3.2. Galaxies with late-type morphologies as the major evolving component
Computation of the N(z) redshift distributions and luminosity functions for different morphological types has shown (Brinchmann et al. 1998, hereafter Paper 1) that the degree of evolutionary change increases from elliptical through spiral to irregular/peculiar. Furthermore, the steep number count N(m) of the irregular/peculiar galaxies in the earlier studies is now clearly seen to be due to the appearance of these galaxies at high redshifts rather than the alternative possibility of a steep local luminosity function. An important point is that the redshift information for individual galaxies now enables us to quantify the effects of the redshift on the morphologies (i.e., the morphological k-correction). The decreasing rest-wavelength generally makes objects appear of later type at higher redshifts, but in Paper 1, we show that this effect plays only a small part in the observed shift to irregular/peculiar morphologies at high redshift.
Of course, the morphology of a galaxy can change with time, and the irregular/peculiar galaxies seen at high luminosities at high redshift are not necessarily the antecedents of galaxies with similar appearance seen in the local Universe at the present epoch. The morphology of a galaxy really tells us only about its dynamical state and the nature of star-formation activity at the time that it is observed. The largest change in the galaxy population is associated with star-formation activity that is occurring in irregular morphological structures.