With the discovery of the nature of galaxies, the first hypothesis developed to explain their existence was one of gravitational collapse in the primordial gas. As the forming galaxies grew smaller, the gas tended to fall into a flat plane, with fragmentation into stars occurring during both the collapse phase and continuing after formation of the final disk. The formation of a galaxy was completed when the mass distribution came into equilibrium between motions and gravity. The differentiation between the types of galaxies was thought to have been the result of initial conditions. If lots of angular momentum were present, a disk galaxy was produced. If initially there was little angular momentum, all matter became stars during the collapse phase, resulting in an elliptical galaxy.
Observational and theoretical work in more recent times has shown that galaxy formation is a much more complicated process. First, the efficiency of star formation is low. As a result, elliptical galaxies cannot be produced as was once thought; galaxy formation produces disk galaxies with significant interstellar material left over. Second, interactions between galaxies over the history of the universe can be significant. Galaxies do merge, and they cannibalize smaller companions. Violent interactions between disk galaxies appear to randomize motions and also to efficiently convert colliding interstellar gas into stars, leaving behind gas-free elliptical galaxies. Galaxies that may have grown in size, but avoided major disruptive encounters, appear to have evolved into the spectrum of spiral galaxies that exist today. Gentle encounters between two gassy disk galaxies are possible, and these encounters leave their fundamental stellar distributions unchanged but result in the gas being swept out, thus producing the relative rare, flat, gas-free galaxies known as S0s.
It is now hypothesized that the early era of galaxies was much more turbulent than today's universe. The process of producing equilibrium galaxies was associated with the growth of massive, nonstellar black holes in the nuclei. The liberation of tremendous energies during their formative stages is observed as quasars, but quasars died when galaxies achieved their equilibrium structures and ended mass infall into the centers. When new mass falls into the centers of galaxies, the central black hole phenomena can be re-ignited, explaining the active galactic nuclei of the present day.
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