Population Studies of the Galaxy — Constraints on the Thin Disk, the Thick Disk, and the Halo

Abstract

Twenty years ago Olin Eggen arrived at Mount Stromlo as Director. I remember it well : I had just begun my post graduate studies at that institution. The ink was barely dry on his (arguably perhaps) most famous paper, with Lynden-Bell and Sandage, “Evidence from the Motions of Old Stars that the Galaxy Collapsed” (1962, hereafter ELS). Two years later Allan Sandage spent a year at Mount Stromlo. It should take little to convince the reader that I was soon persuaded to the view that our Galaxy collapsed in a certain well-defined way. From a study of a kinematically selected sample of high velocity stars, ELS had found a remarkable correlation between chemical abundance and orbital eccentricity, in the sense that “the stars with largest (ultraviolet) excess (i.e., lowest metal abundance) are invariably moving in highly elliptical orbits, whereas stars with little or no excess move in nearly circular orbits”. They also found a similar correlation between abundance and the motion of stars perpendicular to the Galactic plane. To explain these relationships they proposed their hypothesis that the Galaxy collapsed from a protocloud to a thin disk on a timescale of a few times 10⁸ years, with progressive chemical enrichment as the collapse proceeded.