John Kormendy, UT Austin
Supermassive Black Holes: Coevolution (Or Not) of Black Holes and Host Galaxies
Kormendy & Ho 2013, ARA&A, 51, 511 review the observed demographics and
inferred evolution of supermassive black holes (BHs) found by dynamical
modeling of spatially resolved kinematics. Most influential was the
discovery of a tight correlation between BH mass and the velocity dispersion
of the host-galaxy bulge. It and other correlations led to the belief that
BHs and bulges coevolve by regulating each other's growth.
New results are now replacing this simple story with a richer and more plausible
picture in which BHs correlate differently with different galaxy components. BHs
are found in pure-disk galaxies, so classical (elliptical-galaxy-like) bulges
are not necessary to grow BHs. But BHs do not correlate with galaxy disks. And
any correlations with disk-grown pseudobulges or dark matter halos are so weak
as to imply no close coevolution. These results enable a substantial revision
in our picture of BH-host coevolution:
We suggest that there are four regimes of BH feedback.
1 - Local, stochastic feeding of small BHs in mainly bulgeless galaxies involves
too little energy to result in coevolution.
2 - Global feeding in major, wet galaxy mergers grows giant BHs in short,
quasar-like "AGN" events whose feedback does affect galaxies. This makes
classical bulges and coreless-rotating ellipticals.
3 - At the highest BH masses, maintenance-mode feedback into X-ray gas has the
negative effect of helping to keep baryons locked up in hot gas. This happens
in giant, core-nonrotating ellipticals. They inherit coevolution magic from
smaller progenitors.
4 - Independent of any feedback physics, the averaging that results from successive
mergers helps to engineer tight BH-host parameter correlations.