Collaborative Research: The Structure and Origin of the Galactic Halo

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AST 0098435MorrisonThis is a collaborative research project involving scientists at 3 institutions: Dr. Edward Olszewski, at the University of Arizona, Dr. Mario Mateo, at the University of Michigan, and Dr. Heather Morrison, at Case Western Reserve University. These researchers are mapping a region of our Galaxy that was hitherto unreachable. They will complete an imaging survey that has been underway for several years, and they will carry out a comprehensive spectroscopic follow-up of halo-star candidates identified in the survey. The survey is undertaken to directly assess the degree to which accretion has formed the majority of the Galaxy's halo. The main focus of the survey is finding candidate halo stars that inhabit the outer parts of the Galactic halo. The final sample of halo stars from the survey will allow placement of strong limits on the existence of halo substructure and will very likely provide direct evidence of such structure if it comprises more than 10% to 20% of the luminous halo. A profound change is underway in how astronomers believe galaxies, such as ours, formed. Older notions of a single, monolithic collapse of the outer Galaxy on a relatively short timescale have begun to be superseded by the idea that much of the Galaxy formed from the accretion of much smaller systems over the entire age of the Universe. Most data supporting this accretion-driven model is somewhat indirect and involves expectations of sophisticated models of how galaxies form in an expanding Universe. Recently, the Sagittarius dwarf galaxy has provided strong evidence that at least part of our halo stars and clusters have come from an accreted satellite. ***