Formation Scenarios

Abstract

I discuss various proposed formation scenarios for the metal-poor components of the Milky Way Galaxy, emphasising the stellar halo and the thick disk. Interactions and accretion played a significant role in Galactic evolution, in particular at earlier epochs. The present observations favour a scenario by which the thick disk formed through the heating of a pre-existing thin stellar disk, with the heating mechanism being the merging of a satellite galaxy. A remnant `moving group' of the satellite would provide strong support for this scenario, and may have been detected. The field stars in the stellar halo probably formed in early small-scale star-forming regions, which subsequently disrupted. Late accretion is not important for the bulk of the stellar halo. The stellar initial mass function shows no evidence of variations, and indeed shows evidence of being invariant, even in companion satellite galaxies.