Dynamics of binary galaxies. II. Close pairs

Abstract

As part of a comprehensive study of binary galaxies, broadband optical Charge Coupled Device (CCD) images and high resolution H I maps are presented for six close pairs of spiral galaxies. A previous paper Chengalur, et al., (1993) presented the results of a study of binary galaxies with wide separations. The current sample of close pairs was chosen from the Center for Astrophysics (CfA) redshift catalog using selection criteria that were objective and completely independent of any previously known morphological peculiarities. Nonetheless, all the galaxies show some sign of interaction, implying that the selection criteria were sufficiently well tailored to choose physically associated paris. All the galaxies show kinematical disturbances and have asymmetric rotation curves. Tidal tails and bridges are clearly detected for some, and two pairs have a large common H I envelope. This common H I envelope also shows a large scale radial velocity gradient and gives the impression of being in rotation about a single kinematical axis. There is relatively good agreement between the optical and H I images, with similar tidal features being seen in both. The H I features, however, extend to a much larger galactocentric radius than the corresponding optical features. In at least one case, however, the stellar disk appears relatively normal, while the H I disk appears disturbed even towards the center of the galaxy. The systematic velocity difference for the galaxy pairs has been measured quite accurately either from the integrated H I spectral profile, or from a global fit to the galaxy velocity fields. The median velocity difference for the six galaxy pairs is very small, ~ 20 km/s⁻¹. Detailed analysis of the data will be presented in a separate paper.