Abstract

Using the Gaussian+Schechter composite luminosity functions measured from the ESO-Sculptor Survey (de Lapparent et al. 2003, A&A, 404, 831) and assuming that these functions do not evolve with redshift out to z ?- 1, we obtain evidence for evolution in the late spectral class containing late-type Spiral (Sc+Sd) and dwarf Irregular (dI) galaxies. There are indications that the Sc+Sd galaxies are the evolving population, but we cannot exclude that the dI galaxies also undergo some evolution. This evolution is detected as an increase of the Sc+Sd+dI galaxy density which can be modeled as either n(z) ? 1 + 3(z - 0.15) or n(z) ? (1 + z)2 using the currently favored cosmological parameters ?m = 0.3 and ? ?? = 0.7; the uncertainty in the linear and power-law evolution rates is of the order of unity. For ?m = 1.0 and = ??? = 0.0, the linear and power-law evolution rates are ?4 ± 1 and ?2.5 ± 1 respectively. Both models yield a good match to the ESS BVRc redshift distributions to 21-22 mag and to the number-counts to 23-23.5mag, which probe the galaxy distribution to redshifts z ? 0.5 and z ? 1.0 respectively. The present analysis shows the usefulness of the joint use of the magnitude and redshift distributions for studying galaxy evolution. It also illustrates how Gaussian+Schechter composite luminosity functions provide more robust constraints on the evolution rate than pure Schechter luminosity functions, thus emphasizing the importance of performing realistic parameterizations of the luminosity functions for studying galaxy evolution. The detected density evolution indicates that mergers could play a significant role in the evolution of late-type Spiral and dwarf Irregular galaxies. However, the ESO-Sculptor density increase with redshift could also be caused by a ?1mag brightening of the Sc+Sd+dI galaxies at z ? 0.5 and a ? 1.5-2.0 magbrightening at z ? 1, which is compatible with the expected passive brightening of Sc galaxies at these redshifts. Distinguishing between luminosity and density evolution is a major difficulty as these produce the same effect on the redshift and magnitude distributions. The detected evolution rate of the ESO-Sculptor Sc+Sd+dI galaxies is nevertheless within the range of measured values from the other existing analyses, whether they provide evidence for density or luminosity evolution.