Wednesday, September 28, 2005

Astronomy Paper of the "Day"

Well, maybe day isn't quite the proper term, as I don't know when exactly I'll be able to update this, but at least I'll try. I figure this might be a good way to clear the stack of papers out of my astronomy folder and to contain my notes.

I get my papers from astro-ph, which is over there on the right of the blog, under science sites. The papers there are mostly pre-prints, so hunting down the official reference might be tough at times, but I'll do the best I can. You can also go to ADS to look up the authors. Most of these will be stuff that I'm interested in, so expect alot of disk galaxies and bars.

To begin with, here's a short article by a friend, a mentor, and some guy I've never met.

"Anti-Truncation of Disks in Early-Type Barred Galaxies"
P. Erwin, J. Beckman, and M. Pohlen
ApJ Letters

Well, that's alot to start off with. Since I've never done this before, I'm going to define a couple terms. First off, a galaxy is a large collection of stars, gas, dust, and dark matter (unless your my former advisor). For this article, we're only going to worry about galaxies that are disk shaped (like the ones in the previous picture). And further, we're only going to worry about galaxies that are barred. A galactic bar is an oblong structure of stars and dust in the center of a galaxy. In the previous picture it is the creamy, brown structure that runs basically horizontal through the middle of the galaxy. Oh, and an early type galaxy tends to have tightly wrapped spiral arms.

Hope that's a good start.

So, what does it mean for a disk galaxy to be truncated. Well, basically, the galaxy has to come to an end somewhere. If you think about a whispy sort of water cloud here in the Earth's atmosphere, there's always a point at the edge of the cloud where it doesn't look solid anymore. You may still be able to notice some fuzz at the edge of the cloud, but you can see the blue sky behind it. The effect here is caused by the decreasing density of water molecules as you further out from the center of the cloud. Really, even if you go a little further out from the center, you might think that you're looking at clear blue sky, but really there are a few water molecules there still associated with the cloud. It's just that your eyes are not sensitive enough to notice the tiny amount of light that they're blocking.

Galaxies are much the same way, except they are made of stars giving off light instead of water molecules blocking light. The traditional view has been that galaxies end very abruptly, call this truncation. You can sort of see this on the pictures above. As you look outward from the center, the galaxy is going along emitting light steadily, and suddenly the galaxy stops. (In reality, the amount of emission at the outer edge is less than that at the center, but the drop off after the outer edge in a truncated galaxy is significantly greater.)

However, if you look more deeply with a telescope, you can find different behaviour. In this paper, Erwin et al. looked at a number of early type barred galaxies. What they found was that the majority of their galaxies were not truncated at all. These galaxies simply faded away. The decrease in the amount of light from the center of the galaxy to the outside remained constant. This is primarily an observational effect. The outside edge of the galaxy has become very dim, even though it hasn't truncated yet. The amount of telecope observing time needed to see further out in the disk becomes very large and impossible to obtain. With bigger telescopes and more time, you might be able to see edge effects.

Erwin et al. found that less than 12% of the galaxies showed typical truncation effects, but that about 25% of the galaxies showed something called "anti-truncation". What this means is that the outer edge of the galaxy is actually brighter than what we would expect from a truncated disk, and brighter enven than if the galaxy simply faded away as in the previous paragraph (even though the edge is still significantly more dim than the center of the galaxy). This is unexpected. What could be the cause?

Erwin et al. site a process that is beginning to become regarded as very common in galaxies, interactions. This image shows M31 (the Andromeda galaxy) as the big blue/white thing in the center. But M31 has a companion galaxy, M32, that is the reddish blob up and towards the right of center (really it has a number of other companions). Even though M31 is significantly bigger and more massive than M32, M32 excerts a strong influence on the movement of the individual stars within M31. Really, M31 is shaped the way it is because of M32's presence.

Through other visual evidence, Erwin et al. claim that several of the anti-truncated galaxies in their sample do have companion gaalxies. They conclude that it is likely that that anti-truncation in general is a result of interactions with companions. This is a likely conclusion since barred galaxies in general are more likely to have companions, so it is not suprising that we see anti-truncation behaviour in this sample. It is also not suprising that Erwin et al. find more anti-truncated galaxies than truncated ones, as we are finding more and more that galaxies are effected by their outside environments, and that many galaxies previously thought to be isolated are actually part of rich communities of smaller galaxies (even if we can't directly see the smaller galaxies all the time).

Things to take away from all this:
1. Galaxies have an edge somewhere, but the nature of that edge tells us alot about the local environment of the galaxy.

2. Many galaxies have small companions.

3. Even if the small companions are not directly visible, it is possible to observe their effects on a large galaxy, and infer their existence.

Hope you enjoyed.


At 2:29 PM, Blogger maki-girl said...

Very nice hoagie boy. I feel I learned something today. I have a question--were Erwin et al. looking at truncation or anti-truncation in visible light? Or in infrared? Or radio? Are there differences?

At 2:43 PM, Blogger hoagieboy said...

Uh, yeah, I guess that's something that I should have included.

The observations were R band (Visible, red light for the uninitiated.)

There probably shouldn't be a difference in the near infrared, since you're outside the spiral arms at the edge of the galaxy. All the stars there should be old dwarfs and there wouldn't be any younger giants (at least not many.)

Even though I probably should know what happens at the outside of a galaxy in radio (HI), I don't. Perhaps that would be a question for the old advisor. The truncation is defined in to occur with stellar light, so there shouldn't be any reason for the gas to trace the stars, particularly since you aren't forming stars out there. I would bet that the gas just falls off exponentially until we can't see it anymore. Of course, if the gas were to be truncated, it probably would happen significantly further out from the center than the truncation of the stars. That would be a nice follow up study to the anti-truncating galaxies.

At 2:52 PM, Blogger maki-girl said...

From what I understood from "our" old advisor, the HI sort of just dwindles away ... and the deeper we look it seems like the further out we can see it. (Which goes with what you were saying about the clouds.) What we found this summer was that the molecular gas sort of just dwindles away too and we did not see any big reservoirs out there like some had thought there might be.

At 3:08 PM, Blogger hoagieboy said...

Whoever would have thought there were large reservoirs of molecular gas in the outer extremes of galaxies? Glad I didn't work with that person.

At 3:14 PM, Blogger maki-girl said...

Um, yeah you did. Besides, every theory has to be tested.

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