STAR COUNTS IN THE HUBBLE DEEP FIELD - CONSTRAINING GALACTIC STRUCTURE MODELS

Stellar sources are identified in the Hubble Deep Field, and accurate colours and magnitudes are presented. The predictions of a Galactic st ar counts model are compared with the faint stellar counts in this fie ld. The model reproduces the observations very well in tile magnitude range 21.0 < V less than or equal to 26.4, while it overpredicts the c ounts by a factor of 4 in the range 26.4 < V less than or equal to 30. 0. The luminosity function for halo objects must be a factor of 2 smal ler 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 g lobular clusters, is therefore confirmed for the halo field population , The possible nature of a group of faint blue objects is also investi gated, concluding that they are most likely non-stellar. The possibili ty that they are QSOs is ruled out, If we insist upon their stellar na ture, they would be halo white dwarfs, with either a very steep halo w hite dwarf luminosity function fur M(v) > + 11.0, or a stellar density 0.4 times that of the disc white dwarfs in the solar neighbourhood.