1999A&A...348...98B

We present an expanded and refined analysis of the Galactic thick disk as observed in seven fields of the new Basel RGU star count and color survey data. Based on the optimized structural models which were obtained in the initial analysis (Buser et al., 1998A&A...331..934B, hereafter Paper I), we now employ the same systematic least-squares algorithm, introducing more realistic luminosity functions for each population component, applying a more adequate matrix of color transformations in simulating the data, and extending the structural parameter ranges in order to determine improved characteristics of the thick-disk model. Compared with the earlier results of Paper I, the present models provide χ²-fits to the data which are improved by about 25%. From these we conclude that, on the one hand, each of the thin disk, thick disk, and halo components has its own local luminosity function, characterized by a distinctly specific shape and metallicity. On the other hand, the above improvement also allows us to derive more reliable estimates of optimized parameter values and constraints for the structural thick-disk model. According to our currently best models, we find the Galactic thick disk to have local density n₁=5.9±3% of the local thin-disk density, exponential scale length d₃=3.0±1.5kpc, and exponential scale height h₄=0.91±0.3kpc. We also confirm the result of Paper I that the data are consistent with a metallicity distribution of the thick disk centered on <[M/H]≳-0.63dex and having dispersion σ_<[M/H]>∼0.4dex. While these results are in remarkable agreement with the majority of independent recent determinations of the global properties of the Galactic thick disk, we cannot yet exclude the possibility that the sizeable dispersions associated with the mean parameter values are indicative of structural differences between the individual fields that may largely be due to features of the real thick disk - reflecting intrinsic deviations from the smooth physical entity described by the model. Still, pending the analysis of the second catalog of homogeneous RGU data in seven additional directions, the present data marginally favor an accretion model of the origin of the thick disk as a major episode in the merger history of formation of the larger Galaxy.