Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies

TL;DR

This review explores the complex relationship between supermassive black holes and their host galaxies, revealing that their coevolution varies across different galaxy types and is influenced by multiple feedback mechanisms.

Contribution

It challenges the traditional coevolution paradigm by showing that black holes correlate differently with various galaxy components, emphasizing multiple feedback regimes.

Findings

Black holes are present in pure-disk galaxies without classical bulges.

Correlations between black holes and galaxy disks or pseudobulges are weak or absent.

Different feedback regimes influence black hole growth and galaxy evolution.

Abstract

We 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 halo dark matter are so weak as to imply no close 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 correlations.