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A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules. Metallicity, velocities and a clean list of RGB members

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Author

  • Daniel Adén
  • Sofia Feltzing
  • A. Koch
  • M.I. Wilkinson
  • E.K. Grebel
  • Ingemar Lundström
  • G.F. Gilmore
  • D.B. Zucker
  • V. Belukurov
  • N.W. Evans
  • Daniel Faria
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Summary, in English

Our aim is to provide as clean and as complete a sample as possible of red giant branch stars that are members of the Hercules dSph galaxy. With this sample we explore the velocity dispersion and the metallicity of the system. Stromgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars (via the Stromgren c_1 index) and their radial velocities. Based on this information we have selected a clean sample of red giant branch stars, and show that foreground contamination by Milky Way dwarf stars can greatly distort the results. Our final sample consists of 28 red giant branch stars in the Hercules dSph galaxy. Based on these stars we find a mean photometric metallicity of -2.35 dex which is consistent with previous studies. We find evidence for an abundance spread. Using those stars for which we have determined radial velocities we find a systemic velocity of 45.2 km/s with a dispersion of 3.72 km/s, this is lower than values found in the literature. Furthermore we identify the horizontal branch and estimate the mean magnitude of the horizontal branch of the Hercules dSph galaxy to be V_0=21.17, which corresponds to a distance of 147 kpc. We have shown that a proper cleaning of the sample results in a smaller value for the velocity dispersion of the system. This has implications for galaxy properties derived from such velocity dispersions.

Publishing year

2009

Language

English

Pages

1147-1168

Publication/Series

Astronomy & Astrophysics

Volume

506

Issue

3

Document type

Journal article

Publisher

EDP Sciences

Topic

  • Astronomy, Astrophysics and Cosmology

Keywords

  • galaxies: fundamental parameters
  • galaxies: dwarf
  • galaxies:
  • individual: Hercules
  • galaxies: kinematics and dynamics
  • photometry

Status

Published

Research group

  • Observational and Theoretical Astrophysics

ISBN/ISSN/Other

  • ISSN: 0004-6361