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 ∼ 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[/Fe] = 0.09 ± 0.03 and 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 [/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.