GALAXY MORPHOLOGY TO I=25 MAG IN THE HUBBLE DEEP FIELD

The morphological properties of galaxies in the range 21 < I < 25 mag in the Hubble Deep Field are investigated using a quantitative classif ication system based on measurements of the central concentration and asymmetry of galaxian light. The class distribution of objects in the Hubble Deep Field is strongly skewed towards highly asymmetric objects , relative to distributions from both the HST Medium Deep Survey at I < 22 mag and an artificially redshifted sample of local galaxies, The steeply rising number count-magnitude relation for irregular/peculiar/ merging systems at I < 22 mag reported by Glazebrook et al. continues to at least I = 25 mag. Although these peculiar systems are predominan tly blue at optical wavelengths, a significant fraction also exhibit r ed U-B colours, which may indicate that they are at high redshift. Bey ond Glazebrook et al,'s magnitude limit, the spiral counts appear to r ise more steeply than high-normalization no-evolution predictions, whe reas those of elliptical/SO galaxies only slightly exceed such predict ions and may turn over beyond I similar to 24 mag, These results are c ompared with those from previous investigations of faint galaxy morpho logy with HST, and the possible implications are briefly discussed, Th e large fraction of peculiar/irregular/merging systems in the Hubble D eep Field suggests that by I similar to 25 mag the conventional Hubble system no longer provides an adequate description of the morphologica l characteristics of a high fraction of field galaxies.