HIGH-REDSHIFT GALAXIES IN THE HUBBLE-DEEP-FIELD - II - COLORS AND NUMBER COUNTS

We discuss the deep galaxy counts from the Hubble Deep Field (HDF) ima ging 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 obs erved below U-300 approximate to 26 mag is not caused by incompletenes s 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 approximate to 0.5 mag is observed at faint flu xes in the colour-magnitude diagram, (U-300 - V-606)(eff) versus V-606 We interpret these results as the effect of intergalactic attenuation on distant galaxies. At flux levels tin the AB system) of AB approxim ate to 27 mag and in agreement with the fluctuation analysis and the c olour-magnitude relation, about 7 per cent of the sources in U-300, 30 per cent in B-450 and 35 per cent in V606 are Lyman-break 'dropouts', i.e. candidate star-forming galaxies at z > 2. By integrating the numb er counts to the limits of the HDF survey we find that the mean surfac e brightness of the extragalactic sky is dominated by galaxies that ar e relatively bright and are known to have (z) similar to 0.6. To AB ap proximate to 29 mag, the integrated light from resolved galaxies in th e I-band is 2.1(-0.3)(+0.4) x 10(-20) erg cm(-2) s(-1) Hz(-1) sr(-1), and its spectrum is well described by a broken power law (I-nu proport ional to lambda(2) from 2000 to 8000 Angstrom and I-nu proportional to lambda from 8000 to 22 000 Angstrom). We discuss the predictions for the counts, colours and luminosity densities from standard low-go pure -luminosity-evolution models without dust obscuration, and find that t hey are unable to reproduce all the observed properties of faint field galaxies.