The Star Formation History and Extended Structure of the Hercules Milky Way Satellite
Abstract
We present imaging of the recently discovered Hercules Milky Way satellite and its surrounding regions to study its structure, star formation history and to thoroughly search for signs of disruption. We robustly determine the distance, luminosity, size, and morphology of Hercules utilizing a bootstrap approach to characterize our uncertainties. We derive a distance to Hercules via a comparison to empirical and theoretical isochrones, finding a best match with the isochrone of M92, which yields a distance of 133 ± 6 kpc. As previous studies have found, Hercules is very elongated, with epsilon = 0.67 ± 0.03 and a half-light radius of rh sime 230 pc. Using the color-magnitude-fitting package StarFISH, we determine that Hercules is old (>12 Gyr) and metal-poor ([Fe/H] ~ -2.0), with a spread in metallicity, in agreement with previous spectroscopic work. This result is robust with respect to slight variations in the distance to Hercules and mismatches between the observed Hercules color-magnitude diagram and theoretical isochrones. We infer a total absolute magnitude of MV = -6.2 ± 0.4. Our innovative search for external Hercules structure both in the plane of the sky and along the line of sight yields some evidence that Hercules is embedded in a larger stream of stars. A clear stellar extension is seen to the northwest with several additional candidate stellar overdensities along the position angle of Hercules out to ~35' (~1.3 kpc). While the association of any of the individual stellar overdensities with Hercules is difficult to determine, we do show that the summed color-magnitude diagram of all three is consistent with Hercules' stellar population. Finally, we estimate that any change in the distance to Hercules across its face is at most ~6 kpc, and the data are consistent with Hercules being at the same distance throughout.
Based on data acquired using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the US, Italy, and Germany. LBT Corporation partners are the University of Arizona, on behalf of the Arizona university system; Instituto Nazionale do Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute of Postdam, and Heidelberg University; Ohio State University, and the Research Corporation, on behalf of the University of Notre Dame, the University of Minnesota, and the University of Virginia.- Publication:
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The Astrophysical Journal
- Pub Date:
- October 2009
- DOI:
- 10.1088/0004-637X/704/2/898
- arXiv:
- arXiv:0906.4017
- Bibcode:
- 2009ApJ...704..898S
- Keywords:
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- galaxies: dwarf;
- Local Group;
- Astrophysics - Cosmology and Extragalactic Astrophysics;
- Astrophysics - Galaxy Astrophysics
- E-Print:
- 50 pages, 15 figures, submitted to the Astrophysical Journal