GHOSTS: the Nature of Galactic Stellar Halos from Resolved Stellar Populations

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In recent years we have started to appreciate that the outskirts of galaxies contain valuable information about the formation process of galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are thought to be the result of accretion of minor satellites, predominantly in the earlier assembly phases. The size, metallicity, and amount of substructure in current day halos are therefore directly related to issues like the small scale properties of the primordial power spectrum of density fluctuations and the suppression of star formation in small dark matter halos.To exploit this fossil record of the galaxy formation process I have initiated the GHOSTS imaging survey of the resolved stellar populations of 18 nearby, massive disk galaxies. Here I request funding to expand this program of 200+ HST orbits with ground-based Large Binocular Telescope imaging and analyse the combined data sets. We will use bright tracer RGB stars to measure halo surface brightness profiles to extremely low equivalent surface brightness levels out to ~ 60 kpc. The colors of these old RGB stars are also excellent metallicity indicators putting further constraints on formation history. Younger Main Sequence, He burning and AGB stars constrain evolution of outer disk regions. The amount of halo substructure will be compared against the latest hierarchical galaxy formation models.