* Astronomy

Title: Segue 2: A Prototype of the Population of Satellites of Satellites
Authors: V. Belokurov (1), M.G. Walker (1), N.W. Evans (1), G. Gilmore (1), M.J. Irwin (1), M. Mateo (2), L. Mayer (3), E. Olszewski (4), J. Bechtold (4), T. Pickering (5) ((1) Cambridge, (2) Univ of Michigan, (3) Zurich, (4) Steward Observatory, (5) MMT Observatory)

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius of 34 pc and a systemic velocity of -40 km/s. Our MMT data also provides evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about 7 km/s. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites -- survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionisation epoch.

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