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

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)5343-5361
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume479
Issue number4
DOIs
StatePublished - Oct 1 2018

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Keywords

  • Galaxies: dwarf
  • Galaxy: halo
  • Globular clusters: general
  • Local group
  • Magellanic Clouds
  • Stars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Koposov, S. E., Walker, M. G., Belokurov, V., Casey, A. R., Geringer-Sameth, A., Mackey, D., Da Costa, G., Erkal, D., Jethwa, P., Mateo, M., Olszewski, E. W., & Bailey, J. I. (2018). Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge. Monthly Notices of the Royal Astronomical Society, 479(4), 5343-5361. https://doi.org/10.1093/mnras/sty1772