Abstract

The quest for understanding the physics of galaxy halos begins with the primary statement from Eggen, Lynden-Bell and Sandage (1962, ApJ 136, 748): “the time required for stars in the (Milky Way halo) galactic system to exchange their energies and momenta is very long compared with the age of the galaxy. Hence, knowledge of the present energy and momenta of individual objects tells us something of the initial dynamic conditions under which they were formed.” This statement, which is now more than half a century old, illustrates very clearly the motivation to study the dynamics and chemical composition of stars in the halos of galaxies and their implication for the models of galaxy formation. Measuring the physical parameters of the structural components of the galaxy halos with imaging and spectroscopic surveys from the ground and space allows us to trace back the formation history of these components, and to assess the relative importance of the competing mechanisms, i.e. the accretion of stellar systems, and the dissipative collapse of the galaxies’ own gas. In a hierarchical universe both processes operate simultaneously, on different time-scales across galaxies, so to understand galaxy assembly we need to compare observations against theoretical simulations of structure formation, which follow the interplay between the complex physical process (merging, gas flows, AGN formation, feedback) that lead to the formation of different structural components seen in our own Milky Way as well as in external galaxies. This task is far from trivial and requires inter-disciplinary collaborations. In the proposed workshop, close interaction between observational astronomers and model developers will single out those crucial quantities whose measurements provide strong constraints on the models, and will help us learn more about the major physical process which govern the formation of baryonic galactic halos.