The Structure of the Galactic Halo

We used the star counts in 21 BATC fields obtained with the National Astronomical Observatories (NAOC) 60/90cm Schmidt Telescope to study the structure of the Galactic halo. Adopting a de Vaucouleurs r(1/4) law halo, we found that the halo is somewhat flatter (c/a similar to 0.4) towards the Galactic center than in the anticentre and antirotation direction (c/a > 0.4). We also notice that the axial ratios are smaller (flatter) towards the low latitude fields than the high latitude fields, except for a few fields. We provide robust limits on the large-scale flattening of the halo. Our analysis shows that the axial ratio of the halo may vary with distance and the observation direction. At large Galactocentric radii, the halo may not have a smooth density distribution, but rather, it may be largely composed of overlapping streams or substructures, which provides a. support for the hybrid formation model.