What makes a galaxy active?

A galaxy is called active when its central black hole is accreting gas vigorously enough that the nucleus outshines, or rivals, the combined light of all the galaxy's stars, producing distinctive emission across the electromagnetic spectrum.

Are quasars and active galactic nuclei the same thing?

Quasars are the most luminous type of active galactic nucleus, so distant and bright that they appear starlike. All quasars are active galactic nuclei, but the broader class also includes less luminous types such as Seyfert galaxies and radio galaxies.

Active Galactic Nuclei — ScholarGate Atlas

Definition

An active galactic nucleus is a compact, highly luminous region at the center of a galaxy whose energy output, far exceeding that of the surrounding stars, is produced by the release of gravitational energy as matter accretes onto a central supermassive black hole.

Scope

This area covers the physics of accretion onto supermassive black holes, the spectacular phenomena of quasars and blazars and their relativistic jets, the unification scheme that explains the diversity of active galaxies as orientation effects, and the feedback by which active nuclei influence their host galaxies.

Sub-topics

Core questions

What powers the enormous luminosity of active galactic nuclei?
What are quasars and blazars, and how do their jets arise?
How does a single underlying engine produce so many observational types?
How do active nuclei affect the galaxies that host them?

Key theories

Accretion-powered nuclei: Lynden-Bell argued that the luminosity of active nuclei comes from gas accreting onto a supermassive black hole, releasing gravitational energy far more efficiently than nuclear fusion.
AGN unification: Many distinct classes of active galaxy can be understood as the same accreting black hole viewed from different angles, with an obscuring torus and relativistic jets producing orientation-dependent appearances.
AGN feedback: The energy released by active nuclei can heat and expel gas from the host galaxy, linking black hole growth to the regulation of star formation.

Clinical relevance

Active galactic nuclei are the most luminous persistent sources in the universe, serve as beacons for probing the distant cosmos and the intervening gas, and play a key role in galaxy evolution through feedback that connects black hole growth to host galaxy properties.

History

Seyfert identified galaxies with bright, broad-lined nuclei in 1943, and the 1963 discovery of quasar redshifts revealed extreme distances and luminosities. Lynden-Bell's 1969 accretion model identified supermassive black holes as the engine, and the unified models of the 1980s and 1990s organized the bewildering variety of active galaxies.

Key figures

Donald Lynden-Bell
Maarten Schmidt
Robert Antonucci
Carl Seyfert

Seminal works

lyndenbell1969
antonucci1993
peterson1997

Frequently asked questions

What makes a galaxy active?: A galaxy is called active when its central black hole is accreting gas vigorously enough that the nucleus outshines, or rivals, the combined light of all the galaxy's stars, producing distinctive emission across the electromagnetic spectrum.
Are quasars and active galactic nuclei the same thing?: Quasars are the most luminous type of active galactic nucleus, so distant and bright that they appear starlike. All quasars are active galactic nuclei, but the broader class also includes less luminous types such as Seyfert galaxies and radio galaxies.

Active Galactic Nuclei