Morphological number counts and redshift distributions to I=25 from the Hubble Deep Fields: constraints on cosmological models from early-type galaxies

We combine magnitude and photometric redshift data on galaxies in the Hubble Deep Fields with morphological classifications in order to separate out the distributions for early-type galaxies. The updated morphological galaxy number counts down to I = 25 and the corresponding redshift distributions are used as joint constraints on cosmological models; in particular, on the values of the density parameter Omega (0) and the normalized cosmological constant lambda (0). We find that an Einstein-de Sitter universe with simple passive evolution gives an excellent fit to the counts and redshift data at all magnitudes. An open, low-Omega (0) model with no net evolution (and conservation of the number of ellipticals), which fits the counts equally well, is somewhat less successful, predicting slightly lower mean redshifts and, more significantly, the lack of a high-z tail. A number-conserving model with a dominant contribution from lambda (0), on the other hand, is far less successful, predicting a much narrower distribution than is seen. More complex models are obviously possible, but we conclude that if large-scale transmutation between types does not occur, then the lambda-dominated models provide a very poor fit to the current data.