Supermassive Black Holes and Accretion

Supermassive black holes power active galactic nuclei by drawing in surrounding gas, which forms a hot accretion disk that radiates prodigious amounts of energy.

Definition

A supermassive black hole is a black hole of millions to billions of solar masses residing at a galaxy's center; accretion is the inflow of gas that forms a hot, luminous disk around it, releasing gravitational energy as the radiation that powers active galactic nuclei.

Scope

This topic covers the evidence for supermassive black holes at galaxy centers, the physics of accretion disks and the conversion of gravitational energy into radiation, the Eddington luminosity limit, and the characteristic spectral signatures of accretion such as the big blue bump and X-ray emission.

Core questions

• What evidence shows that galaxy centers host supermassive black holes?
• How does an accretion disk convert gravitational energy into radiation?
• What sets the maximum luminosity an accreting black hole can sustain?
• What spectral features reveal accretion around supermassive black holes?

Key theories

The standard accretion disk

Shakura and Sunyaev developed the model of a geometrically thin, optically thick disk in which viscosity transports angular momentum and heats the gas, radiating a characteristic multi-temperature spectrum.

Black hole engine for AGN

Rees synthesized the case that accretion onto a massive black hole is the only mechanism able to produce the compact, energetic, and variable output of active nuclei.

The Eddington limit

Radiation pressure from the accretion luminosity opposes inflow, setting a characteristic maximum luminosity proportional to black hole mass that regulates how fast black holes can grow.

Clinical relevance

Accretion onto supermassive black holes is among the most efficient energy-release processes known, underlies all active galactic nuclei, and links the growth of black holes to their host galaxies and to the radiation that lights up the distant universe.

History

After quasars revealed enormous luminosities from tiny regions, Lynden-Bell and others pointed to accreting black holes, and the 1973 Shakura-Sunyaev disk model gave a quantitative description of accretion. Rees's 1984 review consolidated the black hole paradigm, which subsequent observations across the spectrum have confirmed.

Key figures

• Nikolai Shakura
• Rashid Sunyaev
• Martin Rees
• Donald Lynden-Bell

Seminal works

• shakura1973
• rees1984
• peterson1997

Frequently asked questions

How can a black hole be so luminous if light cannot escape it?

The light comes not from inside the black hole but from the accretion disk of gas spiraling toward it. Friction heats this gas to extreme temperatures before it crosses the event horizon, making the region around the black hole shine brilliantly.

How massive are supermassive black holes?

They range from about a million to several billion times the mass of the Sun. The most massive lie in the centers of giant galaxies, while smaller ones, like that in the Milky Way, sit at the lower end of the range.