active galactic nucleus

History

The first active nucleus was noticed in 1908 as a strange bright center of a galaxy, but its nature was understood later. In 1963, astronomer Maarten Schmidt showed that quasars are distant active nuclei.

How it works

Like a whirlpool in a bathtub: water (gas) swirls around the drain (black hole), accelerates, and creates noise and splashes (radiation and jets).

History

The first active nucleus was noticed in 1908 as a strange bright center of a galaxy, but its nature was understood later. In 1963, astronomer Maarten Schmidt showed that quasars are distant active nuclei.

How it works

Like a whirlpool in a bathtub: water (gas) swirls around the drain (black hole), accelerates, and creates noise and splashes (radiation and jets).

History

In 1908, Edward Fath discovered emission lines in the nucleus of galaxy M87, but recognition came later. Carl Seyfert in 1943 systematized galaxies with bright nuclei. Maarten Schmidt in 1963 interpreted the spectrum of quasar 3C 273 as highly redshifted, proving its extragalactic nature and enormous luminosity. Yakov Zeldovich and Edwin Salpeter independently proposed accretion onto a black hole as the energy source. The modern picture emerged in the 1970s–80s thanks to X-ray observations and the development of disk and jet models.

How it works

Accretion of matter onto a black hole through a disk, where gravitational energy converts into heat and radiation. Some of the energy can be extracted from the hole's rotation via the Blandford–Znajek mechanism (an electromagnetic process involving the ergosphere), producing jets. The unified model assumes the central source is surrounded by a dust torus; the visible type depends on whether we see the disk directly (Seyfert I) or through the torus (Seyfert II). The beam velocity of jets is close to the speed of light, explaining superluminal motions due to relativistic aberration. Principles: accretion follows the equations of hydrodynamics and magnetohydrodynamics, radiation follows the laws of thermal radiation and synchrotron mechanism, relativistic radiation is affected by special relativity effects.

Open problems & fun facts

Mechanisms of jet launching and collimation down to parsec scales.; The nature of variability on ultrafast timescales.; The role of magnetic fields in transferring angular momentum in the disk.; The link between AGN evolution, galaxy growth, and star formation.; Formation of the dusty torus and its geometry.

💡 Blazar APM 08279+5255 contains a supermassive black hole that consumes the mass of the Sun every few hours, and its jet is aimed directly at Earth.