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

S. M. Walsh, B. Willman, D. Sand, J. Harris, A. Seth, D. Zaritsky, H. Jerjen

Research output: Contribution to journalArticlepeer-review

50 Scopus citations


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 rh ≃ 36 ± 9 and 33 ± 10 pc, respectively, and lower-limit inferred luminosities of MV ≃ -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.

Original languageEnglish (US)
Pages (from-to)245-253
Number of pages9
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 20 2008


  • Galaxies: dwarf
  • Local group

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'Boötes II reboöted: An MMT/Megacam study of an ultrafaint Milky Way satellite'. Together they form a unique fingerprint.

Cite this