Some of the grandest encounters in the Universe occur when galaxies pass by one another. In the jetsam of their tidal debris, we often find circumgalactic evidence for close encounters. Sometimes the outer parts of a galaxy pair are distorted into long bridges and tails. We also find large irregular galaxies which have multiple nuclei, perhaps the recent result of a merger.

For a long time, interest in the subject of galaxy merging was quenched by a straightforward nσvT calculation which gave a very small merger probability over the Hubble age T of the Universe. This result does not depend much on the actual value of the Hubble constant. If the observed value of H were larger the volume would decrease, so the observed number density n ∝ H3. Supposing the merger cross section to be roughly proportional to the observed visible cross section gives σ ∝ H-2. (Increasing H means the galaxy is closer and has a smaller linear visible diameter.) The Hubble age T≈H-1. Therefore the resulting probability remains small, regardless of the uncertain current value of H.

The apparent conclusiveness of this argument, however, resulted from the unjustified simplicity of its assumptions. Merging encounters of galaxies are not like random collisions in a gas of particles. For one thing, most mergers within groups and clusters of galaxies occur among the more massive galaxies which tend to collect in the center through mass segregation (Section 46).