Tidal Evolution of Dwarf Spheroidal Satellites




Borukhovetskaya, Alexandra

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Dwarf spheroidal galaxies populate the faintest end of the galaxy luminosity function and yet they apparently reside in massive dark matter halos. With mass-to-light ratios as high as M/L ~ 10^4 , they are the key to understanding the nature of dark matter on galactic scales. Of particular interest are nearby dwarf spheroidals that appear at odds with predictions from LCDM, the standard model of cosmological structure formation. These are galaxies which exhibit unusually large sizes and low line-of-sight velocity dispersions, suggestive of surprisingly underdense dark matter halos, perhaps occurring as a result of tidal interactions with the Milky Way. In this dissertation I present a detailed study of three carefully selected such cases: the Fornax, Crater II and Antlia II dwarf spheroidal satellites of the Milky Way. Fornax is chosen for its relatively low mass-to-light ratio, which is suggestive of a lower mass halo than indicated by abundance matching. Crater II and Antlia II, termed ‘feeble giants’, are both chosen for their particularly low values of velocity dispersion, which are especially anomalous given their large half-light radii and low luminosities. Using N-body simulations, we investigate the evolution of these objects under the effects of Galactic tides imparted by the Milky Way potential. Our study leads us to conclude that the low measured velocity dispersions of these dwarf galaxies are indeed consistent with a tidal interpretation in the context of predictions put forth by cosmological simulations of the Local Group and recent measurements of the galaxies’ sky positions, proper motions, distances, and radial velocities. The large sizes of Crater II and Antlia II are much more difficult to reconcile in this scenario, however these are still possible to reproduce under the effect of Galactic tides, provided that initially the stellar binding energy distributions had a minimum “cutoff”. Such a limit, which may have been imposed by baryonic effects during the formation of the galaxy, or by the presence of a constant density dark matter core, leads to transient stages with large sizes and low velocity dispersions comparable to those of Crater II and Antlia II. Detailed observations of galaxies’ density profiles, corresponding logarithmic slopes, and velocity dispersion profiles may help provide insight into the likelihood of such a formation scenario for Crater II, Antlia II, and the emerging population of other feeble giant galaxies.



astronomy, dark matter, dwarf galaxies