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

[abridged] We present MMT/Megacam imaging in Sloan $g$ and $r$ of the extremely low luminosity Bo"otes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Bo"otes II’s distance, luminosity, size, and morphology. We show that Bo"otes II’s stellar population is old and metal-poor ([Fe/H] $\text{lta}$ -2). Assuming a stellar population 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 distance revision, combined with a more robust measurement of Bo"otes II’s structure with a Plummer model (exponential model) results in a more compact half-light size 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 move Bo"otes 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 Bo"otes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Bo"otes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated). However, we can not rule out that Bo"otes II is a star cluster on the verge of disruption, such as Palomar 5.