The apparent morphology of peculiar galaxies at intermediate to high redshifts

To understand the range of morphologies observed in the galaxy population at high redshift, it is instructive to evaluate the appearance of local peculiar galaxies when placed at cosmological distances. To this end, we use rest-frame ultraviolet (UV), B-and V-band images of five nearby (z<0.02) interacting and/or starbursting galaxies to simulate deep HST observations of peculiar galaxies at medium to high redshifts. In particular, we simulate Hubble Deep Field (HDF) observations in the F606W and F814W filters of starburst galaxies in the redshift range z similar to 0.5-2.5 by explicitly accounting for the combined effects of band-shifting and surface brightness dimming. We find that extended morphological features remain readily visible in the long exposures typical of the HDF out to redshifts of similar to 1. For systems above z approximate to 1.5, the simulated morphologies look remarkably similar to those of the faint objects found in the HDF and other deep HST fields. Peculiar starburst galaxies therefore appear to be the best local analogs to the highest redshift galaxies in terms of morphology, star formation rates, and spectral energy distributions. Nevertheless, photometric measurements of the z>1.5 images fail to recover the true global properties of the underlying systems. This is because the high-z observations are sensitive to the rest-frame UV emission, which is dominated by the most active star forming regions. The extended distribution of starlight from more evolved populations would not be detected. Therefore, while the observed morphologies provide information on the distribution of star formation within the systems, they may reveal very little about their true dynamical states. We conclude that imaging observations in the rest-frame UV alone cannot reveal whether high-z systems (z greater than or similar to 1.5) are proto-galaxies, proto-bulges, or starbursts within a pre-existing population. Definitive statements regarding the global properties and dynamical states of these objects require deep imaging observations at longer wavelengths. (C) 1997 American Astronomical Society.