Papers › Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy
Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy
Jonathan R. Hargis, B. Kimmig, B. Willman, N. Caldwell, M. G. Walker, J. Strader, D. J. Sand, C. J. Grillmair, J. H. Yoon
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The Eastern Banded Structure (EBS) and Hydra~I halo overdensity are very nearby (d ∼ 10 kpc) objects discovered in 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 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_([α/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 subsequently lost more than 99.99% of its stellar mass.
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