QUANTITATIVE MORPHOLOGY OF MODERATE-REDSHIFT GALAXIES - HOW MANY PECULIAR GALAXIES ARE THERE

The advent of the Hubble Space Telescope (HST) has provided images of galaxies at moderate and high redshifts and changed the scope of galax y morphologies considerably. It is evident that the Hubble sequence re quires modifications in order to incorporate all the various morpholog ies one encounters at such redshifts. We investigate and compare diffe rent approaches to quantifying peculiar galaxy morphologies on images obtained from the Medium Deep Survey (MDS) and other surveys using the Wide Field Planetary Camera 2 (WFPC2) on board the HST, in the I band (filter F814W). We define criteria for peculiarity and put them to us e on a sample of 978 galaxies, classifying them by eye as either norma l or peculiar. Based on our criteria and on concepts borrowed from dig ital image processing, we design a set of four purely morphological pa rameters, which comprise the overall texture (or ''blobbiness'') of th e image; the distortion of isophotes; the filling factor of isophotes; and the skeletons of detected structures. We also examine the paramet ers suggested by Abraham and coworkers. An artificial neural network ( ANN) is trained to distinguish between normal and peculiar galaxies. W hile the majority of peculiar galaxies are disk dominated, we also fin d evidence for a significant population of bulge-dominated peculiar ga laxies. Consequently, peculiar galaxies do not all form a ''natural'' continuation of the Hubble sequence beyond the late spirals and the ir regulars. The trained neural network is applied to a second, larger sa mple of 1999 WFPC2 images, and its probabilistic capabilities are used to estimate the frequency of peculiar galaxies at moderate redshifts as 35% +/- 15%.