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

February 2016 • 2016ApJ...818...39H

Authors • 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 }_{[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 has subsequently lost more than 99.99% of its stellar mass.

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Carl Grillmair

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