On the morphology of the HST faint galaxies

Deep imaging with the Hubble Space Telescope (HST) has revealed a population of rapidly evolving galaxies, which account for less than or similar to 50% of the total counts at I less than or similar to 22.5, are well distinct from the passively evolving normal ellipticals and spirals, and have morphologies that elude the traditional Hubble classification scheme. This classification has been derived from the morphological properties of local galaxies observed at optical wavelengths. Since galaxy morphology is a function of the wavelength and of the localization and intensity of the star-formation activity, the appearance of galaxies at large redshifts is subject to k-correction and evolutionary effects of the stellar populations, even if the underlying dynamics does not change significantly. In addition, the strong dependence of the surface brightness on redshift as sigma similar to(1+z)(-4) implies that the observed morphology of distant galaxies observed at UV wavelengths with the Ultraviolet Imaging Telescope (UIT) would appear to HST if placed at cosmological distances, with the UV light redshifted to the optical wavelengths. The simulated distant galaxies have morphologies that are of later type of more irregular than their local (optical), counterparts, and some are in qualitative agreement with those revealed by the faint HST surveys, suggesting that dynamical evolution has played a minor role in the evolution of the majority of the galaxies over a large fraction of the Hubble time. However, the dependence of galaxy morphology on the star-formation activity and on the wavelength must be properly understood before any conclusion on the overall morphological evolution of galaxies can be derived. (C) 1996 American Astronomical Society.