Wednesday, July 20, 2011

Globular Clusters: Tracers of Galactic Evolution

Globular clusters are a tight aggregation of hundreds of thousands to millions of stars. They are very old, devoid of gas, and have stellar populations that share the same age and chemical composition. To give you an idea of how old these systems are, it’s known that the Milky Way's globular clusters have ages ranging from 10-15 billion years. This means that these clusters could be relics of the formation of the Milky Way itself!

Empirical observations show that globular cluster luminosities are roughly proportional to the stellar mass of their host galaxy, which suggests that the formation of globular clusters and galaxies are related. However, how they are related is currently not well understood.

In addition, globular cluster populations in the Milky Way exhibit a curious bimodal distribution in their metallicities, where one group belongs to a metal-poor, spherical halo surrounding our galaxy, and another group belongs to a metal-rich, flattened distribution in the bulge and disk portion of the galaxy. Metallicity in astronomy simply refers to how much iron (or metals) is in proportion to hydrogen - the higher the metallicity, the higher the fraction of metals and vice versa. West et al (2004) provided the shown histogram of cluster metallicities for three galaxies. Again, why there are two distinct populations of globular clusters remains in disagreement. What is known is that the vast majority of other galaxies also have metallicty distributions with two or more peaks.

Although globular clusters are interesting in their own right, and can fill many books, I'm particularly interested in what they can tell us about galaxy evolution. There are two competing theories about cluster formation, with different implications about galactic evolution depending on which model is correct:

Model 1: Globular clusters are formed early in their respective galaxies, but are captured or cannibalized from subsequent mergers. If two galaxies collide, it is not unusual for the smaller galaxy to be ripped apart or absorbed by the larger galaxy. In this case, even though the progenitor galaxy would be destroyed, the globular clusters within it would remain intact. There are many such clusters in our galaxy that we think could have been captured this way.The image to the right shows the remains of galaxies absorbed by a giant elliptical galaxy in Abell 3827.

Model 2: The different populations of clusters correspond to distinct periods of star formation. The hydrogen rich, metal-poor clusters formed first, and then, after supernovae enriched the galaxy with metals, the metal-rich population came after. But how is this star formation triggered? It’s expected that star formation occurs steadily throughout a galaxy’s lifetime. However, if that were the case, the globular cluster populations should have metallicities that smear out into many different peaks. The double peaked distribution suggests that specific periods of star formation are needed, with the bulk of cluster formation occurring within relatively short bursts of time. This behavior could be explained by a merger of gas-rich galaxies. Shocks from two colliding galaxies could compress gas enough to form new globular clusters. In some interacting galaxies, massive star clusters that look tantalizingly like young clusters have been observed.The image to the left shows a Hubble image of a pair of colliding galaxies, which seems to have sparked a burst of star formation and over 1,000 massive star clusters (recognized by big blue clumps).

My suspicion is that clusters form in both these ways. The question, then, is which of these processes dominate cluster formation?

If Model 1 is correct, then the large star formation processes that produced globular clusters must have ceased a long time ago. Could this give us information about how galaxies evolved in the young universe? What would give rise to such vigorous star formation in the past, but not today?

If Model 2 is correct, could we then infer the metallities and kinematics of progenitor galaxies? What kinds of progenitor galaxies merged to form the large galaxies we see today? Is it possible large galaxies formed from a couple of gas-rich mergers? In that case, we'd expect to see over densities in globular cluster populations. Or, do large galaxies form from many mergers? If this is the case, the gases in the progenitor galaxies may have been used up in star formation from the multiple merging events, which could effectively deplete the materials needed for cluster formation. Perhaps this could provide another avenue to study the poorly understood physics of galactic mergers in general, such as gas heating and cooling, feedback from star formation, and the complex interplay between them.

I do not have the spectroscopy needed to date globular clusters, or to distinguish between different populations. However, I do have survey data of a large number of galaxies (on the order of thousands) from the Spitzer Space Telescope. I plan on looking at globular cluster populations to see if there are any over densities, and if so, what those over densities are related to. I do have some candidates that look promising, but at this point, it's too early to tell. Automating the detection and data reduction process has been difficult these past few months, but the prize is well worth the effort: a glimpse into galactic history.

Note: All of these images (except the top one of M80, which I jacked off of Wikipedia) come from the work of West, Cote, Marzke, and Jordan. They wrote a wonderful review paper in Nature on this very topic. If you're interested, I suggest you click here:

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