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

TO Phil Plate edited by Lee Billings

Standing on Earth and looking up at the night sky, you might think that our Milky Way galaxy is relatively calm. Yes, of course, from time to time there are supernovae and small disturbances when huge clouds of gas collide and begin to form stars. But overall, the vast space region in which we live seems majestic. Most of the nearby galaxies we see look the same, just quietly going about their cosmic business. Calm.

But this does not apply to all galaxies. Centaurus A is an overly enthusiastic weirdo. approximately 13 million light years from Earth. It appears to be an elliptical galaxy, shaped like a round cotton ball, but with a striking dark band down the middle. In the 1940s, astronomers discovered that it was inexplicably emitting radio waves from its core, and subsequent studies in subsequent decades revealed that its center also emits high-energy X-rays and even gamma rays. It's clear that there's a lot more going on with Centauri A than might be expected at first glance. Observers eventually discovered many other similar objects, collectively called Seyfert galaxies (after the American astronomer Carl Seifertwho discovered several of them).

Then, in the 1960s. everything just got weirder. Astronomers have begun to discover objects that emit powerful radio waves but, unlike most Seyfert galaxies, are very faint in visible light. Many of these newly discovered objects were incredibly far away and therefore extremely luminous, but were so star-like that they were called quasi-stellar radio sources, or quasars for short. Deep images taken with large telescopes showed that each observed quasar was an unusually bright central point of light, significantly eclipsing the much dimmer surrounding galaxy.

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While astronomers were puzzling over the mysteries of quasars, another galaxy caught their attention, exhibiting another strange behavior: BL Lizards (or BL Lac for short). Nearly a billion light years away, it is also an incredibly bright powerhouse, but surprisingly, it also changes its brightness dramatically over short periods of time, sometimes in just a few hours. It was the first of a class of galaxies called “blazars”—a clever triple combination of BL Lac, blaze, and quasar. Like Seyferts and quasars, most of their light comes from their very centers. Together, all three varieties are grouped into a broad category called active galactic nuclei, or AGNs for short.

It didn't take long for astronomers to figure out what could have caused such a powerful and concentrated burst of energy: a supermassive black hole that absorbs a lot of matter. And yet, if this is the engine of all AGNs, then why are quasars, Seyferts and blazars so different from each other?

In the 1990s, astronomers came up with the brilliant idea of ​​combining these disparate characteristics. Although there is some physical diversity among these galaxies, most of their different properties can be explained rather 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 this happens essentially comes down to what exactly is happening near the central supermassive black hole.

Far from the black hole, thousands to tens of thousands of light years away, lies a relatively normal galaxy not too different from the Milky Way. But closer to us, where the gravity of a black hole dominates, things are completely different.

Directly surrounding the monstrous black hole is a flat disk of matter (called an accretion disk) from which it feeds. This accretion disk may extend over several trillion kilometers, a good fraction of a light year. hot. Material very close to the black hole orbits at speeds approaching the speed of light, but material beyond that moves much more slowly. This creates enormous friction in the disk, which can heat the material to millions of degrees. At this temperature and density, the material is incredibly bright and could easily outshine all the stars in the galaxy.

The disk has a powerful magnetic field built into it. As matter in the disk approaches the black hole and increases its orbital speed, the embedded magnetic field can coil like thread onto a spool. This further enhances the magnetic field that can become so powerful (combined with a fancy effect called frame drag). in which the rotation of a black hole itself drags the fabric of space-time around itself) this material is ejected from the disk and exploded in a pair of rays called jets. These jets are highly focused and incredibly powerful, and they can create huge internal shock waves, which in turn release streams of gamma rays, the highest energy form of light in space. In some cases, the jets can travel hundreds of thousands of light years.extending far beyond the galaxy itself.

It's important to note that further away from the accretion disk and jets, in some cases on a scale of tens to hundreds of light years, lies a dust torus—a donut-shaped cloud of tiny grains of rock and carbonaceous material. This material is dense and opaque to visible light, and if thick enough, it can also block higher energy forms of radiation.

In a unified AGN model, the angle at which we see these structures explains almost everything we see. If the jets are directed more or less toward Earth, we see light across the entire electromagnetic spectrum, from gamma rays to radio waves. These are blazars. If we see the AGN at a slightly lower angle, the jet will be pointed away and the beam of gamma rays will miss us, but we will still be able to detect high-energy X-rays; these are quasars.

At even smaller angles, the dust torus begins to block the highest energy light from reaching us.. In these cases, most of the AGN emission is so dim that the surrounding galaxy becomes easier to see. These are Seyferts, and they tend to be bright in radio waves and infrared because their dust is heated by a hellish brew closer to the center, creating infrared heat radiation.

The situation is a little similar parable of the blind men and the elephant: What you think you see depends on which part you see, and it is only when you put the pieces together that the true picture emerges. For active galaxies, a single model does a good job of explaining the wide differences between observed classes of galaxies.

Although, to be honest, a unified model doesn't explain everythingand it may be difficult for him to reproduce many of the details we see. But this is not surprising: any the model will be incomplete and will not be able to explain every thing found in every observation. The idea is to have a general understanding of the processes and structures involved to explain most of what is seen. The model can then be extended to explain outliers.

What about our Milky Way? There is a supermassive black hole at the center of our home galaxy, but like many of our galactic neighbors, it is dormant, meaning it is not currently feeding on matter. The key word here is “currently”: it is likely that in the distant past the central black hole of the Milky Way also periodically oversaturated itself with matter, each time erupting as an AGN. And since we tend to see AGN at greater distances – when the Universe was younger – this means that most large galaxies have similarly turbulent youth. But fortunately, we live in peace at the moment.