Abstract

A density-based hierarchical group-finding algorithm is used to identify stellar halo structures in a catalog of M-giants from the Two Micron All Sky Survey (2MASS). The intrinsic brightness of M-giant stars means that this catalog probes deep into the halo where substructures are expected to be abundant and easy to detect. Our analysis reveals 16 structures at high Galactic latitude (greater than 15 degrees), of which 10 have been previously identified. Among the six new structures, two could plausibly be due to masks applied to the data, one is associated with a strong extinction region, and one is probably a part of the Monoceros Ring. Another one originates at low latitudes, suggesting some contamination from disk stars, but also shows protrusions extending to high latitudes, implying that it could be a real feature in the stellar halo. The last remaining structure is free from the defects discussed above and hence is very likely a satellite remnant. Although the extinction in the direction of the structure is very low, the structure does match a low-temperature feature in the dust maps. While this casts some doubt on its origin, the low-temperature feature could plausibly be due to real dust in the structure itself. The angular position and distance of this structure encompass the Pisces overdensity traced by RR Lyraes in Stripe 82 of the Sloan Digital Sky Survey (SDSS). However, the 2MASS M-giants indicate that the structure is much more extended than what is visible with the SDSS, with the point of peak density lying just outside Stripe 82. The morphology of the structure is more like a cloud than a stream and reminiscent of that seen in simulations of satellites disrupting along highly eccentric orbits. This finding is consistent with expectations of structure formation within the currently favored cosmological model: assuming the cosmologically predicted satellite orbit distributions are correct, prior work indicates that such clouds should be the dominant debris structures at large Galactocentric radii (similar to 100 kpc and beyond).