The asymmetric thick disk: A star-count and kinematic analysis. I. The star counts

We report a statistically significant asymmetry in the distribution of thick-disk and possibly inner halo stars interior to the solar orbit 1-2.5 kpc from the Sun. We have compared the star counts in the 120 POSS I fields, 40 each above and below the Galactic plane in quadrant I with the 40 complementary fields above the plane in quadrant IV. We find a spatially extended region in quadrant I with a significant excess, 20% to 25%, in the numbers of blue- and intermediate-colored stars. While the region of the asymmetric distribution is somewhat irregular in shape, it is also fairly uniform, stretching over several hundred square degrees on the sky. It is therefore a major substructure in the Galaxy due to more than small-scale dumpiness in the thick disk or inner halo. In this first paper, we describe the observations and our star-count and statistical analysis. We also discuss three possible explanations for the asymmetry: the fossil remnant of a merger, a triaxial thick disk or halo, and interaction of the thick-disk/inner halo stars with the bar in the disk.