A multivariate analysis of galaxies in the Hubble Deep Field-North

We use the ultraviolet and optical WFPC2 and near-infrared NICMOS images of the Hubble Deep Field-North to measure and statistically compare an array of parameters for over 250 of the galaxies it contains. These parameters in clude redshift, rest-frame visible asymmetry and concentration, bolometric luminosity, and extinction-corrected star formation rate. We find only one strong correlation, between bolometric luminosity and star formation rate, from which early-type galaxies noticeably deviate. When our asymmetry measu rements are combined with those of a sample of nearby galaxies covering the full Hubble sequence, we find a weak correlation between redshift and rest -frame visible asymmetry, consistent with the qualitative evidence of galax y morphological evolution from these and other deep Hubble Space Telescope images. The mean values of these asymmetry measurements show a monotonic in crease with redshift interval over the range 0 less than or similar to z le ss than or similar to 2, increasing by a factor of approximately 3. If this trend is real, it suggests that galaxy morphological evolution within the last similar to 70% of the Hubble time is a gradual process that is continu ing through the present cosmological epoch. There is evidence that the domi nant source of this evolution is the "minor" mergers of disk galaxies with smaller companions, which could also transform late-type spiral galaxies to early-type spiral galaxies. Interestingly, in contrast to local galaxies w e find no correlations between galaxy star formation rate and either UV or visible asymmetry. This could arise if the star formation of high-redshift galaxies proceeds in episodes that are short (similar to 100 Myr) relative to the timescales over which galaxy mergers produce strong asymmetries (sim ilar to 500 Myr), a result suggested by the high star formation rates of Ly man break galaxies.