COUNTS OF GALAXIES IN A MERGER MODEL

A model for the photometric evolution of galaxies has been developed a nd has been applied to the problem of galaxy counts. The integrated co lors of galaxies are calculated using the most recently computed evolu tionary tracks from Maeder and collaborators complemented with evoluti onary tracks derived by other authors. The asymptotic giant branch lif etime is left as a free parameter. A series of cosmological models usi ng different values of the cosmological constant, LAMBDA0, and the den sity parameter, OMEGA0, have been computed. The universality hypothesi s of the luminosity function of galaxies has been abandoned. The influ ence of galaxy merging on the counts has been considered in a simple m anner by assuming that the number of strongly interacting galaxies in a comoving volume increases with redshift as a power law given by (1 z)3.8. Taking a Schechter parametrization for the luminosity function of the different types of galaxies, we are able to reproduce the obse rvations reasonably well. We have also considered models with a Gaussi an distribution for the luminosity function of the brighter galaxies t hat provide a poorer fit to the observations. It is shown that galaxy count data are not yet able to make unambiguous cosmological statement s since evolutionary assumptions are critical. In particular, an OMEGA 0 = 1, LAMBDA0 = 0 cosmology is shown to be consistent with the data.