Exploring Halo Substructure with Giant Stars: A Diffuse Star Cloud or Tidal Debris around the Milky Way in Triangulum-Andromeda

We report here the discovery of an apparent excess of 2MASS M giant candidates with dereddened 0.85<J-K_s<1.2 spanning a considerably large area of the celestial sphere between, at least, 100^deg<l<150^deg and -20^deg>b>-40^deg and covering most of the constellations of Triangulum and Andromeda. This structure does not seem to be preferentially distributed around a clear core, but rather lies in a tenuous, clumpy cloudlike structure tens of kiloparsecs away. The reduced proper-motion diagram, as well as spectroscopy of a subsample, shows these excess stars to be real giants, not contaminating dwarfs. Radial velocity measurements indicate among those M giants the presence of a coherent kinematical structure with a velocity dispersion σ<17 km s^-1. Our findings support the existence of a quite dispersed stellar structure around the Milky Way that, because of its coreless and sparse distribution, could be part of a tidal stream or a new kind of satellite galaxy.