Spiral galaxies in pairs

From seven indicators of central velocity, the internal scatter is delta_vv = 34 km/s. Of these, the velocity weighted by Halpha intensity along the slit shows a systematic offset of about 20 km/s with respect to the others for the Karachentsev pairs, in the sense that this measure is redshifted with respect to the other indicators. This is in the sense (but not of the total magnitude) required to account for statistical asymmetries in pair velocity differences.

Individual scatter between the velocity indicators taken pairwise ranges from delta = 20 to 52 km/s. These results imply that emission-line data such as these cannot specify the center of mass or nuclear redshift at a level more accurate than this, even for arbitrarily precise velocity measurements, because it is not clear how the observed quantities relate to the desired measurement in a physical sense. No useful predictor of which galaxies have large or small scatter among velocity measures was found, except that the scatter is small for the class of "nonrotating" galaxies with small overall velocity amplitudes.

Projected separation, separation normalized to disk scale length, and morphological disturbance do not correlate with velocity scatter.