What Is a Quasar? The Brightest Objects in the Universe

Every large galaxy has a black hole at its centre, of millions to billions of solar masses, and most of the time it sits quiet, as the Milky Way's does. When gas falls toward it, from a galaxy merger, a passing cloud or a disturbed disc, the gas cannot fall straight in; it has too much rotation, and it spirals inward through an accretion disc, a flat swirl in which friction between the orbiting streams heats it to millions of degrees. The disc radiates across the spectrum, from radio to X-rays, and it converts matter to light with an efficiency of around ten percent of the mass, twenty times the efficiency of the nuclear fusion that powers stars; a black hole swallowing a few solar masses a year shines with the light of a trillion suns. Some quasars also fire jets of particles from their poles at nearly the speed of light, launched by the twisted magnetic fields of the disc, that reach millions of light years into space.

Radio astronomers in the 1950s catalogued sources that matched no visible object, and when the positions were refined the counterparts were faint blue star-like points whose spectra baffled everyone until Maarten Schmidt recognised, in the object 3C 273, the hydrogen lines shifted by 16 percent. The implications came at once: the redshift meant cosmological distance, the distance meant enormous luminosity, and the fact that the brightness varied over weeks meant that the source was no larger than a few light weeks across, since a thing cannot flicker faster than light can cross it, so that the light of a hundred galaxies was coming from a region the size of the solar system. Only gravity could do that, and Donald Lynden-Bell and Martin Rees argued in 1969 that a supermassive black hole with an accretion disc was the answer, which the next fifty years confirmed. The properties:

• •Luminosity from a hundred to a hundred thousand times that of the Milky Way, from a region a few light days across
• •Redshifts from near zero to above 7, meaning light that left them when the universe was under a billion years old
• •A black hole of a hundred million to ten billion solar masses at the centre
• •About one in ten radio-loud, with jets; the rest quiet in radio and named quasi-stellar objects, though everyone says quasar
• •A peak in number about ten billion years ago, when galaxies were merging and gas was plentiful, and a decline since as the fuel ran short

Quasars are lighthouses for cosmology. Because they are bright and far, their light passes through every cloud of gas between them and us, and the absorption lines those clouds imprint, the Lyman-alpha forest, map the hydrogen of the early universe and the growth of the structure that became galaxies. Their numbers over time trace how black holes grew, and the discovery of billion-solar-mass quasars at redshifts above 7, less than 700 million years after the Big Bang, is a puzzle, since a black hole cannot easily grow that large that fast from the remains of the first stars. And they connect the black hole to its galaxy: the mass of the central black hole tracks the mass of the galaxy's bulge across every galaxy measured, and the energy that quasars blast into their surroundings is thought to regulate the growth of the galaxy around them, so that the quiet black hole at the centre of every galaxy is a quasar that has finished eating.

3C 273, the first identified, is the nearest bright quasar at about 2.4 billion light years, and it can be seen in an amateur telescope of moderate size as a thirteenth-magnitude point in Virgo, the most distant object that such an instrument can show. The Milky Way's own black hole, Sagittarius A star, at four million solar masses, is dormant, and the Andromeda galaxy's, at a hundred million, is too; when the two merge in four billion years the gas thrown inward may light one of them, and the merged galaxy will have, for a few tens of millions of years, a quasar of its own, visible from wherever anything is left to look.

A quasar is the active core of a distant galaxy, where a supermassive black hole is swallowing gas through an accretion disc that heats to millions of degrees and shines with the light of a trillion stars from a region no larger than the solar system, sometimes firing jets at near light speed. Identified in 1963 by their huge redshifts, they peaked ten billion years ago when galaxies were merging, they map the early universe by the gas their light passes through, and the quiet black hole at the centre of every galaxy is one that has stopped feeding.