Milky Way Dwarf Galaxies: A Search for Stellar Substructure

Abstract

The Milky Way and its satellite population provides the nearest laboratory for the study of galaxy formation and evolution. As a result, it is the subject of detailed analysis, in order to better understand both the formation history of our own galaxy, as well as galaxy formation in the cosmological context. The dwarf galaxy population is of particular interest due to its perceived high dark matter content inferred from velocity dispersion measurements, making the dwarf galaxies candidates for the primordial building blocks of galaxy formation under the ΛCDM cosmological paradigm. As close as the Milky Way satellite population is, much of it remains a mystery. This is largely due to the difficulty in its observation; not only are the satellites resolved into their individual stars, blending in with the foreground of the Milky Way, they are extremely faint in many cases and require time intensive telescope allocation to obtain science grade data. This has resulted in small-scale, piecemeal observations and varying data sets, making the development of accurate theoretical models difficult. The advent of the digital survey camera era has changed this somewhat, and provides a new opportunity to delve deep into the Milky Way satellite population. This theses presents homogeneous observations, in the form of deep, wide-field photome- try, of three of the Milky Ways satellite dwarf galaxies: Hercules, Sextans and BootesI. The wide-field nature of these observations enables a thorough search for stellar substruc- ture associated with these dwarfs, in an attempt to better understand their level of tidal interaction with the Milky Way and how this influences our understanding of their role in galaxy formation. Each of the three dwarfs is found to possess extended stellar substructure, to varying degrees. The brightest, Sextans, demonstrates the least extreme substructure, and poten- tially has the most circular orbital path. The two fainter dwarfs are more representative of the ultra-faint regime and display more elongated structure, with most extreme of the two, Hercules, most likely to have the minimal peri-galactic distance. Interestingly, Hercules also has the most recent infall time, giving it less time to complete multiple orbits than Sextans or BootesI. This pattern suggests that while the size and infall of each galaxy is important, it is the orbital eccentricity and peri-galactic distance which play a larger role in the level of tidal influence. It is also important to note that in this sample, the two dwarfs closest to the ultra-faint regime are also the two showing the most extreme structure. If this finding is representative of the rest of the ultra-faint population, it may indicate a need to review mass-to-light ratios which are based on the assumption that the dwarfs are in dynamic equilibrium. Show more