Galactic structure studies from BATC survey

Abstract

We present an analysis of the photometric parallaxes of stars in 21 BATC fields carried out with the National Astronomical Observatories (NAOC) 60/90 cm Schmidt Telescope in 15 intermediate-band filters from 3000 to 10000 Å. In this study, we have adopted a three-component (thin disk, thick disk and halo) model to analyze star counts information. By calculating the stellar space density as a function of distance from the Galactic plane, we determine that the range of scale height for the thin disk varies from 220 to 320 pc. Although 220 pc seems an extreme value, it is close to the lower limit in the literature. The range of scale height for the thick disk is from 600 to 1100 pc, and the corresponding space number density normalization is 7.0-1.0% of the thin disk. We find that the scale height of the disk may be variable with observed direction, which cannot simply be attributed to statistical errors. Possibly the main reasons can be attributed to the disk (mainly the thick disk) is flared, with a scale height increasing with radius. The structure is consistent with merger origin for the thick disk formation. Adopting a de Vaucouleurs $r^{1/4}$ law halo, we also find that the axis ratio towards the Galactic center is somewhat flatter ($\sim 0.4$), while the shape of the halo in the anticentre and antirotation direction is rounder with $c/a> 0.4$. Our results show that star counts in different lines of sight can be used directly to obtain a rough estimate of the shape of the stellar halo. Our solutions support the Galactic models with a flattened inner halo, possibly it is formed by a merger early in the Galaxy's history.