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

TL;DR

This study uses MMT/Megacam imaging to refine the properties of Bo"otes II, revealing it as a compact, old, metal-poor satellite with implications for its classification as a dwarf galaxy or star cluster.

Contribution

The paper provides a more accurate distance, size, and luminosity measurement of Bo"otes II, and discusses its nature using structural and tidal analysis, which was not previously established.

Findings

Bo"otes II is closer than previously thought at 42 kpc.

It has a smaller size and lower luminosity than earlier estimates.

Its properties place it among recently discovered faint satellites like Willman 1 and SEGUE 1.

Abstract

[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] $\lta$ -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.