Thin, self-gravitating disks which have a high vrot/sigmav can be very susceptable to growing instabilities, particularly the onset of bars. This bar instability has been characterized a number of ways, but the key parameter appears to be the ratio of mass in a rotating disk to that in a non-rotating halo. There are several methods by which the bar instability has been parameterized as a function of disk mass, halo mass, and disk rotation properties. Toomre's X and Q parameters are used to describe the instability as a function of local properties of the disk, while the Ostriker-Peebles criteria seeks to describe the instability in terms of the global properties of the galaxy.
In this project, you will run several models of disk galaxies, varying some of the parameters of the models to understand when disk galaxies are stable or not stable against growing bar modes. You will then see how well your results compare with the various instability criteria which have been put forth in the literature.
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As small satellite galaxies orbit in the halo of a bigger host galaxy they can be affected in different ways. First, by a process known as dynamical friction, the interaction between the satellite and the host causes the small galaxy to spiral into the center of the larger galaxy. Whether or not it actually makes it to the center depends on how effective gravitational tides are at stripping stars from the satellite as it spirals inwards. Both dynamical friction and tidal stripping depend on the properties of the host and of the satellite galaxy, and analytic approximations exist which describe these processes. For example, the Chandrasekhar formula parameterizes dynamical friction, while a simple formula for tidal radii describes the extent over which stars remain bound to the satellite.
This project involves modeling the interaction between satellite galaxies and a large galaxy such as our Milky Way. You will run a variety of encounters, looking at the conditions under which satellites are disrupted and the speed at which mergers take place. Again, you will then compare your results to the analytic formula that exist, to assess their validity.
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Stars often form in clusters -- either open or globular clusters -- but they don't necessarily stay there. Various processes act to remove stars from a cluster; you will explore two of them here. The first is tidal stripping, in which stars gain energy as the cluster moves through the potential of the galaxy, eventually unbounding stars from the cluster. The second process you'll look at is "disk shocking", which happens when orbiting star clusters plunge through the galactic disk. Because they suddenly feel themselves in a region of high density as they enter the disk, then quickly leave it, they receive a strong gravitational "jolt" which affects the orbits of the stars and unbinds some of them. (Other important processes, evaporation, ejection and core collapse, involve collisional dynamics, and are beyond the scope of these models...)
As with the other projects, there are analytic formulae which approximate these processes. You will simulate the evolution of a few star clusters, and compare the rate at which they lose stars to the rates predicted analytically.
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