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

The Eastern Banded Structure (EBS) and Hydra. I halo overdensities are very nearby (d similar to 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 (similar to 5-6 Gyr) and metallicity ([Fe/H] = -0.9 dex). We measure statistically significant spreads in both the iron and alpha-element abundances of sigma([Fe/H]) = 0.13 +/- 0.02 dex and sigma([alpha/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 [alpha/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.