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

Sergey E. Koposov, Matthew G. Walker, Vasily Belokurov, Andrew R. Casey, Alex Geringer-Sameth, Dougal Mackey, Gary Da Costa, Denis Erkal, Prashin Jethwa, Mario Mateo, Edward W. Olszewski, John I. Bailey

Research output: Contribution to journalArticlepeer-review


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 MV ∼-4.7 and size ∼ 50 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 kms-1and find tentative evidence for a radial velocity gradient consistent with 3 kms-1rotation. Hydrus 1's velocity dispersion in-dicates that the system is dark matter dominated, but its dynamical mass-to-light ratio M/L ∼ 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 plausi-ble 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 promis-ing 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]∼-3.2 and [C/Fe] ∼ +3.0.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Apr 17 2018


  • Galaxies: Dwarf
  • Galaxies: Local Group
  • Galaxy: Globular clusters: General
  • Galaxy: Halo
  • Magellanic Clouds
  • Stars: General

ASJC Scopus subject areas

  • General

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