Subproject A2: Milky Way Satellites as Cosmological Probes

A prediction of recent cosmological models is that the most significant contributors to the stellar mass of the inner halo were several massive satellites that were accreted early on. The Magellanic Clouds may be present-day counterparts of such early contributors. If so, their old stellar populations should be chemically similar to halo stars. However, since these irregular galaxies are dominated by intermediate-age populations, it is a challenge to identify genuinely old stars. Using RR Lyrae stars, which only occur in old populations, we discovered the most metal-poor stars known in the Magellanic Clouds known so far. We showed that their element abundance ratios are consistent with those of similarly metal-poor stars in the halo. While progresss was made on the characterization of Galactic satellites, the work towards the completion of the census of Galactic satellites during the first funding period did not proceed as planned since the photometric survey data from the “Panoramic Survey Telescope and Rapid Response System” (Pan-STARRS) only recently reached a homogeneity sufficient for this purpose.

In the second funding period, apart from Pan-STARRS, we will apply a novel approach to detect and map known and new satellites and tidal features around the Milky Way using small, extremely wide-field amateur telescopes. This approach is based on integrated surface brightness, no longer on individual stars and has already delivered impressive maps of tidal features and substructure. Special emphasis will be placed on searches in the area of the proposed “great plane” of Galactic satellites. In combination with tailored, state-of-the-art hydrodynamical simulations we will explore whether the observed apparent satellite alignment is consistent with the current cosmological scenario, constrain the overall potential of the Milky Way based on the detected streams, explore the use of the stellar streams to detect the predicted dark halos via their gravitational interaction, and investigate the impact of satellites on dynamical heating and (sub)structure in the thin and thick disk.