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Nils Ryde. Photo.

Nils Ryde

Professor

Nils Ryde. Photo.

Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies

Author

  • Sergey E. Koposov
  • Andrew R. Casey
  • Vasily Belokurov
  • James R. Lewis
  • Gerard Gilmore
  • Clare Worley
  • Anna Hourihane
  • S. Randich
  • Thomas Bensby
  • A. Bragaglia
  • M. Bergemann
  • G. Carraro
  • M. T. Costado
  • E. Flaccomio
  • P. Francois
  • U. Heiter
  • V. Hill
  • P. Jofre
  • C. Lando
  • A. C. Lanzafame
  • P. de Laverny
  • L. Monaco
  • L. Morbidelli
  • L. Sbordone
  • S. Mikolaitis
  • Nils Ryde

Summary, in English

We report on VLT/GIRAFFE spectra of stars in two recently discovered ultra-faint satellites, Reticulum 2 and Horologium 1, obtained as part of the Gaia-ESO Survey. We identify 18 members in Reticulum 2 and five in Horologium 1. We find Reticulum 2 to have a velocity dispersion of 3.22(-0.49)(+1.64) km s(-1) , implying a mass-to-light ratio (M/L) of similar to 500. The mean metallicity of Reticulum 2 is [Fe/H] = -2.46, with an intrinsic dispersion of similar to 0.3 dex and alpha-enhancement of similar to 0.4 dex. We conclude that Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 with [Fe/H] similar to -2 and V-hel similar to 220 km s(-1), far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of sigma (V) = 4.9(-0.9)(+2.8) km s(-1) and a M/L ratio of similar to 600, leading us to conclude that Horologium 1 is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum 2 ([Fe/H] = -2.76) and is similarly alpha-enhanced: [alpha/Fe] similar to 0.3 dex with a significant spread of metallicities of 0.17 dex. The line-of-sight velocity of Reticulum 2 is offset by 100 km s(-1) from the prediction of the orbital velocity of the Large Magellanic Cloud (LMC), thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward-integrated orbit of the LMC and its halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is possible that Horologium 1 is or once was a member of the Magellanic family.

Department/s

  • Lund Observatory

Publishing year

2015

Language

English

Publication/Series

Astrophysical Journal

Volume

811

Issue

1

Document type

Journal article

Publisher

American Astronomical Society

Topic

  • Astronomy, Astrophysics and Cosmology

Keywords

  • stars: abundances
  • globular clusters: general
  • Galaxy: halo
  • dynamics
  • galaxies: kinematics and
  • galaxies: dwarf
  • galaxies: abundances

Status

Published

ISBN/ISSN/Other

  • ISSN: 0004-637X