Quasars – the energetic cores of distant galaxies – are among the most powerful sources of continuous energy in the Universe. Their light comes to us from a much earlier time – a time when galaxies flared with the birth of new stars and funneled cosmic ingredients down the maws of supermassive black holes in their centers.
In the 1950s, radio astronomy was maturing. Astronomers at the University of Cambridge published the first detailed maps of the entire sky at radio wavelengths. These new observations revealed, among other things, a zoo of mysterious objects in the sky: point-like sources of radio emission which did not, apparently, emit any visible light. Up until then, visible light was all astronomers had to go on; radio astronomy was revealing a previously invisible Universe!
The mystery deepened in 1960 when astronomers Allen Sandage and Thomas Matthews identified what appeared to be a faint blue star sitting on top of one of the radio source “3C 48″. Spectroscopic chemical analysis – the breaking up of light into its component colors or wavelengths – baffled investigators. The chemical signature in no way matched any known objects. The experience was repeated in 1962 when astronomer Maarten Schmidt, using the 200-inch Hale Telescope at Palomar Obseratory, found nearly the same thing with another radio source.
Schmidt figured out why the chemical fingerprint seemed so odd. The object was not made up of exotic elements but rather very mundane elements – hydrogen, mostly – traveling at an enormous speed. With the realization that they were seeing plain old hydrogen, they were able to clock 3C 48′s velocity at 37% the speed of light. This “star” was receding from Earth at nearly 370 million km/hr! And each object that was measured told a similar story: all of these radio sources were caught up in the expansion of the Universe.
These “quasi-stellar radio sources” – or quasars, for short – were not nearby objects at all, but rather extremely luminous entities far out in intergalactic space. Today, astronomers know of rougly 200,000 quasars, the furthest of which sits over 28 billion light-years from Earth. At that distance, the light has been traveling through space for 13 billion years – nearly three times the age of our planet! And this is one of the peculiar things about quasars: none of them are “local”. Most are at least three billion light years from Earth. The Universe appears to have gone through a phase early in its development when it produced many quasars…and then stopped.
With the aid of deep imaging from the largest telescopes, astronomers have learned that quasars are actually the hyper-luminous cores of very distant galaxies. Some of the most luminous shine with the light of two trillions suns — that’s about 100 times the light output of our entire galaxy! All of this energy is coming from a region of space no larger than our Solar System. This kind of energy can not be produced by star light alone. Quasars, therefore, require a phenomenonly powerful engine.
The driver at the heart of every quasar is a supermassive black hole – an exotic creature that contains the combined mass of several million suns within a region so compact that the intense gravity can not allow even light to escape. It’s gravitational tentacles reach out many light-years, drawing in stars, gas, and dust that venture a little too close. But, limited by the dynamics of orbital motion, this cosmic detritus can not simply plop down to the black hole’s surface. Rather, it spirals in, like water running down a drain. Every bit of matter that is funneled down the black hole’s throat must pass through this vortex. In the case where material is collected faster than it can spiral in, a celestial traffic jam ensues and the black hole builds what astronomers call an accretion disk. The high velocities of material running into each other in this disk generates friction that heats the disk to many millions of degrees. And, as it does so, it begins to pump out prodigious amounts of energy at all wavelengths of light: radio waves, infrared, visible light, and x-rays. The disk lights up and a quasar is born.
The observation that quasars proliferated in the early Universe and then slowly died out tells us something about the evolution of galaxies. A quasar needs two things: a supermassive black hole in the galactic center and enough material to continuously feed it. The largest quasars consume material at the rate of 600 Earths a minute! Almost every galaxy we observe – our own included – has a supermassive black hole in its core. What appears to have gone missing is the steady diet of interstellar gas and dust that creates the accretion disk. In the early Universe, galaxy collisions were far more common then they are today. This game of galactic bumper cars provided the cores of these galaxies with fresh material to feed the black holes. As the Universe aged, the galaxies drifted apart, the material got used up, and the quasars turned off. Astronomers hypothesize that the Milky Way may once have housed a quasar in its center that has long since shut down. But, it may reactivate in a few billion years when the Andromeda Galaxy collides with our own and the new combined super-galaxy flares up one more time.
The powerful light from quasars provides astronomers with a unique way to measure changes in the Universe. As the light from a distant quasar crosses through space en route to Earth, it frequently passes through intervening clouds of extragalactic gas. When it does so, the chemical signature of each cloud is imprinted on the spectrum of the quasar’s light. Each cloud leaves its own unique stamp. For the most distant quasars, the light travels for nearly the age of the Universe and every time it passes through one of the clouds, it takes a snapshot of the cosmic stew at that time. This has proven to be a fantastic test of theories describing the origin and evolution of the Universe. By tracing these snapshots back across space and time, we get an unprecedented peek into the chemical makeup of the cosmos from its first billion years of existance.