Application of star count data to studies of galactic structure

Abstract

Star count data from 12 fields analyzed by Jarvis and Tyson (1981) have been used to place constraints on galactic structure. A simple model of the Galaxy has been used to predict star counts; parameters describing the models have been obtained from a least-squares optimizatio program. Evidence is presented that the Wielen luminosity function results in a better modeling of star counts than does the Luyten function. If there is a 'local density enhancement' in the solar neighborhood, it must be thin (scale height less than 200 pc). The scale height of the galactic disk perpendicular to the galactic plane is found to be close to 350 pc in the solar neighborhood. Disk models whose z profiles are exponential provide a much better fit to star count data than do self-gravitating isothermal disks. The existence of a weak intermediate-scale-height ('thick') disk component in the galaxy cannot be ruled out, if that thick disk contributes exceeding about 10 per cent of the integrated surface density of the disk. A halo axial ratio exceeding about 0.9 is found independent of any assumptions regarding the local fraction of halo material. The Schmidt normalization for halo material is, however, confirmed to within a factor of 2.