The Relationship Between the Halo and Disk Populations

Norris, J.E. 1996, in Formation of the Galactic Halo... Inside and Out, eds. H. Morrison & A. Sarajedini, p. 14.

The view that the Galaxy's (luminous) halo and disk are quite distinct entities, with very little overlap in their basic characteristics, is currently under intense scrutiny. While it may be true that the so-called `old disk' and `thick disk' have properties distinct from the Galaxy's most metal-poor material, once thought to be exclusively the realm of its halo, there are a number of indications that a significant fraction of material with [Fe/H] < -1.0 has disklike signature. Further, it appears that the halo itself may possess subcomponents characterized by different age, kinematics, and spatial characteristics. The data and their implications for the relationship between the various halo and disk subpopulations are reviewed and discussed in the light of the competing paradigms of the formation of the Milky Way of Eggen, Lynden-Bell, & Sandage and Searle & Zinn, and recent theoretical modelling of satellite mergers with disk galaxies.