Galactic structure studies from the Beijing-Arizona-Taiwan-Connecticut survey

We present an analysis of the photometric parallaxes of stars in 21 of the Beijing-Arizona-Taiwan-Connecticut Survey 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 disc, thick disc and halo) model to analyse star count information. By calculating the stellar space density as a function of distance from the Galactic plane, we determine that the range of scaleheight for the thin disc 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 scaleheight for the thick disc is from 600 to 1100 pc, and the corresponding space number density normalization is 7.0-1.0 per cent of the thin disc. We find that the scaleheight of the disc may be variable with the observed direction, which cannot simply be attributed to statistical errors. Possibly the main reasons can be attributed to the disc (mainly the thick disc) being flared, with a scaleheight increasing with radius. The structure is consistent with the merger origin for the thick disc formation. Adopting a de Vaucouleurs r^1/4 law halo, we also find that the axis ratio towards the Galactic Centre is somewhat flatter (~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 formed by an early merger in the Galaxy's history.