many masses between that of stellar mass black holes and supermassive black holes.