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.
@article{arxiv.1509.06391,
title = {Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy},
author = {Jonathan R. Hargis and B. Kimmig and B. Willman and N. Caldwell and M. G. Walker and J. Strader and D. J. Sand and C. J. Grillmair and J. H. Yoon},
journal= {arXiv preprint arXiv:1509.06391},
year = {2016}
}