Subproject A3: Mapping the Milky Way's Stellar Outskirts

Accretion events should leave observable signatures in the halo in the form of stellar streams and overdensities. Using the variability and light-curve information from time-domain surveys that became available during the first funding period, large numbers of short-period RR Lyrae and long-period carbon-rich variables could be identified and distances determined. Carbon stars are tracers of intermediate-age populations, which as such are not expected in the Galactic halo. Mapping the C star distribution in the Galactic halo, we recovered a number of previously known stellar streams (such as Sagittarius) as well as identified new overdensities. Among other things, these data will ultimately allow us to constrain the contribution of satellites sufficiently massive to have had extended star formation histories to the halo, and also impose constraints on the time of accretion. Like subproject A2, A3 had been intended to be based largely on Pan-STARRS data. A considerable effort was thus spent on uniformly calibrating the Pan-STARRS data. This time-consuming, but essential work has led to a many-color survey of the Milky Way that is better calibrated than the SDSS and covers a much larger area, including the Galactic disk. We developed novel techniques to identify RR Lyrae and blue horizontal branch stars in Pan-STARRS data, which will permit us to search for substructure in a data set much deeper than other wide-field surveys.

In the second funding period, we will focus on the question of in-situ formation vs. accretion in the Galactic stellar halo. Building on the now calibrated Pan-STARRS data, other time-domain surveys, and the resulting 3-D maps, kinematic, phase-space, and abundance information will be added using ground-based spectroscopic surveys and the Gaia data releases. This will permit us to quantify the amount of stellar substructure, to characterize it in orbital space, to infer its stellar population and abundance properties, and to constrain the properties of the progenitor galaxies. We will obtain a detailed empirical picture of past accretion events that can be fed into the simulation efforts in the subprojects A1 and A2.