Close galaxy pairs in medium density regions: the Northern sky

Abstract

We present HI synthesis and optical CCD data for pairs of spiral galaxies chosen in an objective manner from the CfA redshift catalog. This is a simple extension to higher velocity differences and higher background galaxy density (i.e. to densities comparable to that of loose groups, called 'medium' density here) of an earlier study on field galaxy pairs (Chengalur, Salpeter & Terzian 1994). The extension to higher background density has a dramatic effect on the pair kinematics, with the median velocity difference for pairs in medium density regions being ~ 100 \kms, as compared to the median velocity difference of ~ 30 \kms seen in the low density pairs of CST94. Despite this increase in median velocity difference, many of the pairs in the medium density region show extensive tidal disturbances, including large ( ~ 100 kpc) HI tails, common HI envelopes etc. We also find a number of smaller (typically uncataloged and also gas rich) companion galaxies around the main galaxy pair. Although the sample is small, there appears to be systematically more gas rich companions found around pairs in medium density regions (five out of eight pairs) as compared to pairs in low density regions (two out of eight). In a forthcoming paper we will present data for 15 pairs of galaxies, chiefly from the southern sky and analysis of the properties of the complete galaxy pair sample.