ON THE MORPHOLOGY OF THE HST FAINT GALAXIES

Deep imaging with the Hubble Space Telescope (HST) has revealed a popu lation of rapidly evolving galaxies, which account for less than or si milar to 50% of the total counts at I less than or similar to 22.5, ar e well distinct from the passively evolving normal ellipticals and spi rals, and have morphologies that elude the traditional Hubble classifi cation scheme. This classification has been derived from the morpholog ical properties of local galaxies observed at optical wavelengths. Sin ce galaxy morphology is a function of the wavelength and of the locali zation and intensity of the star-formation activity, the appearance of galaxies at large redshifts is subject to k-correction and evolutiona ry 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) implie s that the observed morphology of distant galaxies observed at UV wave lengths with the Ultraviolet Imaging Telescope (UIT) would appear to H ST if placed at cosmological distances, with the UV light redshifted t o the optical wavelengths. The simulated distant galaxies have morphol ogies that are of later type of more irregular than their local (optic al), counterparts, and some are in qualitative agreement with those re vealed by the faint HST surveys, suggesting that dynamical evolution h as played a minor role in the evolution of the majority of the galaxie s over a large fraction of the Hubble time. However, the dependence of galaxy morphology on the star-formation activity and on the wavelengt h must be properly understood before any conclusion on the overall mor phological evolution of galaxies can be derived. (C) 1996 American Ast ronomical Society.