Boötes II reboöted: An MMT/Megacam study of an ultrafaint Milky Way satellite

We present MMT/Megacam imaging in Sloan g and r of the extremely low luminosity Bootes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and to calculate uncertainties on, Boötes II's distance, luminosity, size, and morphology. Comparisons with theoretical isochrones and empirical globular cluster fiducials show that Boötes II's stellar population is old and metal-poor ([Fe/H] ≲ -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 both a Plummer model and an exponential model, results in more compact inferred physical half-light sizes of r_h ≃ 36 ± 9 and 33 ± 10 pc, respectively, and lower-limit inferred luminosities of M_V ≃ -2.4 ± 0.7 and -2.2 ± 0.7 mag, respectively. Our revised size and luminosity calculations 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 in which Boötes II is a dwarf galaxy (dark matter-dominated) rather than a globular cluster (not dark matter-dominated), although our inferred uncertainty on the M/L for Boötes II is substantial. Moreover, we cannot rule out the possibility that Boötes II is a star cluster on the verge of disruption like Palomar 5.