The Evolution of the Optical Galaxy Luminosity Function -
  The optical luminosity function is a fundamental characterization of the galaxy population. A combination of earlier redshift surveys with two new surveys allows the first accurate determination of the evolution of the luminosity function with redshift, and reveals a marked steepening of the faint end slope. This effect is more profound for star-forming galaxies - there are 5-10 times as many star-forming galaxies at z~0.5 as there are locally. These results, together with high-resolution imaging and linewidth velocity measurements, support the view that the excess of star-forming galaxies at moderate redshift represents a general increase in the star-formation rate of normal galaxies rather than a distinct new population. This increase in star-formation appears in lower-L galaxies at lower redshift and only appears in L* galaxies at z>0.5. Imaging studies provide indirect evidence which suggests that interactions are responsible for a large part of this increased activity. 
Matthew Colless