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

Koposov, Sergey E, Walker, Matthew G, Belokurov, Vasily, Casey, Andrew R, Geringer-Sameth, Alex, Mackey, Dougal, Da Costa, Gary, Erkal, Denis, Jethwa, Prashin, Mateo, Mario , Olszewski, Edward W and Bailey III, John I (2018) Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge Monthly Notices of the Royal Astronomical Society.

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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 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 km s−1 and find tentative evidence for a radial velocity gradient consistent with 3 km s−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 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] standard deviation 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.

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Physics
Authors :
Koposov, Sergey E
Walker, Matthew G
Belokurov, Vasily
Casey, Andrew R
Geringer-Sameth, Alex
Mackey, Dougal
Da Costa, Gary
Jethwa, Prashin
Mateo, Mario
Olszewski, Edward W
Bailey III, John I
Date : 2018
Funders : EU FP7
Copyright Disclaimer : This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society, following peer review. The version of record will be available online at:
Uncontrolled Keywords : Magellanic Clouds – galaxies: dwarf – Galaxy: halo – Galaxy: globular clusters: general – Galaxies: Local Group – stars: general
Depositing User : Melanie Hughes
Date Deposited : 06 Sep 2018 11:14
Last Modified : 11 Dec 2018 11:24

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