paper

Bootes II ReBooted: An MMT/MegaCam Study of An Ultra-Faint Milky Way Satellite

arXiv:0712.3054 · doi:10.1086/592076

Abstract

[abridged] We present MMT/Megacam imaging in Sloan and of the extremely low luminosity Boötes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Boötes II's distance, luminosity, size, and morphology. We show that Boötes II's stellar population is old and metal-poor ([Fe/H] $\lta$ -2). Assuming a stellar population like that of M92, Boötes II is at a distance of 42 2 kpc, closer than the initial published estimate of 60 10 kpc. This distance revision, combined with a more robust measurement of Boötes II's structure with a Plummer model (exponential model) results in a more compact half-light size of pc and lower luminosity of mag. This revised size and luminosity move Boötes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and SEGUE 1. We show that the apparently distorted morphology of Boötes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Boötes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated). However, we can not rule out that Boötes II is a star cluster on the verge of disruption, such as Palomar 5.

arXiv comment10 pages, 8 figures. ApJ accepted. Conclusions are unchanged, but content and figures have changed substantively in this accepted version

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