The Role of Accretion in Forming the Galactic Halo

Abstract

If the Galaxy formed hierarchically through the accretion of smaller satellite galaxies we might hope to find signatures of this in the halo's phase-space distribution. I review theoretical ideas about what form these signatures should take, concentrating in particular on the characteristics of debris from the tidal disruption of satellites less than the size of the Large Magellanic Clouds and at distances greater than 10 kpc. Under these circumstances, tidal debris can remain aligned in streams of stars close to the satellite's original orbit for the lifetime of the Galaxy and we can understand the evolution of its structure analytically. This understanding has many applications to observations and I will discuss a few examples: reconstructing the properties of a long-dead satellite from its debris; measuring the mass loss rate from Galactic satellites through the density of extra-tidal stars; and using observations of motions of tidal stream stars to measure the mass and size of the Milky Way.