The brightest steady lights in the cosmos are black holes eating
Some galaxy centres blaze across every kind of light, from radio waves to gamma rays, far brighter than their stars could manage. These active galactic nuclei are the most luminous persistent objects in the universe, powered by gas spiralling into supermassive black holes. Our own Milky Way's centre was probably active about 8 billion years ago.
Hints appeared long before anyone understood them. Edward Fath spotted bright emission lines from the cores of NGC 1068 and Messier 81 in a 1909 paper, and Heber Curtis found the jet in Messier 87 in 1918. In 1943 Carl Seyfert described nearby galaxies whose brilliant nuclei produced unusually broad lines, and such objects are now called Seyfert galaxies in his honour.
Radio astronomy cracked the problem open. Early radio sources included Messier 87 and Centaurus A, and Walter Baade and Rudolph Minkowski matched Cygnus A to a distorted galaxy racing away at 16,700 kilometres per second. The 3C survey turned up sources that looked almost like single stars, dubbed quasi-stellar radio sources, or quasars. In the early 1950s Viktor Ambartsumian proposed that galactic cores harbour huge masses that blast out matter, an idea he pressed at the 1958 Solvay Conference to a sceptical audience.
In 1963 Maarten Schmidt measured the redshift of quasar 3C 273 at 0.158; if it lay at cosmic distance, it outshone other radio galaxies roughly a hundredfold, and 3C 48 soon proved even farther. No ordinary stars could supply such power. In 1964 Edwin Salpeter and Yakov Zeldovich suggested gas falling onto a supermassive black hole, and in 1969 Donald Lynden-Bell proposed that nearby galaxies hide the black holes of dead quasars. X-ray telescopes in the 1960s and 1970s then showed Seyferts and quasars shining brightly from their inner accretion disks.
What an observer sees depends on the black hole's mass, how fast it feeds, the tilt of its disk, how much dust hides it and whether it fires jets. The mightiest are quasars, and a blazar is one whose jet points at Earth, its glow boosted by relativistic beaming. Because they are so bright, these beacons help astronomers find distant objects and test models of cosmic history.
Source: Active galactic nucleus
