Are head-on collisions cosmologically important?
It is now generally accepted that galaxies were smaller at high redshift and that local universe galaxies (i.e. with small lookback times and hence current objects rather than early universe objects) are the product of collisions and accretion of smaller entities. The details of this picture remain quite unclear, however, but these collisions are mostly tidal rather than the head-on collisions that are the subject of this work. We now consider two environments, field and dense clusters, to briefly explore whether ISM-ISM collisions could play a role in galaxy evolution for a large number of galaxies.
