A CCD Study of High Latitude Galactic Structure: Testing the Model Parameters

Abstract

We interpret published CCD UBVI data to deduce the stellar density distribution and metallicity distribution function in the region from 2-8 kpc from the Galactic Plane, and compare our results to several star count models. A feature of extant star count models is degeneracy between the adopted scale heights of the thin and thick disks, and their local normalisation. We illustrate the utility of this small data set, and future larger sets (e.g. SDSS), by explicitly considering consistency between the derived density laws, and the implied solar neighbourhood luminosity function. Our data set, from Hall et al.(1996) (l=52, b=-39) contains 566 stars, selected to be consistent with stellar loci in colour-colour diagrams. Our analysis supports the parameterisation of the recent SDSS galaxy model of Chen et al.(2001), except in preferring the stellar halo axis ratio to be 0.84. Photometric metal-abundances have been derived for 329 stars using a new calibration. The vertical distance-dependent metallicity distribution function, if parameterised by a single mean value, can be described by a metallicity gradient -0.2$ dex/kpc for the thin disk and thick disk, and -0.1$ dex/kpc for the inner halo. The data are however better described as the sum of three discrete distribution functions, each of which has a small or zero internal gradient. The changing mix of thin disk, thick disk and halo populations with distance from the plane generates an illusion of a smooth gradient.