Mapping Galactic Halo and Dwarf Spheroidal Structure With Giant Stars

The Milky Way and its satellite system represent a unique testbed for present "concordance" Cold-Dark-Matter cosmogonies, which predict that galaxies like the Milky Way form hierarchically from mergers and accretion of smaller subsystems. The work presented in this thesis adds to the current observational evidence that the halo of our Galaxy has been built, at least partly, through the accretion of smaller stellar systems like star clusters and dwarf satellite galaxies, and thus qualitatively supports these currently favored cosmological models. We present observational data and N-body simulations to show that other Galactic dwarf spheroidal (dSph) galaxies, besides the widely accepted example of the Sagittarius (Sgr) dSph, are being tidally stripped by the Milky Way. In particular, we report that both the Carina and Leo I dSphs have characteristics strongly suggesting that they are undergoing luminous tidal disruption, and that it is difficult to explain their observed structure and kinematics in any other way. We also show that the Ursa Minor dSph may well be in a similar state as Carina, Leo I and Sgr. In addition, we present a new spectroscopic survey of red giant, horizontal branch and main-sequence-turn-off stars to V ∼ 17.5 selected from the Grid Giant Star Survey photometric database. The survey was carried out with the Hydra multifiber spectrographs on NOAO telescopes over the past six semesters. These data probe well into the Galactic halo, and the distribution of radial velocities and metallicities show the halo to be substructured, possibly with the remains of tidally disrupting satellite galaxies. In a first foray into exploring the kinematical richness of this newly created database, we have reported the discovery of a potential new stellar stream in the 1 st Galactic quadrant, one of several new potential kinematical features discernible in this survey.

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