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[abridged] We present MMT/Megacam imaging in Sloan $g$ and $r$ of theextremely low luminosity Bo\"otes II Milky Way companion. We use a bootstrapapproach to perform robust measurements of, and uncertainties on, Bo\"otes II'sdistance, luminosity, size, and morphology. We show that Bo\"otes II's stellarpopulation is old and metal-poor ([Fe/H] $\lta$ -2). Assuming a stellarpopulation like that of M92, Bo\"otes II is at a distance of 42 $\pm$ 2 kpc,closer than the initial published estimate of 60 $\pm$ 10 kpc. This distancerevision, combined with a more robust measurement of Bo\"otes II's structurewith a Plummer model (exponential model) results in a more compact half-lightsize of $r_h\simeq 36 (33) \pm 9 (10)$ pc and lower luminosity of$M_V\simeq-2.4 (-2.2) \pm 0.7 (0.7)$ mag. This revised size and luminosity moveBo\"otes II into a region of size-luminosity space not previously known to beoccupied by old stellar populations, but also occupied by the recentlydiscovered Milky Way satellites Willman 1 and SEGUE 1. We show that theapparently distorted morphology of Bo\"otes II is not statistically significantgiven the present data. We use a tidal argument to support a scenario whereBo\"otes II is a dwarf galaxy (dark matter dominated) rather than a globularcluster (not dark matter dominated). However, we can not rule out that Bo\"otesII is a star cluster on the verge of disruption, such as Palomar 5.

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