2007/12/31 by S. M. Walsh, B. Willman, D. Sand +4 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Dwarf galaxy #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Luminosity #Milky Way #Population #Star cluster #Stellar population #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/592076
10 pages, 8 figures. ApJ accepted. Conclusions are unchanged, but content and figures have changed substantively in this accepted version
arxiv created 2008/07/23 · openalex publication_date 2008/11/07 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present MMT/Megacam imaging in Sloan g and r of the extremely low luminosity Boötes 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.