Counting Pairs of Faint Galaxies

Abstract

The number of close pairs of galaxies observed to faint magnitude limits, when compared to nearby samples, determines the interaction or merger rate as a function of redshift. The prevalence of mergers at intermediate redshifts is fundamental to understanding how galaxies evolve and the relative population of galaxy types. Mergers have been used to explain the excess of galaxies in faint blue counts above the numbers expected from no-evolution models. Using deep CFHT ($I\leq24$) imaging of a ``blank'' field we find a pair fraction which is consistent with the galaxies in our sample being randomly distributed with no significant excess of ``physical'' close pairs. This is contrary to the pair fraction of $34\%\pm9\%$ found by Burkey {\it et al.} for similar magnitude limits and using an identical approach to the pair analysis. Various reasons for this discrepancy are discussed. Colors and morphologies of our close pairs are consistent with the bulk of them being random superpositions although, as indicators of interaction, these criteria are uncertain due to contamination of field galaxies and difficulty in seeing structure at intermediate redshifts, respectively. This observed lack of close pairs is probably linked with the decreasing amplitude of the angular correlation function at faint limits. If our faint samples are comprised of galaxies which have a higher average redshift than brighter samples studied by other workers then either the merger rate has been overestimated or there is a change in its behaviour from what is observed at brighter magnitude limits.