A Ghost Story: The Halo of the Milky Way Galaxy

The history of the Galaxy is imprinted in the kinematics and chemical properties of the stars in the stellar halo. Their study allows us partially to reconstruct the Galactic past because the time required for stars in the halo to exchange their energies and momenta is very long compared with the age of the Galaxy. This field has been revolutionised in recent years by data from the Sloan Digital Sky Survey Data, which has revealed a super-abundance of substructure. There are ghostly streams from disrupting dwarf galaxies and globular clusters, analogues of meteor streams along old cometary paths in the Solar. There are ultrafaint phantom galaxies composed of old and faint stars, so puny that the entire galaxy is outshone by a single red giant star. There are smooth, flattened, halo populations, such as the blue horizontal branch stars. Dominating the Galactic halo, there is the magnificent double arch of the Sagittarius stream criss-crossing the whole sky. These tracers enable the study of the assembly of the Galaxy, as well as its present-day mass and gravitational field. The quality of the data permit confrontation -- and conflict -- with modern-day cosmological simulations of galaxy formation.

Followed by wine and cheese in Pupin 1402.

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