What is a peculiar galaxy?

Abstract

Following the recent surge of interest in peculiar galaxies at high redshifts we consider the definition, or lack thereof, of morphological peculiarities on a sample of local Universe galaxies. Studying the morphology of local Universe galaxies is also of interest in trying to understand galaxy dynamics and in quantifying the relations between morphology and environment. We use classifications given by five experts for a sample of 827 galaxies taken from the automated plate measuring (APM) machine equatorial catalogue and find that there is little agreement between them on what qualifies as a peculiar galaxy. We attempt several objective approaches: matching galaxy images to ‘templates’; examining the 180° asymmetry against light concentration (following Abraham et al.); and exploring angle-dependent asymmetry measures. While none of the quantities we use results in a clean distinction between normal and peculiar galaxies, there is a rough correlation between some parameters and image peculiarity. However, the mixing between the two classes is significant. We conclude that the class of peculiar galaxies is not totally distinct from the class of normal galaxies, and that what we are seeing is really a sequence. It is therefore more useful to consider distribution functions of morphological parameters. The current (and possibly other, more accurate) parametrizations require better data, which are becoming available through CCD imaging.