A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules

Free access article

Issue		A&A Volume 506, Number 3, November II 2009


Page(s)		1147 - 1168
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361/200912718
Published online		27 August 2009

A&A 506, 1147-1168 (2009)
DOI: 10.1051/0004-6361/200912718

Metallicity, velocities, and a clean list of RGB members

D. Adén¹, S. Feltzing¹, A. Koch², M. I. Wilkinson², E. K. Grebel³, I. Lundström¹, G. F. Gilmore⁴, D. B. Zucker^{5, 6}, V. Belokurov⁴, N. W. Evans⁴, and D. Faria⁴

¹ Lund Observatory, Box 43, 22100 Lund, Sweden
e-mail: daniel.aden@astro.lu.se
² Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
³ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
⁴ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
⁵ Department of Physics, Macquarie University, North Ryde, NSW 2109, Australia
⁶ Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia

Received 18 June 2009 / Accepted 13 August 2009

Abstract
Context. Dwarf spheroidal (dSph) galaxies are the least luminous, least massive galaxies known. Recently, the number of observed galaxies in this class has greatly increased thanks to large surveys. Determining their properties, such as mass, luminosity and metallicity, provides key information in our understanding of galaxy formation and evolution.
Aims. 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.
Methods. Strömgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars (via the Strömgren c₁ 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.
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 $-2.35 \pm 0.31$ 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 $45.20 \pm 1.09 {\rm\, km\, s^{-1}}$ with a dispersion of 3.72 km s^-1, 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 $V_0=21.17 \pm 0.05$ , which corresponds to a distance of 147⁺⁸_-7 kpc.
Conclusions. When studying sparsely populated and/or heavily foreground contaminated dSph galaxies it is necessary to include knowledge about the evolutionary stage of the stars. This can be done in several ways. Here we have explored the power of the $c_{{\rm 1}}$ index in Strömgren photometry. This index is able to clearly identify red giant branch stars redder than the horizontal branch, enabling a separation of red giant branch dSph stars and foreground dwarf stars. Additionally, this index is also capable of correctly identifying both red and blue horizontal branch stars. 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.

Key words: galaxies: dwarf -- galaxies: fundamental parameters -- galaxies: individual: Hercules -- galaxies: kinematics and dynamics -- galaxies: photometry

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.