Are stellar halos the leftovers from galactic cannibalism?

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This project aims to clarify the assembly histories of galaxies, focusing in particular on their stellar halos. In the current conceptual picture of structure formation in the early universe, small gravitationally bound systems dominate in number, and the present population of galaxies is formed by multiple mergers of these protogalactic objects. Cosmological simulations predict that the extended stellar components of galaxies are built up by the merging and tidal disruption of dwarf galaxies. Since orbital times are long in these regions, a memory of this accretion should persist in the form of coherent structures in space and/or velocity. The Principal Investigator and collaborators have identified and quantified these structures in the halo of the Milky Way. In this project they will carry out an observational program on the 6.5-meter Magellan telescope to map the faint, extended stellar halos in 4 nearby spiral galaxies, tripling the number of galaxies mapped to this depth. Using N-body simulations of dwarf galaxy accretion, they will develop quantitative measures of halo structure that are effective indicators of the accretion history. They will then apply the new metrics both to their new observations of external galaxies and to large-scale survey data on the halo of the Milky Way from the Sloan Digital Sky Survey and from the Pan-STARRS 1 survey. The goal is to sensitively test theories of stellar halo formation. The project will involve the professional training and mentoring of a postdoctoral researcher and undergraduate students. The Principal Investigator will also participate in outreach through visits to elementary school classrooms and a coordinated effort to improve the astronomical content of the on-line reference Wikipedia.