HALO MASS FUNCTION Low-mass Stars in Deep Fields

Deep fields are unique probes of the Galactic halo. With a limiting magnitude of, e.g., I = 30 mag, all stars down to the hydrogen-burning limit are detected out to distances of 10 kpc, while stars with 0.5 solar masses could be traced out to distances of 400 kpc. Thus deep fields provide an opportunity to study both the structure of the Galactic halo and the mass function of population (Pop) II and Pop III stars. I summarize previous work on the Galactic stellar halo utilizing the Hubble Deep Fields North and South, supplemented by a preliminary analysis of the recently released Hubble Ultra-Deep Field. 1. Motivation The structure and stellar content of the Galactic Halo directly points back to the formation of the Milky Way. Initial studies naturally focused on the Pop II main-sequence stars in the solar neighbourhood. These limits have been gradually extended towards fainter (i.e. lower mass and/or more distant) halo stars, including evolved stars, white dwarfs and a search for the baryonic dark matter component (e.g. Paczynski 1986, Alfonso et al. 2003, and references therein). Studies of stellar populations in deep fields aim at addressing the following issues: Stellar Content and Origin of the Galactic Halo (Global) Structure of the Galactic Halo Search for a (still self-luminous) baryonic component of the ”dark matter” halo Fraction of Pop III stars in the Galactic Halo, in particular stars with [Fe/H] < −4 (see, e.g., Oey 2003)