Wednesday, November 16, 2005

Another Astronomy Paper

Today's paper is:

"Three-Dimensional Bar Structure and Disc/Bulge Secular Evolution"
Bureau, M., Aronica, G., and Athanassoula, E.
astro-ph/0508616v1
2005

This paper (by some of the top people in the galaxy dynamics world) deals with boxy/peanut shaped galaxy bulges. Yes, peanut is an astronomical term. Compare the previous picture to the central regions of these images.

Alright, I guess you have to have an astronomer's imagination to see it, but trust me, the central regions of the galaxies do look like a peanut.

In the paper, the authors discuss near-infrared observations of 30 galaxies with this type of peanut morphology. They are trying to determine how bars and bulges form in relation to the disk galaxies that they are in.

--Oh, in order to see the peanut, the galaxy has to be viewed edge on, since the peanut is caused by matter moving up and down away from the disk. This galaxy could have a peanut structure (althought it probably doesn't), but we can't tell, since we're looking down at the disk.--

Back in the old days, people divided disk galaxies into three parts. First there was the disk. Well, duh, there has to be a disk in a disk galaxy. Second there was the bar (unfortunately not labled in this diagram, but you can consider the nucleus to be the bar). And third there was the bulge/halo. As Bureau et al. discuss in their introduction, bulges were thought to be a mini-elliptical (inside the larger disk galaxy) that were created through accretion of smaller galaxy. Everything was thought to be a separate entity that really didn't interact much with each other.

Well, times have changed. Now, we're more worried about secular evolution (try this definition to ensure you that I'm not bringning religion into this discussion).

It seems that numerical/computer models show that anytime you set up a disk of stars, and a bar forms at the center (another story entirely), the bar evolves into a bulge - it sort of spreads out and up/down out of the plane of the galaxy. In the astronomical vocabulary, the bar heats up and becomes a bulge.

The observations made by the authors point to this same scenario. Obviously, no one can observe a galaxy evolving. To see any real change in a galaxy, you'd have to sit around for at least 10,000,000 years, and even then, you wouldn't see much. What we can do is look for circumstantial evidence of change. With very sensitive images, the authors found that galaxies with peanut-shaped bulges had evidence of stellar orbits related to bars.

What are orbits related to bars? (Now this part would get really technical if I wanted to write it, but here's a link to a technical page on disk galaxies written by one of the authors). Think about it for a second. A bar/bulge is composed of stars that are rotating around the center of a galaxy. If the stars were moving about randomly in the disk, they wouldn't be able to hold any collective shape -- the stars would melt back into the disk. But bars are special. Bars are formed because stars get trapped on certain orbits that look much more ovalish. These are called x1 family orbits. As long as most of the stars in the bar region are on one of these orbits, you see a bar.

The observations of bulges in this paper observe bar orbits in the bulge. Thus, the conclude that the bulge is actually a result of the evolution of a bar.

Here's the sequence: disk -> disk with bar -> disk with bar and bulge -> disk with bulge

Now, this never really struck me as odd, but apparently, it has been a tough leap for the astronomical conclusion, namely because it's very esoteric and no one studies it, but also because the observations are so hard to make. You'll notice that the authors used near-infrared (K-band) observations of the galaxies. This is because near infrared cameras can see through the dust at the center of the galaxies, and they also see the older stars that have settled onto the well formed orbits. Really, near infrared observations have only been good for 15 years or so, so galaxy dynamics is an evolving science.

Things to take away:
1. Everything inside a galaxy is connected. Even though we might label a galaxy as having a disk and a bulge, they're really part of one big hole.
2. Bars and bulges are formed by stars on particular/well defined orbits around the center of the galaxy. Bars/bulges would never appear if stars moved randomly about a galaxy.
3. If you want to see the inner parts of a galaxy, try to observe in the infrared.

Next time, I'll try to write about something a little more exciting.

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