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 giantcandidates with dereddened 0.85 < J-K_S < 1.2 spanning a considerably largearea of the celestial sphere between, at least, $100\degr < l < 150\degr$ and$-20\degr > b > -40\degr$, and covering most of the constellations ofTriangulum and Andromeda. This structure does not seem to be preferentiallydistributed around a clear core, but rather lies in a tenuous, clumpycloud-like structure tens of kiloparsecs away. The reduced proper-motiondiagram as well as spectroscopy of a subsample shows these excess stars to bereal giants, not contaminating dwarfs. Radial velocity measurements indicateamong those M giants the presence of a coherent kinematical structure with avelocity dispersion $\sigma < 17$ km s$^{-1}$. Our findings support theexistence of a quite dispersed stellar structure around the Milky Way that, dueto its coreless and sparse distribution, could be part of a tidal stream or anew kind of satellite galaxy.Comment: 14 pages, 4 figures, 1 tabl