Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy

Creators: Hargis, Jonathan R.; Kimmig, Brian; Willman, Beth; Caldwell, Nelson; Walker, Matthew G.; Strader, Jay; Sand, David J.; Grillmair, Carl J.; Yoon, Joo Heon

Abstract

The Eastern Banded Structure (EBS) and Hydra I halo overdensities are very nearby (d ~ 10 kpc) objects discovered in Sloan Digital Sky Survey (SDSS) data. Previous studies of the region have shown that EBS and Hydra I are spatially coincident, cold structures at the same distance, suggesting that Hydra I may be the EBS's progenitor. We combine new wide-field Dark Energy Camera (DECam) imaging and MMT/Hectochelle spectroscopic observations of Hydra I with SDSS archival spectroscopic observations to quantify Hydra I's present-day chemodynamical properties, and to infer whether it originated as a star cluster or dwarf galaxy. While previous work using shallow SDSS imaging assumed a standard old, metal-poor stellar population, our deeper DECam imaging reveals that Hydra I has a thin, well-defined main sequence turnoff of intermediate age (~5–6 Gyr) and metallicity ([Fe/H] = −0.9 dex). We measure statistically significant spreads in both the iron and alpha-element abundances of σ_[(Fe)/H}= 0.13 ± 0.02 dex and σ_[ɑ/Fe] = 0.09 ± 0.03 dex, respectively, and place upper limits on both the rotation and its proper motion. Hydra I's intermediate age and [Fe/H]—as well as its low [α/Fe], apparent [Fe/H] spread, and present-day low luminosity—suggest that its progenitor was a dwarf galaxy, which has subsequently lost more than 99.99% of its stellar mass.

Additional Information

© 2016 American Astronomical Society. Received 2015 September 25; accepted 2015 December 7; published 2016 February 5. B.W., J.R.H., and B.K. were supported by an NSF Faculty Early Career Development (CAREER) award (AST-1151462). M.G.W. is supported by National Science Foundation grants AST-1313045, AST-1412999. J.S. acknowledges support from NSF grant AST-1514763. J.R.H. would like to acknowledge useful conversations with Keith Hawkins, Ting Li, and Jennifer Marshall during the course of this work. J.R.H. would also like to thank Kathy Vivas both for useful conversations and observing support during the DECam observing at CTIO. We thank the anonymous referee for very helpful comments on the manuscript. This work was supported in part by National Science Foundation Grant No. PHYS-1066293 and the hospitality of the Aspen Center for Physics. This project used data obtained with the Dark Energy Camera, which was constructed by the Dark Energy Survey collaboration. Funding for the DES Projects has been provided by the DOE and NSF(USA), MISE(Spain), STFC(UK), HEFCE(UK). NCSA(UIUC), KICP(U. Chicago), CCAPP(Ohio State), MIFPA(Texas A&M), CNPQ, FAPERJ, FINEP (Brazil), MINECO(Spain), DFG(Germany) and the collaborating institutions in the Dark Energy Survey, which are Argonne Lab, UC Santa Cruz, University of Cambridge, CIEMAT-Madrid, University of Chicago, University College London, DES-Brazil Consortium, University of Edinburgh, ETH Zurich, Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE Barcelona, Lawrence Berkeley Lab, LMU Munchen and the associated Excellence Cluster universe, University of Michigan, NOAO, University of Nottingham, Ohio State University, University of Pennsylvania, University of Portsmouth, SLAC National Lab, Stanford University, University of Sussex, and Texas A&M University.

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