Tuesday, October 28, 2014

A Tour of an Active Galactic Nucleus

In my last post I showed an image of an active galactic nucleus. AGN are some of the strangest objects in the universe. Let’s take a tour and see what we find.

Here on Earth, far, far away from even the nearest AGN, what we often see in the sky are single points of light - objects that look much like stars. But these objects have very strange spectra. Spreading the light out over all its colors and wavelengths (spectroscopy), we see that these objects are not like stars at all. They have spectra that are very different from stars. Many show radio emission - not like stars at all. This led to these objects being names quasars - a mashup of ‘quasi stellar radio sources’.

Let’s see what happens if we get closer to the quasar. Astronomers have a couple ways of looking at objects in more detail. We can ‘zoom-in’ by getting better angular resolution; and we can ‘go deeper’ to see faint features in the object. If we do this, we often see that the quasars are just one part of an entire galaxy. It’s amazing - what we saw before - just one point of light - is brighter than the entire galaxy where it resides! And because astronomers like to classify and re-classify objects, we get to rename them. No longer are they ‘quasi-stellar’ - they appear in the center of entire galaxies. It’s a nucleus of a galaxy and it’s doing something, so a better name would be Active Galactic Nucleus (or Nuclei if plural). I know, we’re so creative :P

If we look a little more closely at the host galaxies, we can find entire swaths of gas that are ionized in a strange way. The atoms in the gas get ionized (loose electrons) when energetic photons (light particles) smash into them, and that happens around bright stars. But this gas is ionized in a way that stars can’t make happen. So what is ionizing this gas in the galaxies? It all seems to come from the active nucleus, so let’s get closer to that.

It is practically impossible to resolve the nucleus of AGN, but we can learn a lot about it using other methods. If we could zoom in, we would see some really funky stuff.

We’d see a large toroidal structure of dark, dusty clouds. Because the dust is shaped in a torus, like a doughnut or a fat bike tire, it blocks our vision of the very center of the AGN along some lines of sight, but not others. We think the torus isn’t exactly smooth - it’s more likely made up of lots of individual clouds that travel around the nucleus itself. There just may be fewer of these clouds around the polar regions of the AGN, allowing more light to pass through more lines of sight.

Within the torus we find the “broad line region”, where clouds are orbiting something very massive and very small at the center. These clouds can travel with velocities up to ten thousand kilometers per second. By comparison, the International Space Station travels at about 7.7 kilometers per second (thanks google), and the ISS flies all the way around the Earth in just 90 minutes. So these “broad line clouds” are traveling about 1000 times faster than the ISS.

Getting closer still to the center we find a very bright, very hot disk of material. This is the source of all the light energy that is shinning from the AGN. It is so hot that it glows in the ultraviolet wavelength range of the light spectrum. Just think - your cooking pan gets hot, but doesn’t glow. You’ve probably seen videos of hot metal on the internet that is heated to the point that it glows a bright orange color. And surely you’ve seen fires with blue flames. Well this gas is so hot that it glows in the ultraviolet.

But why is all this gas so hot and traveling with such high velocities anyway? Answer: Within the very center of the hot gas disk, there is a black hole. Not just any black hole - a supermassive one. As I mentioned in my last post, supermassive black holes can be up to a billion times more massive than our sun. And that black hole is pulling very hard on all the gas and dust in the disk surrounding it. The material of the disk is actually falling down onto the black hole itself, making it even more massive.

And if you zoom back out and think about all that gas and dust in the host galaxy that is experiencing the radiation from the active nucleus, you might think that supermassive black hole can have a big effect on the host galaxy. You might wonder if it affects the galaxy’s ability to form stars, or if it affects the shape of the galaxy in some way. And then you might have to get a PhD in astronomy to figure it all out . . . :D

That’s the tour of the AGN, see you next time!

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