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!

Friday, October 10, 2014

Astro Notations: The M-σ Relation

This week I thought I would discuss the M-σ relation.

The M-σ relation is a puzzle of modern astronomy. You see, it shouldn’t exist, or rather - we don’t understand why it exists. The M-σ relation is a correlation between the mass of supermassive black holes (M) and the velocity dispersion of the stars in the host galaxy (σ).

What the heck does that mean? I hear you asking. Well I’ll tell you.

First, the stellar velocity dispersion. This is a measurement of the range of velocities that stars have while flying around the galaxy. An observer from Earth sees some of the stars flying toward her, while others are flying away, and many are traveling with velocities somewhere in-between directly toward and directly away. The range of those velocities is what we call the dispersion, and it is directly determined by the amount of mass within the orbits of the stars.

Now what about the mass of the black holes? Well, black holes are really, really massive. Some are so massive, they are dubbed supermassive, and those supermassive black holes rest at the center of nearly every galaxy in the universe. The supermassive ones have more mass than a million of our Suns, and some have more mass than a billion. A billion. And that means that they have a lot of gravity. Mass leads to gravity, and gravity binds galaxies together.

But as big as supermassive black holes are, they are nothing compared to the galaxies in which they reside. And all the stars and all the gas and dust in the galaxy (what we call baryonic matter) is still small compared to the dark matter in the halos of the galaxies.

But before this turns into a discussion about the largest structures of the universe, let’s scale back to the supermassive black holes. While those black holes are massive enough to influence the orbits of the stars that are very nearby, they are not massive enough to influence the orbits of stars that are further away in the bulge of the host galaxy. The galaxy is so much bigger than the black hole, that the black hole can only influence - can only pull on - the stars closest to it. And that is precisely why the M-σ relation should not exist.

We measure the velocity dispersion of the galaxy, which should not be influenced by the black hole, and we measure the mass of the black hole. When we examine the measurements from many different galaxies, we find that the more massive the black hole is, the bigger the velocity dispersion is in the host galaxies. But the galaxies are so much bigger than the black holes - how do they "know" about the mass of the black hole at the center? Why does that relationship even exist?

There are a couple explanations that might connect the supermassive black holes to the host galaxies. One involves smashing galaxies together and merging the black holes within them. The other adds a layer of feedback from black hole growth into the host galaxy. Neither of these ideas are definitively confirmed yet, but there is evidence for both. I’ll leave that for another day.

For now, check out this cool artwork of an active galactic nucleus. What is it? Where did it come from? What is it doing? Although, I have discussed the Teacup AGN in a previous post, I'll keep discussing AGN in the future. I'll even discuss how we can use AGN to study the M-σ relation.