High-redshift galaxies in the Hubble Deep Field - II. Colours and number counts

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) imaging survey. At faint magnitudes, the slope of the differential number-magnitude relation is flatter than 0.2 in all four HDF bandpasses. In the ultraviolet, a fluctuation analysis shows that the flattening observed below U300 ≈ 26 mag is not caused by incompleteness and is more pronounced than in the other bands, consistent with the idea that a redshift limit has been reached in the galaxy distribution. A reddening trend of ≈ 0.5 mag is observed at faint fluxes in the colour-magnitude diagram, (U300 - V606)eff versus V606. We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels (in the AB system) of AB ≈ 27 mag and in agreement with the fluctuation analysis and the colour-magnitude relation, about 7 per cent of the sources in U300, 30 per cent in B450 and 35 per cent in V606 are Lyman-break 'dropouts', i.e. candidate star-forming galaxies at z > 2. By integrating the number counts to the limits of the HDF survey we find that the mean surface brightness of the extragalactic sky is dominated by galaxies that are relatively bright and are known to have (z) ∼ 0.6. To AB ≈ 29 mag, the integrated light from resolved galaxies in the I-band is 2.1+0.4-0.3 × 10-20 erg cm-2 s-1 Hz-1 sr-1, and its spectrum is well described by a broken power law (Iv ∝ λ2 from 2000 to 8000 Å and Iv ∝ λ from 8000 to 22 000 Å). We discuss the predictions for the counts, colours and luminosity densities from standard low-q0 pure-luminosity-evolution models without dust obscuration, and find that they are unable to reproduce all the observed properties of faint field galaxies.