Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge
  We report the discovery of a nearby dwarf galaxy in the constellation of
Hydrus, between the Large and the Small Magellanic Clouds. Hydrus 1 is a mildy
elliptical ultra-faint system with luminosity $M_V\sim$ -4.7 and size $\sim$ 50
pc, located 28 kpc from the Sun and 24 kpc from the LMC. From spectroscopy of
$\sim$ 30 member stars, we measure a velocity dispersion of 2.7 km/s and find
tentative evidence for a radial velocity gradient consistent with 3 km/s
rotation. Hydrus 1's velocity dispersion indicates that the system is dark
matter dominated, but its dynamical mass-to-light ratio M/L $\sim$ 66 is
significantly smaller than typical for ultra-faint dwarfs at similar
luminosity. The kinematics and spatial position of Hydrus~1 make it a very
plausible member of the family of satellites brought into the Milky Way by the
Magellanic Clouds. While Hydrus 1's proximity and well-measured kinematics make
it a promising target for dark matter annihilation searches, we find no
evidence for significant gamma-ray emission from Hydrus 1. The new dwarf is a
metal-poor galaxy with a mean metallicity [Fe/H]=-2.5 and [Fe/H] spread of 0.4
dex, similar to other systems of similar luminosity. Alpha-abundances of Hyi 1
members indicate that star-formation was extended, lasting between 0.1 and 1
Gyr, with self-enrichment dominated by SN Ia. The dwarf also hosts a highly
carbon-enhanced extremely metal-poor star with [Fe/H] $\sim$ -3.2 and [C/Fe]
$\sim$ +3.0.
