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 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 ( \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 / { Fe } ] } = 0.09 \pm 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 .