Abstract

Overdensities in the distribution of low-latitude, 2MASS giant stars are revealed by systematically peeling away from sky maps the bulk of the giant stars conforming to "isotropic" density laws generally accounting for known Milky Way components. This procedure, combined with a higher resolution treatment of the sky density of both giants and dust, allows us to probe to lower Galactic latitudes than previous 2MASS giant star studies. While the results show the swath of excess giants previously associated with the Monoceros ring system in the second and third Galactic quadrants at distances of 6-20 kpc, we also find a several times larger overdensity of giants in the same distance range concentrated in the direction of the ancient constellation Argo. Isodensity contours of the large structure suggest that it is highly elongated and inclined by about 3 degrees to the disk, although details of the structure-including the actual location of highest density, overall extent, true shape-and its origin remain unknown because only a fraction of it lies outside highly dust-obscured, low-latitude regions. Nevertheless, our results suggest that the 2MASS M giant overdensity previously claimed to represent the core of a dwarf galaxy in Canis Major (l similar to 240 degrees) is an artifact of a dust extinction window opening to the overall density rise l similar to 240 degrees to the more significant Argo structure centered at larger longitude (l similar to 290 degrees +/- 10 degrees, b similar to -4 degrees +/- 2 degrees).