Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Here Be Monsters: AGN and quasars

After studying galaxies in the optical for several decades, astronomers noticed that a small fraction of galaxies are somewhat peculiar:

These peculiar entities were called by several names: Seyfert galaxies or Markarian galaxies (after astronomers who made catalogues of them), or, more generally, Active Galactic Nuclei (AGN for short). It was clear that something wierd was going on at their centers, but what?

In the 1950s and 1960s, astronomers began to use radio telescopes to scan the skies. They noticed that most galaxies didn't emit much energy at these wavelengths -- but a few galaxies were very bright in the radio. As radio telescopes improved, astronomers were able to discern the details of these radio sources ... and they were very surprised! Many galaxies look VERY different in the radio:

Here are some examples:

Meanwhile, the radio astronomers found a very few bright radio sources which didn't appear to associated with galaxies, but instead with faint, blueish objects which looked like stars. They called these quasi-stellar sources, or quasars for short.

Recent optical observations of these sources, both from space (with HST) and from the ground (with adaptive optics) has showed that most quasars are, in fact, surrounded by large conglomerations of stars and gas; surrounded, in fact, by galaxies. Often, the galaxies around quasars have very close companions.

When X-ray telescopes first made surveys of the sky, in the late sixties and seventies, they detected a number of galaxies. AGNs and quasars both emitted lots of X-rays.

At this point, there were several models for the "central engines" that power AGNs and quasars. But all the existing observations could be explained by the different models -- so we still didn't know for sure what was going on at their centers.

Now, X-rays can be produced by stars, or stellar remnants (white dwarfs). But photons of higher energy, gamma rays, are created when protons and electrons moving at relativistic speeds smash into low-energy photons. Stars cannot accelerate matter to such high speeds. So astronomers were very interested in using gamma-ray telescopes to look at AGNs and quasars -- we might learn something about their central engines if we detected very high-energy photons coming from them.

Gamma rays from space cannot pass through the Earth's atmosphere and reach the ground ... so we cannot detect them directly with ground-based telescopes. We have put some gamma-ray telescopes in orbit

but they are so small that they can detect only the very brightest sources. AGNs and quasars are almost all too faint for those gamma-ray satellites to see. But, with a little ingenuity, we can use big telescopes on the ground to detect gamma rays from faint sources!

After a decade of use, the gamma-ray telescopes have shown that several quasars and AGNs emit photons with energies of up to 1 Tera-electron-Volt (TeV) and higher. That's more than one million times the energy of ordinary X-rays, and one hundred billion times more energetic than optical photons. Mind you, they don't emit many of these super-photons: the Whipple telescope detects about 1 gamma-ray photon from the galaxy Markarian 501 every six minutes.

To sum it up, astronomers have identified a class of strange galaxies which

What's going on?

Additional resources:

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.