The Morphologically Divided Redshift Distribution of Faint Galaxies

We have constructed a morphologically divided redshift distribution of faint field galaxies using a statistically unbiased sample of 196 galaxies brighter than I=21.5 for which detailed morphological information (from the Hubble Space Telescope) as well as ground-based spectroscopic redshifts are available. Galaxies are classified into three rough morphological types according to their visual appearance (E/S0s, spirals, Sdm/dE/Irr/Pec galaxies), and redshift distributions are constructed for each type. The most striking feature is the abundance of low- to moderate-redshift Sdm/dE/Irr/Pec galaxies at I<19.5. This confirms that the faint-end slope of the luminosity function (LF) is steep (α<-1.4) for these objects. We also find that Sdm/dE/Irr/Pec galaxies are fairly abundant at moderate redshifts, and this can be explained by a strong luminosity evolution. However, the normalization factor (or the number density) of the LF of Sdm/dE/Irr/Pec galaxies is not much higher than that of the local LF of Sdm/dE/Irr/Pec galaxies. Furthermore, as we go to fainter magnitudes, the abundance of moderate- to high-redshift Irr/Pec galaxies increases considerably. This cannot be explained by strong luminosity evolution of the dwarf galaxy populations alone: these Irr/Pec galaxies are probably the progenitors of present-day ellipticals and spiral galaxies that are undergoing rapid star formation or merging with their neighbors. On the other hand, the redshift distributions of E/S0s and spirals are fairly consistent with those expected from passive luminosity evolution and are only in slight disagreement with the nonevolving model.