What Is a Quasar? The Answer Depends on Your Point of View

When a Galaxy Erupts, What We See Depends on How We See It

By Phil Plait edited by Lee Billings

Standing on Earth and gazing out into the night sky, you’d think our Milky Way galaxy is relatively calm. Oh sure, there’s the occasional supernova and a bit of unrest as huge gas clouds collide and start to form stars. But as a whole, the vast cosmic neighborhood in which we live feels stately. Most nearby galaxies we can see look that way as well, just quietly going about their cosmic business. Tranquil.

But that’s not the case for all galaxies. Centaurus A is an overly enthusiastic oddball approximately 13 million light-years from Earth. It appears to be an elliptical galaxy shaped like a roundish cotton ball but with a striking dark lane across its middle. In the 1940s astronomers discovered it was inexplicably blasting out radio waves from its core—and subsequent studies across the ensuing decades showed that its center was also emitting high-energy x-rays and even gamma rays. Clearly Centaurus A has a lot more going on than you’d suspect at first glance. Observers eventually found many other similar objects, which were collectively given the catchall name Seyfert galaxies (after American astronomer Carl Seyfert, who discovered several of them).

Then, in the 1960s, things got weirder. Astronomers started discovering objects that emitted powerful radio waves but, unlike most Seyfert galaxies, were very faint in visible light. Many of those newfound objects were incredibly far away and therefore extremely luminous, but looked so much like stars that they were dubbed quasi-stellar radio sources, or quasars for short. Deep images taken with large telescopes revealed each observed quasar to be an extraordinarily bright central point of light drastically outshining a much fainter surrounding galaxy.

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As astronomers were wrapping their heads around the mysteries of quasars, yet another galaxy exhibiting a different sort of strange behavior drew their attention: BL Lacertae (or BL Lac for short). At nearly a billion light-years away, it too is a ridiculously luminous powerhouse, but amazingly it also changes its brightness dramatically over short periods of time, sometimes just a few hours. It was the first of a class of galaxies given the name blazars, a clever triple portmanteau of BL Lac, blazing and quasar. Like Seyferts and quasars, the majority of their light comes from their very centers. Together all three flavors are lumped into a broad category called active galactic nuclei, or AGN for short.

It didn’t take astronomers long to figure out what could possibly power such fierce and concentrated energy emission: a supermassive black hole gobbling down lots of matter. Still, if this is the engine behind all AGN, why do quasars, Seyferts and blazars all appear so different from each other?

In the 1990s astronomers came up with a brilliant idea to unify these disparate characteristics. While there is some actual physical diversity among these galaxies, the majority of their differing properties could be explained more by how we see them.

I mean this literally: the angle at which we view their centers greatly affects the resulting light we see. Understanding why that is essentially boils down to what exactly is happening near that central supermassive black hole.

Far out from the black hole, thousands to tens of thousands of light-years away, is a relatively normal galaxy not too different from the Milky Way. But closer in, where the gravity of the black hole holds sway, things are very different.

Immediately surrounding the monster black hole is a flat disk of matter (called an accretion disk) upon which it feeds. This accretion disk can stretch for several trillion kilometers, a decent fraction of a light-year, and it’s hot. The material very close to the black hole is orbiting at speeds approaching that of light, but farther out, the material is moving much more slowly. This creates immense friction in the disk that can heat matter up to millions of degrees. At this temperature and density, the material is incredibly bright and can easily outshine all of the galaxy’s stars.

The disk has an intense magnetic field embedded in it. As matter in the disk approaches the black hole and increases its orbital speed, the embedded magnetic field can coil up like thread around a spool. This further strengthens the magnetic field, which can become so powerful (coupled with a bizarre effect called frame dragging in which the black hole’s spin itself drags the fabric of spacetime around it) that material is expelled from the disk and blasted away in a pair of beams called jets. These jets are highly focused and stupendously powerful, and they can create immense internal shock waves that in turn unleash torrents of gamma rays—the highest-energy form of light in the cosmos. The jets can flow for hundreds of thousands of light-years in some cases, extending well outside the galaxy itself.

Crucially, farther out from the accretion disk and jets, on a scale of dozens to hundreds of light-years across in some cases, there is a dust torus—a doughnut-shaped cloud of tiny grains of rock and carbonaceous material. This stuff is dense and opaque to visible light, and if thick enough, it can block higher-energy forms of radiation as well.

In the unified model of AGN, the angle at which we see these structures explains almost everything we see. If the jets are pointed more or less toward Earth, we see light from across the electromagnetic spectrum, from gamma rays down to radio waves. Those are the blazars. If we see an AGN at a slightly lower angle, then the jet is aimed away, and the beamlike gamma-ray emissions miss us, but we can still detect high-energy x-rays; these are the quasars.

At lower angles still, the dust torus starts to the block the highest-energy light from reaching us. In these cases, much of an AGN’s emission is so dimmed that its surrounding galaxy is more readily visible. These are the Seyferts, and they tend to be bright in radio waves and infrared because their dust is warmed by the hellish brew closer in to the center, producing infrared thermal radiation.

The situation is a bit like the parable of the blind men and the elephant: what you think you see depends on which part you’re seeing, and it’s only when you put the pieces together that the true picture emerges. For active galaxies, the unified model does a good job at explaining the broad differences between the classes of galaxies observed.

To be honest, though, the unified model doesn’t explain everything, and it can struggle to reproduce many details that we see. But that’s not surprising: any model is going to be incomplete and isn’t going to explain every single thing found in every observation. The idea is to have a general sense of the processes and structures involved to explain a majority of what’s seen. Then extensions to the model can be introduced to explain the outliers.

And what of our own Milky Way? We have a supermassive black hole at the heart of our home galaxy, but like many of our galactic neighbors, it’s quiescent, meaning it’s not currently feasting on matter. The key word there is “currently”: It’s likely that in the distant past the Milky Way’s central black hole has episodically glutted itself on matter, too, each time erupting as an AGN. And because we tend to see AGN at large distances—when the universe was younger—this implies that most large galaxies have a similarly tempestuous youth. But happily, for now, we are at peace.