High-resolution imaging of distant clusters. I - Close pairs, interactions, and the 'Butcher-Oemler' effect at Z = 0.4

Abstract

New ground-based high-resolution images are presented of 23 blue galaxies whose membership has been confirmed spectroscopically in the two distant rich clusters Abell 370 and CL 0024+1654. The images have a stellar FWHM of 0.45 arcsec. Ten of these blue galaxies are found to have close companions, with separations on the scale of 1 to 2 arcsec. Several of the remaining blue galaxies also have peculiar morphologies and features, such as tidal tails, suggestive of interactions/mergers. At least 11 of the 23 blue galaxies are disk systems. It is suggested that galaxy-galaxy interactions, and possibly mergers, play an important role in the starburst mechanism responsible for the Butcher-Oemler effect at a redshift of about 0.4. It is concluded that galaxy-galaxy interactions/mergers are a primary mechanism responsible for the Butcher-Oemler effect over the range of redshifts studied to date.