Mathematics – Probability
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt........13w&link_type=abstract
Proquest Dissertations And Theses 2007. Section 0128, Part 0606 145 pages; [Ph.D. dissertation].United States -- Michigan: Mich
Mathematics
Probability
1
Globular Clusters, M87, Star Clusters, Dynamical Evolution
Scientific paper
Globular clusters are massive compact groups of stars, with masses that range beyond 10 6 [Special characters omitted.] . Because they are so large, they can remain bound together as they orbit their host galaxies. They are also very luminous, which ensures that they can be seen at distances far beyond the point where individual stars are no longer visible. The combination of these two qualities makes them wonderful test particles to explore how the dynamical interactions of stars in the cluster change the observed parameters. The evolution of these clusters has not been very well constrained by observations. They must lose mass as they orbit, but the exact way that this mass loss changes their observed properties is not well known.
M87 is a massive galaxy located 16 Mpc away from the Milky Way. This makes it much farther than other galaxies that have clearly resolved globular clusters. However as M87 is so massive (M ~ 10 12 [Special characters omitted.] ), its globular cluster population is much more numerous than those of other closer galaxies. Only about 150 globular clusters have been detected in the Milky Way, whereas M87 should have close to 10000 clusters. This large population allows for any observed relations to be less influenced by statistical uncertainty.
The core of M87 was imaged with the Advanced Camera for Surveys on the Hubble Space Telescope as part of a 50 orbit program in 2005 and 2006. During each orbit, multiple exposures were taken in the infrared F814W and red F606W filters, giving total exposure times of 73,800s in F814W and 24,500s in F606W. These very long exposures provide some of the deepest data ever taken with HST.
As the data used in this project can resolve the faintest clusters, we can use it to investigate the luminosity and mass functions of the globular clusters in M87. The final sample contains 2091 clusters, with a luminosity function that matches well with previously published results. The mass function generated from these clusters shows the signature of mass loss from two-body relaxation. Much theoretical work has been done to investigate this evolution, but since there are few galaxies in which large numbers of clusters can be observed, these theoretical predictions have been difficult to test in the past. The change in the mass to light ratio between clusters of different ages and metallicity is an important complication in the shape of the mass function. However, by correcting for these changes, this sample shows that the different color groups of the M87 globular clusters indicate different formation epochs.
These data also provide much higher angular resolution than previously available for populous extragalactic systems. This resolution ensures that the clusters are broader than just simple point sources, allowing them to be fit with theoretical models of the cluster structure. Such fits show that the relations between the cluster structure and luminosity appear to be universal, as those found for M87 match well with the Milky Way, the only other complete sample that exists. These structure fits also show that the probability of the formation of low mass X-ray binaries in a cluster is influenced by the rate of stellar interactions.
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