Tuesday, August 9, 2011

Thick Disk Formation and Finding Missing Baryons

I read a recent paper some days ago, entitled “Thick Disks of Edge-On Galaxies Seen Through the Spitzer Survey of Stellar Structure in Galaxies (S4G): Lair of Missing Baryons?” by Comeron et al. I decided to read this paper mainly because the authors were using the S4G survey, which is the same survey I'm using. The S4G survey is an infrared survey using 3.6 micron and 4.5 micron data from IRAC. These wavelengths are sensitive to older stellar populations and have reduced contamination from dust and star formation.

These authors looked at 46 edge on galaxies, to determine properties of the thick and thin disks. A thick disk in a galaxy is characterized as having a larger scale height (or, roughly, thicker cross section) than its flatter thin disk. The thick disk also tends to have older stars and a lower surface density (7% of that of the thin disk for the Milky Way). It's been a bit of a puzzle why these two disk populations exist, but an understanding of how the two disk components are related to galaxy formation will help us unravel clues to the processes that drive galactic evolution (hmmm... seems to be my favorite subject, but too bad! It's my blog!).

There are a couple of interesting things these authors found. First, they claim that thick disks may have a larger stellar population than previously thought, and second, they claim to have a better understanding of how thick disks formed.

Most of the paper is on the specifics of their model, which is way too complicated for a blog post. If you're interested in their modeling techniques, I suggest you read their paper. Long story short, the authors fit two stellar populations together to represent the thick and thin disks, Then, after they decided on what to assume for the numbers of small stars formed vs massive stars, they generated a mathematical description of how the luminosity of these galaxies should look like. They generated multiple models, assuming different stellar populations in the disks. Then, they simply compared those luminosity models to the actual data.

What they found was that the best fit to their models was a thick disk comprised of ¾ to as much as 3 times what we had previously thought, depending on which model was used. Of course, this may be highly model dependent, but all the models agreed that there is a higher stellar population in the thick disk. Now, why is this important? It's important because we haven't been able to account for all the universe's baryons. There was some speculation that the missing baryons were driven into the intergalactic medium from supernova feedback, but that scenario can't account for all the missing baryons. Perhaps this is the missing reservoir (hence the title of their paper!).

Also, the authors propose a formation scenario for thick disks. There are four different models for thick disk formation, which I'll abbreviate here:

Model 1: The clumpiness of galaxies can create overdensities that may perturb stellar orbits. These overdensities may come from spiral arms or molecular clouds. Astronomers do believe that young galaxies may have had a clumpier structure, but this model has difficulty describing the highest velocity stars.

Model 2: The young galaxies could have passed through dark matter clouds or other galaxies. This scenario, though, cannot produce a a thick enough thick disk when large quantities of gas is present. Also, these interactions usually produce a flared disk, which is not seen in the galaxies in this survey.

Model 3: Thick disk formation could have been a result of in situ star formation. The best candidate for this is . . . surprise . . . galactic mergers. The gas forming the thick disk would be dynamically hot from the merger, which could produce a thicker disk. The only problem with this model is that the accretion of external gas shrinks the disk, making it virtually indistinguishable from the thin disk. However, if you increase the mass in the thick disk, the net shrinkage would not be such a problem.

Model 4: Interactions with other galaxies may have donated stars via tidal stripping. However, this mechanism can't produce a large enough stellar population at present.

The authors suggest, in light of their observations, a thick disk formation scenario favoring in situ star formation. They believe that young protogalctic clusters may have merged, producing a hot disk of stars. Since they detected a higher population of thick disk stars, the later accretion of external material did not shrink the size of the thick disk by much. Then, once the galaxy aged and accreted cold gas, the thin disk was formed.

I was pleasantly surprised by this paper. I initially thought it would have nothing to do with my research, but in the end, it may. The conclusions seem to support galactic mergers as an important mechanism for galactic evolution, which is something I'm looking for. It would be interesting to see if thick disks vary depending on cluster environment, and, if so (or if not!), why that would be the case.

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