It can be argued that no single event in the universe is as impressive and as physically altering as the collision of two galaxies. Galactic interactions and mergers are now recognized as playing a critical role in galactic evolution. Considering the immense vastness of space, collisions of these cosmic giants occurs more often than one might think. Due to their enormous size the average distance between galaxies is only 20 times greater than their diameter. The statistical outcome is that collisions will occur at least a few times in the life of an average galaxy. Collisions or mergers can be characterized as "major" when two equal mass galaxies collide, or "minor" when objects of lesser mass like a dwarf galaxy is absorbed by a larger neighbor. Galactic collisions may take hundreds of millions or even billions of years to complete. Close encounters that do not involve collisions can create less catastrophic changes such as distortion of spiral arms, tidal tails and plumes of expelled gas, and bursts of star formation. These may be the only signs of a previous encounter.
The merging galaxies NGC 4038 and 4039, popularly referred to as the "Antennae" are the prototypical model of merging galactic systems. The interaction between the two galaxies began between 300 and 450 million years ago and will likely end in the full merger of the two in some 300 million years. The "Antennae" have been investigated in detail at almost every wavelength. The effects of the merger can be observed on two separate fronts. The disturbed inner region comprising the central mass of the two colliding galaxies contain many young clusters of giant O and B type stars and even superclusters of massive stars arising from recently formed giant molecular clouds. The formation of superclusters of stars are a phenomenon clearly associated with galactic mergers. The sweeping tidal tails of the two galaxies contain gas and stars, mutually stripped from both galaxies that extend out almost 500,000 light years from their respective centers. Detailed studies of the tidal tails reveal many older stars present before the encounter but also new star clusters triggered as a direct result of the massive collision. There appears to be a very large gravitationally bound mass of new stars near the end of the southern tail. This stellar mass is so large it may qualify as an independent "Tidal Dwarf Galaxy", a new dwarf galaxy created through the tidal effects of the collision. This remains to be confirmed.
Simulations of galactic collisions and mergers are consistent with what we observe occurring in the Antennae merger. As the galaxies come together there is an inflow of gaseous material towards the inner merger remnant followed by periods of intense star formation which eventually depletes the gas over several hundred million years. As the gas collects centrally there is a tendency for a stellar bar to form suggesting that collisions may account for the origin of bar formation in the nuclei of galaxies. Along with the inflows, gas is ejected from the galaxies in giant arcs that can extend several hundred thousand light years into space. Star formation occurs in the arcs which is exactly what we see occurring in the Antennae merger.