Why is it so plausible that galactic mergers and tidal interactions were more frequent in the past? Several theoretical reasons come to mind:
Hierarchical clustering, in which small objects are progressively incorporated into larger structures , is common to many accounts of galaxy formation. In the ``core-halo'' picture , clustering of dark matter creates galaxy halos which subsequently accumulate cores of baryons, forming visible galaxies.
Tidal encounters generate short-lived features; a population of binary galaxies with highly eccentric orbits is required to explain the peculiar galaxies observed today . If these binaries have a flat distribution of binding energies, their merger rate has declined with time as t-5/3, and the 10 or so merging galaxies in the NGC catalog are but the most recent additions to a population of about 750 remnants .
Observations, though not always reaching the redshift range emphasized in this meeting, also imply rapid merging at high redshift:
Peculiar morphology becomes more common with increasing redshift . For example, the fraction of irregular galaxies in the CFRS survey increases from about 10% at z ~ 0.4 to a third at z ~ 0.8 .
Thus, both theory and observation support the notion that there was ``a great deal of merging of sizable bits and pieces (including quite a few lesser galaxies) early in the career of every major galaxy'' . But the nature of these early mergers is not so clear; were the objects involved dominated by dark matter, by gas, or by stars? And can we learn anything about early mergers by studying present-epoch examples?