CLOSE GALAXY AIRS IN MEDIUM DENSITY REGIONS - THE NORTHERN SKY

We present H I synthesis and optical CCD data for pairs of spiral gala xies chosen in an objective manner from the CfA redshift catalog. This is a simple extension to higher Velocity differences and higher backg round galaxy density (i.e., to densities comparable to that of loose g roups, called ''medium'' density here) of an earlier study on held gal axy pairs [Chengalur et al., AJ. 107, 1984 (1994)]. The extension to h igher background density has a dramatic effect on the pair kinematics, with the median velocity difference for pairs in medium density regio ns being similar to 100 km s(-1), as compared to the median velocity d ifference of similar to 30 km s(-1) seen in the low density pairs of C ST94. Despite this increase in median velocity difference, many of the pairs in the medium density region show extensive tidal disturbances, including large (similar to 100 kpc) H I tails, common H I envelopes, etc. We also find a number of smaller (typically uncataloged and also gas rich) companion galaxies around the main galaxy pair. Although th e sample is small, there appears to be systematically more gas rich co mpanions found around pairs in medium density regions (five out of eig ht pairs) as compared to pairs in low density regions (two out of eigh t). In a forthcoming paper we will present data for 15 pairs of galaxi es, chiefly from the southern sky and analysis of the properties of th e complete galaxy pair sample. (C) 1997 American Astronomical Society.