Mapping the asymmetric thick disk. I. A search for triaxiality

A significant asymmetry in the distribution of faint blue stars in the inner Galaxy, Quadrant 1 (l = 20°-45°) compared to Quadrant 4 was first reported by Larsen & Humphreys in 1996. Parker et al. greatly expanded the survey to determine its spatial extent and shape and the kinematics of the affected stars. This excess in the star counts was subsequently confirmed by Jurić et al. using Sloan Digital Sky Survey data. Possible explanations for the asymmetry include a merger remnant, a triaxial thick disk, and a possible interaction with the bar in the disk. In this paper, we describe our program of wide field photometry to map the asymmetry to fainter magnitudes and therefore larger distances. To search for the signature of triaxiality, we extended our survey to higher Galactic longitudes. We find no evidence for an excess of faint blue stars at l ≥55° including the faintest magnitude interval. The asymmetry and star count excess in Quadrant 1 is thus not due to a triaxial thick disk.