Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge




Koposov, Sergey
Walker, Matthew G
Belokurov, Vasily
Casey, Andrew
Geringer-Sameth, Alex
Mackey, Dougal
Da Costa, Gary
Erkal, D.
Jethwa, Prashin
Mateo, Mario

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Oxford University Press


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.



stars: general, globular clusters: general, Galaxy: halo, galaxies: dwarf, Local Group, Magellanic Clouds



Monthly Notices of the Royal Astronomical Society


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