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 galaxy morphologies considerably. It is evident that the Hubble sequence requires modifications in order to incorporate all the various morphologies one encounters at such redshifts. We investigate and compare different 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 use on a sample of 978 galaxies, classifying them by eye as either normal or peculiar. Based on our criteria and on concepts borrowed from digital image processing, we design a set of four purely morphological parameters, which comprise the overall texture (or ''blobbiness'') of the image; the distortion of isophotes; the filling factor of isophotes; and the skeletons of detected structures. We also examine the parameters suggested by Abraham and coworkers. An artificial neural network (ANN) is trained to distinguish between normal and peculiar galaxies. While the majority of peculiar galaxies are disk dominated, we also find evidence for a significant population of bulge-dominated peculiar galaxies. Consequently, peculiar galaxies do not all form a ''natural'' continuation of the Hubble sequence beyond the late spirals and the irregulars. The trained neural network is applied to a second, larger sample of 1999 WFPC2 images, and its probabilistic capabilities are used to estimate the frequency of peculiar galaxies at moderate redshifts as 35% +/- 15%.