The abundance distribution in the inner spheroid

Abstract

We present the metallicity distribution perpendicular to the Galactic plane, to distances of order 2 kpc. This has been derived by comparison of new photometry and existing spectroscopic samples of stars in situ and local samples of kinematically selected stars. These data may be used to constrain the enrichment history of the inner spheroid, where an intermediate component apparently dominates. The metal-licity distribution we find is best characterized by a sum of three independent components, each with a well-defined mean abundance and little or no gradient in this mean, but with very different spatial density distributions. These components are identified as the Galactic thin disk, the young stars of which have mean abundance [Fe/H] ~ + 0.0, <7 [Fe/H] ~0.15, and (exponential) scale height ~100 pc, while the older stars have mean abundance [Fe/H] ~-0.3, <7 [Fe/H] c^0.2 and (exponential) scale height ^¿300 pc; the Galactic thick disk, with mean abundance [ Fe/H-0.6, a [Fe/H] c^0.3 and (exponential) scale height £ 1 kpc; and the Galactic extreme spheroid, with mean abundance [Fe/H] ~-1.5, 0-[Fe/H] c^O.5 and (exponential) scale height £4 kpc, consistent with an R 1/4 law distribution. The relative number of stars in the solar vicinity in each of these populations are very approximately 0.2 : 1.0 : 0.02 : 0.001 for the young disk, the old disk, thick disk, and spheroid, respectively. The thick disk is shown to have kinematic properties intermediate between the thin disk and spheroid. The consequences of these results for theories of Galaxy formation are briefly discussed.