Dynamics of binary galaxies. III. Details of the close pairs

Abstract

In a previous paper [Chengalur et al. ApJ, 107, 1984(1994)] H I synthesis data and CCD images were presented for an objectively chosen sample of six close spiral-spiral galaxy pairs. In this paper we present a detailed analysis of the data. Although the presence of tidal features was not an explicit selection criterion, five of our six pairs show tidal tails and bridges. The remaining pair also shows signs of interaction: both galaxies in the pair have disturbed velocity fields and one of the two galaxies has a central concentration of molecular gas and is undergoing a starburst. The morphology of three of the six pairs implies that the galaxies are on high eccentricity, low total energy orbits, and for one of these pairs there is some evidence that the impact parameter is as small as ~1 disk radius. For the remaining three pairs the tidal features are not as pronounced, and the observations do not yield as much information on the orbit. However, even for these pairs, the observed morphology is consistent with the galaxies being on slow, small impact parameter orbits, provided the galaxies are at a relatively early phase of the orbit, i.e., prior to the first close encounter. We also find weak evidence for preferential alignment in the pairs (with the lack of preferential alignment being ruled out at the 97% level), in the sense that the major axis of a galaxy tends to point toward the companion galaxy. This sample of close pairs is complemented by a larger sample of wide pairs (Chengalur et al. 1993). The median velocity difference {DELTA}V of the sample of close pairs in low-density regions is not much different from that for the wide pairs sample, in particular, there is no evidence for positive energy impacts. The effect of dynamical friction on infall velocities is controversial and will be discussed in the following paper [Chengalur et al., in preparation (1995)] which combines the data on close and wide pairs.