Mapping the asymmetric thick disk: The hercules thick-disk cloud

The stellar asymmetry of faint thick-disk/inner-halo stars in the first quadrant (l p 20°-45°) that was first reported by Larsen & Humphreys and investigated further by Parker et al. has recently been confirmed by the Sloan Digital Sky Survey (SDSS). Their interpretation of the excess in the star counts as a ringlike structure, however, is not supported by critical complementary data in the fourth quadrant, which is not covered by the SDSS. We present stellar density maps from the Minnesota Automated Plate Scanner Catalog of the POSS I showing that the overdensity does not extend into the fourth quadrant. The overdensity is most probably not a ring. It could be due to interaction with the disk bar, or it could be evidence of a triaxial thick disk or a merger remnant/stream. We call this feature the Hercules Thick-Disk Cloud.