HIGH-RESOLUTION IMAGING OF FAINT BLUE GALAXIES

We have used HRCam on the CFHT to obtain subarcsec images of 26 galaxies with z = 0.1-0.7 from the redshift survey of Colless et al. The 17 galaxies of the primary sample have enhanced star formation indicated by [O II] equivalent widths greater than 20 angstrom, while the nine galaxies of the comparison sample have equivalent widths of less than 10 angstrom. By fitting exponential discs or r1/4 bulges to B, V and I images, we derive scalelengths for the blue and red stellar populations and so establish the location of the star formation (in the nucleus or the disc) for each galaxy. We have also searched for nearby faint companions in order to determine whether the star formation might be linked to tidal interactions or mergers. We find that these moderate-redshift galaxies generally have straightforward low-redshift analogues, in that their colours, sizes and luminosities are consistent with those of various types of z almost-equal-to 0 galaxies. The star-forming objects have structural components consistent with the full range of present-day disc galaxies, and absolute magnitudes spanning the range M* - 1 to M* + 5. Some of these galaxies have star formation concentrated in their nuclei, but most have star formation occurring across the entire disc. We find companions at projected distances closer than 10 h-1 kpc for 30 per cent of the galaxies with enhanced star formation, whereas none of the galaxies in the comparison sample has such close companions. This fraction is very similar to the 40 per cent excess in the number of star-forming galaxies found in the redshift survey of Colless et al., and provides the first direct evidence linking interactions or mergers to the increased fraction of field galaxies with enhanced star formation at moderate redshifts.