Star counts in the Hubble Deep Field: constraining galactic structure models.

Abstract

Stellar sources are identified in the Hubble Deep Field, and accurate colours and magnitudes are presented. The predictions of a Galactic star counts model are compared with the faint stellar counts in this field. The model reproduces the observations very well in the magnitude range 21.0 < V ≤ 26.4, while it overpredicts the counts by a factor of 4 in the range 26.4 < V ≤ 30.0. The luminosity function for halo objects must be a factor of 2 smaller than that predicted by an extrapolation of the solar neighbourhood luminosity function for disc stars (with 95 per cent confidence level). This result, seen before in deep Hubble Space Telescope images of globular clusters, is therefore confirmed for the halo field population. The possible nature of a group of faint blue objects is also investigated, concluding that they are most likely non-stellar. The possibility that they are QSOs is ruled out. If the authors insist upon their stellar nature, they would be halo white dwarfs, with either a very steep halo white dwarf luminosity function for M_v> +11.0, or a stellar density 0.4 times that of the disc white dwarfs in the solar neighbourhood.