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\degr<l<150\degr and −20\degr>b>−40\degr, 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 cloud-like 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, due to its coreless and sparse distribution, could be part of a tidal stream or a new kind of satellite galaxy.
@article{arxiv.astro-ph/0405437,
title = {Exploring Halo Substructure with Giant Stars: A diffuse star cloud or tidal debris around the Milky Way in Triangulum-Andromeda},
author = {Helio J. Rocha-Pinto and Steven R. Majewski and M. F. Skrutskie and Jeffrey D. Crane and Richard J. Patterson},
journal= {arXiv preprint arXiv:astro-ph/0405437},
year = {2009}
}