What is the M-sigma relation?

It is the observed correlation between the mass of a galaxy's central black hole and the velocity dispersion of stars in its bulge. The tightness of this relation suggests the black hole and the galaxy influenced each other's growth.

How can a tiny black hole affect an entire galaxy?

Although the black hole is minuscule compared with the galaxy, the energy released as it accretes gas is enormous. Even a small fraction of that energy, coupled to the surrounding gas through winds, radiation, or jets, can heat or expel gas and regulate star formation.

AGN Feedback and Galaxy Coevolution

Tight correlations between supermassive black holes and their host galaxies, together with the energy released by active nuclei, point to a deep link between black hole growth and galaxy evolution.

Definition

AGN feedback is the influence an accreting supermassive black hole exerts on its host galaxy through radiation, winds, and jets, while coevolution refers to the observed correlations and presumed mutual regulation between black hole growth and the formation of the surrounding galaxy.

Scope

This topic covers the scaling relations between black hole mass and host galaxy properties, the energetic feedback by which active nuclei drive winds and heat surrounding gas, the distinction between radiative and mechanical feedback modes, and the role of feedback in quenching star formation in massive galaxies.

Core questions

What scaling relations connect black hole mass to host galaxy properties?
How does an active nucleus deposit energy into its host galaxy?
What is the difference between radiative and mechanical feedback?
Does feedback quench star formation in massive galaxies?

Key theories

Black hole host scaling relations: The mass of a central black hole correlates tightly with the velocity dispersion and mass of the host galaxy's bulge, suggesting their growth is physically linked.
The M-sigma relation: The strong correlation between black hole mass and bulge stellar velocity dispersion is a leading piece of evidence for coevolution and a target for feedback models.
Radiative and mechanical feedback: Active nuclei influence their hosts both through radiation-driven winds in luminous phases and through jets that heat cluster gas, the two modes operating in different regimes.

Clinical relevance

AGN feedback is invoked in galaxy formation models to halt the growth of the most massive galaxies and explain why they are old and red; the black hole host correlations make supermassive black holes central characters in the story of galaxy evolution.

History

Surveys in the late 1990s established that massive galaxies generically harbor central black holes whose masses scale with the host, and the 2000 discovery of the M-sigma relation sharpened this link. Observations of AGN-driven winds and the heating of cluster gas then established feedback as a key ingredient of galaxy evolution.

Key figures

John Kormendy
Andrew Fabian
Laura Ferrarese
David Merritt

Seminal works

magorrian1998
ferrarese2000
fabian2012

Frequently asked questions

What is the M-sigma relation?: It is the observed correlation between the mass of a galaxy's central black hole and the velocity dispersion of stars in its bulge. The tightness of this relation suggests the black hole and the galaxy influenced each other's growth.
How can a tiny black hole affect an entire galaxy?: Although the black hole is minuscule compared with the galaxy, the energy released as it accretes gas is enormous. Even a small fraction of that energy, coupled to the surrounding gas through winds, radiation, or jets, can heat or expel gas and regulate star formation.