HIGH-RESOLUTION IMAGING OF FAINT BLUE GALAXIES

We have used HRCam on the CFHT to obtain subarcsec images of 26 galaxi es 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 g alaxies of the comparison sample have equivalent widths of less than 1 0 angstrom. By fitting exponential discs or r1/4 bulges to B, V and I images, we derive scalelengths for the blue and red stellar population s 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 c ompanions in order to determine whether the star formation might be li nked to tidal interactions or mergers. We find that these moderate-red shift galaxies generally have straightforward low-redshift analogues, in that their colours, sizes and luminosities are consistent with thos e of various types of z almost-equal-to 0 galaxies. The star-forming o bjects have structural components consistent with the full range of pr esent-day disc galaxies, and absolute magnitudes spanning the range M - 1 to M + 5. Some of these galaxies have star formation concentrate d in their nuclei, but most have star formation occurring across the e ntire disc. We find companions at projected distances closer than 10 h -1 kpc for 30 per cent of the galaxies with enhanced star formation, w hereas none of the galaxies in the comparison sample has such close co mpanions. This fraction is very similar to the 40 per cent excess in t he number of star-forming galaxies found in the redshift survey of Col less et al., and provides the first direct evidence linking interactio ns or mergers to the increased fraction of field galaxies with enhance d star formation at moderate redshifts.