“The Smooth and the Rough”
There is an enormous amount of information stored in the positions, velocities and chemistry of resolved stellar populations in the Milky Way. A comprehensive theory of galaxy formation can only be formed through the complementary information provided by the Milky Way and by external galaxies. There is currently a revolution taking place in Milky Way astronomy, sparked by the European Space Agency satellite Gaia, which will release a transformational dataset in April. Here, we show how to use SDSS photometry to provide first epoch for the Gaia Data Release 1 sources and hence build a catalogue that is a proxy for the dataset that will come in a few months time.
This SDSS-Gaia catalogue is used to study both the smooth and the rough portions of the Milky Way stellar halo. When decomposed into action space, the smooth portion is built from at least four components, whose admixture changes systematically with metallicity and betrays evidence of very ancient phases of accretion and merging. The rough portion is composed of halo substructure, such as streams and clumps. We carry out a search for such substructure and identify convincing examples, which are matched in a library of accreted remnants. The substructures are the residue of progenitors with masses about the size of largish dwarf galaxies that fell into the Milky Way about 9 Gyr ago. This provides proof of principle that the Gaia Data Release 2 will enable us to uncover the entire history of build-up of structure in the Milky Way halo back to earliest times.