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 (LMC) and the Small Magellanic Clouds (SMC). Hydrus 1 is a mildly elliptical ultrafaint system with luminosity MV ~-4.7 and size 53 ± 3 pc, located 28 kpc from the Sun and 24 kpc from the LMC. From spectroscopy of ~30 member stars, we measure a velocity dispersion of 2.7 ± 0.5 kms-1 and find tentative evidence for a radial velocity gradient consistent with 3 kms-1 rotation. Hydrus 1's velocity dispersion indicates that the system is dark matter dominated, but its dynamical mass-to-light ratio M/L=66+29 -20 is significantly smaller than typical for ultrafaint 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 kinematicsmake it a promising target for darkmatter 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] standard deviation of 0.4 dex, similar to other systems of similar luminosity. α abundances of Hyi 1 members indicate that star formation was extended, lasting between 0.1 and 1 Gyr, with self-enrichment dominated by supernova Ia. The dwarf also hosts a highly carbon-enhanced extremely metal-poor star with [Fe/H]~-3.2 and [C/Fe] ~ +3.0.ARC acknowledges support from the Australian Research Council (ARC) through Discovery Project grant DP160100637. DM is supported by an ARC Future Fellowship (FT160100206). DM and GDC acknowledge support from ARC Discovery Project DP150103294. MM acknowledges support from NSF grant AST1312997. MGW acknowledges support from NSF grant AST1412999. E.O. acknowledges support from NSF grant AST1313006. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement number 308024