The stellar content of the Hamburg/ESO survey^*

V. The metallicity distribution function of the Galactic halo

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: $\!$[tschoerck;fzickgraf;dreimers]$\!$@hs.uni$\!$-hamburg$\!$.$\!$de
² Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany e-mail: N.Christlieb@lsw.uni-heidelberg.de
³ Department of Physics and Astronomy, Uppsala University, Box 515, 75120 Uppsala, Sweden e-mail: [barklem;be]@fysast.uu.se
⁴ Palomar Observatory, Mail Code 105-24, California Institute of Technology, Pasadena, CA 91125, USA e-mail: jlc@astro.caltech.edu
⁵ Department of Physics and Astronomy, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA e-mail: beers@pa.msu.edu
⁶ Carnegie Observatories of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA e-mail: [shec;ian;andy]@ociw.edu
⁷ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611, Australia e-mail: [bessell;jen]@mso.anu.edu.au
⁸ Centro de Astrofisica da Univ. Porto, Rua das Estrelas, 4150-762 Porto, Portugal e-mail: jorge@astro.up.pt
⁹ IPAC, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125, USA e-mail: solange@ipac.caltech.edu
¹⁰ Anglo-Australian Observatory; PO Box 296, Epping, NSW 1710, Australia e-mail: [dmj;cpc;mh;ksr;fgw]@aao.gov.au
¹¹ MPI for Gravitational Physics, Albert-Einstein-Institute, Am Mḧlenberg, 14476 Golm, Germany e-mail: Berit.Behnke@aei.mpg.de
¹² Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany e-mail: cfech@astro.physik.uni-potsdam.de
¹³ McDonald Observatory, The University of Texas at Austin, 1 University Station, C1400, Austin, TX 78712-0259, USA e-mail: anna@astro.as.utexas.edu
¹⁴ Astrophysical Institute Postsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: lutz@aip.de

Received: 6 September 2008
Accepted: 7 September 2009

Abstract

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely “best and brightest stars first”. Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., ) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., ) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for , but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of reproduces the sharp drop at present in the HES MDF. Although currently about ten stars at are known, the evidence for the existence of a tail of the halo MDF extending to is weak from the sample considered in this paper, because it only includes two stars . Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.