Tracing the Galactic Anticenter Stellar Stream with 2MASS M Giants

The recently discovered, ringlike structure just outside the Galactic disk in Monoceros is detected and traced among 2MASS M giant stars. We have developed a method to recover the signature of this structure from the distance probability density function of stars along a given line of sight. Application of this method reveals the presence of a large group of M giant stars at a Galactocentric distance of 18 ± 2 kpc, over +36° < b < +12° and 100° < l < 270°. Evidence that the stream extends to high negative latitudes is also found. That the structure contains M giants shows that it contains populations of at least an order-of-magnitude higher abundance than the [Fe/H] = -1.6 mean metallicity previously reported for this system. The structural characteristics of the stellar stream as traced by M giants do not support the interpretation of this structure as a homogeneously dense ring that surrounds the Galaxy but possibly as a merging dwarf galaxy with tidal arms, like the Sagittarius dwarf galaxy.