The new Basel high-latitude field star survey of the Galaxy V. The metallicity distributions in the inner-Galaxy fields SA 107 and NGC 6171

Abstract

We discuss star count and three-color data for the two inner-Galaxy fields SA 107 and NGC 6171 observed in the new Basel RGU high-latitude field star survey. Our analysis is based on the structural models of the Galactic population components that were derived from the previous studies of seven fields mainly in the outer Galaxy (Buser et al. 1998, 1999, hereafter Papers I and II, respectively). Apart from the canonical structural parameters, we here explore some of the special conditions prevailing in the transition region from high to low latitudes approaching the Galactic bulge, such as interstellar reddening, preponderant sampling of giants, and possible large-scale radial metallicity gradients. We show that the: data are consistent on a high confidence level with a model incorporating an old-thin disk With local mean metallicity ([M/H]) = -0.13 +/- 0.21 dex at galactocentric distance R similar to 7.3 kpc and radial abundance gradient partial derivative [M/H] = -0.078 +/- 0.016 dex/kpc, and a thick disk with mean metallicity ([M/H]) = -0.73+/-0.15 dex and vertical metallicity gradient partial derivative [M/H]partial derivativez = -0.07+/-0.03 dex/kpc. Any radial gradient of the thick disk is shown to be marginal, 0 greater than or equal to partial derivative [M/H]/partial derivativex greater than or equal to -0.02 dex/kpc. For the inner halo, we find a gradient-less metal-weak component with mean metallicity ([M/H]) similar to -1.8 +/- 0.3 dex but also evidence of a - possibly localized - metal-rich component having ([M/H]) = -1.0+/-0.2 dex. These results provide an important further step toward a comprehensive and detailed mapping of the larger-scale density and metallicity structures of the Galaxy, to be derived from the full-survey data in 14 fields.