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 Hubbl e Deep Fields with morphological classifications in order to separate out t he distributions for early-type galaxies. The updated morphological galaxy number counts down to I = 25 and the corresponding redshift distributions a re used as joint constraints on cosmological models; in particular, on the values of the density parameter Omega (0) and the normalized cosmological c onstant lambda (0). We find that an Einstein-de Sitter universe with simple passive evolution gives an excellent fit to the counts and redshift data a t all magnitudes. An open, low-Omega (0) model with no net evolution (and c onservation of the number of ellipticals), which fits the counts equally we ll, is somewhat less successful, predicting slightly lower mean redshifts a nd, more significantly, the lack of a high-z tail. A number-conserving mode l with a dominant contribution from lambda (0), on the other hand, is far l ess 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 model s provide a very poor fit to the current data.