In this paper, we explore the plausible luminosity evolution of early-type
galaxies in different cosmological models by constructing a set of pure
luminosity evolution (PLE) models via the choices of the star formation rate
(SFR) parameters and formation redshift zfβ of galaxies, with the
observational constraints derived from the Hubble Space Telescope (HST)
morphological number counts for elliptical and S0 galaxies of the Medium Deep
Survey (MDS) and the Hubble Deep Field (HDF). We find that the number counts of
early-type galaxies can be explained by the pure luminosity evolution models,
without invoking exotic scenarios such as merging or introducing an additional
population. But the evolution should be nearly passive, with a high zfβ
assumed. The conclusion is valid in all of the three cosmological models we
adopted in this paper. We also present the redshift distributions for three
bins of observed magnitudes in F814w pass-band, to show at which redshift are
the objects that dominate the counts at a given magnitude. The predictions of
the redshift distribution of 22.5<bjβ<24.0 are also presented for comparison
with future data.Comment: Plain tex, 15pages, 9 eps figures, plus an extra figure fig2c.eps,
with the tex-macro mn.tex. MNRAS, accepte