The nature of the faint galaxies in the Hubble Deep Field

Abstract

We present a study of the galaxies found in the Hubble Deep Field (HDF). A high proportion of HDF galaxies are undergoing a strong episode of star formation, as evidenced by their very blue colours. A wide range of morphological types is found, with a high proportion of peculiar and merger morphologies. Fitting the multiband spectra with redshifted spectral energy distributions of galaxy types E to HII, we predict the spectral types and redshifts of galaxies detected in the HDF. We predict a median redshift of 2.1, with 72 per cent having <e1>z<~</e1>1 and 5 per cent with <e1>z<~</e1>3. The I-band absolute magnitude distributions as a function of galaxy type show a plausible trend of decreasing luminosity towards later types. The derived I-band luminosity function agrees well with that from the Canada-France survey for <e1>z<</e1>1, and shows evidence for luminosity evolution (at <e1>M_I<</e1>-21) in the range 2<<e1>z<</e1>3, smaller than the rate seen in quasars and starburst galaxies. We have predicted infrared and submillimetre fluxes, assuming that most of the galaxies are undergoing a strong starburst. Several planned space-borne and ground-based deep surveys are capable of detecting interesting numbers of HDF galaxies.