Chemical Fingerprinting and Chemical Analysis of Galactic Halo Substructure

Chou, Mei-Yin, Department of Astronomy, University of Virginia
Majewski, Steven, Department of Astronomy, University of Virginia

It is now known that the halo of the Milky Way was formed from the accretion of dwarf galaxies, which have left behind long-lived, dynamical halo substructure. One can investigate these substructures through "Galactic archaeology" to learn the nature of these accreted subhalo fragments. In particular, the chemistry of halo substructure allows us to glean insights into the metallicities and star formation histories of the absorbed progenitors, and to identify distinct Galactic stellar populations with specific accreted bodies. In this thesis we present an investigation of the chemical abundance patterns of halo substructures using high-resolution spectroscopic measurements. In particular, we study the abundances of the α-like element titanium (Ti) and the s-process elements yttrium (Y) and lanthanum (La) for M giant candidates of (a) the Sagittarius (Sgr) dwarf spheroidal (dSph) + tidal tail system, (b) the Galactic Anticenter Stellar Structure (GASS), also known as the Monoceros Ring, and (c) the TriangulumAndromeda (TriAnd) Star Cloud. Targets are pre-selected to be likely members of each of these systems on the basis of both three-dimensional position and radial velocity. As expected, the majority of the Sgr stars show peculiar abundance patterns compared to those of nominal Milky Way stars, but as a group the stars form a coherent picture of chemical enrichment of the Sgr dSph from [Fe/H] = −1.4 to solar abundance. The overall [Ti/Fe], [Y/Fe], [La/Fe] and [La/Y] patterns with [Fe/H] of the Sgr stream plus Sgr core do, for the most part, resemble those seen in the Large Magellanic Cloud (LMC) and other current Milky Way satellites, only shifted by ∆[Fe/H]∼+0.4 from the LMC and by ∼+1 dex from Milky Way dSph satellites; these relative shifts reflect the faster and/or more efficient chemical evolution of Sgr compared to the iii other satellites, and show that Sgr has had an enrichment history more like the LMC than the dSph satellites. By tracking the evolution of the abundance patterns along the Sgr stream we can follow the time variation of the chemical make-up of dSph stars donated to the Galactic halo by Sgr. This evolution demonstrates that while the bulk of the stars currently in the Sgr dSph are quite unlike those of the Galactic halo, an increasing number of stars farther along the Sgr stream have abundances like Milky Way halo stars, a trend that shows clearly how the Galactic halo could have been contributed by present day satellite galaxies even if the present chemistry of those satellites is now different from typical halo field stars. We also analyze the chemical abundances of a moving group of M giants among the Sgr leading arm stars at the North Galactic Cap, but having radial velocities unlike the infalling Sgr leading arm debris there. Through use of "chemical fingerprinting", we conclude that these mostly receding northern hemisphere M giants also are Sgr stars, likely trailing arm debris overlapping the Sgr leading arm in the north. We also apply "chemical fingerprinting" to the GASS/Monoceros Ring and TriAnd Star Cloud, to explore the origins of the two systems and the hypothesized connections between them. GASS has been debated either to originate from the Galactic accretion of a satellite creating a tidal stream, or as a part of the disk, dynamically induced through warping or resonances, etc. Our exploration shows that GASS is indeed made of stars from a dSph, and that it is distinct in chemistry from outer disk stars. And whereas the TriAnd Star Cloud has been assumed to come from the tidal disruption of the same accreted Milky Way satellite as the GASS/Monoceros Ring, our comparison of the abundance patterns in Monoceros and TriAnd M giants suggests that the TriAnd Star Cloud is likely an independent halo substructure unrelated to Monoceros.

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PHD (Doctor of Philosophy)
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