The Eastern Banded Structure (EBS) and Hydra~I halo overdensity are very nearby (d $sim$ 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 EBSs progenitor. We combine new wide-field DECam imaging and MMT/Hectochelle spectroscopic observations of Hydra I with SDSS archival spectroscopic observations to quantify Hydra Is 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 ($sim 5-6$ Gyr) and metallicity ([Fe/H] = $-0.9$ dex). We measure statistically significant spreads in both the iron and alpha-element abundances of $sigma_{[Fe/H]} = 0.13 pm 0.02$ dex and $sigma_{[alpha/{rm Fe}]} = 0.09 pm 0.03$ dex, respectively, and place upper limits on both the rotation and its proper motion. Hydra~Is 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 subsequently lost more than $99.99%$ of its stellar mass.
Download