THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 106:27-40, 1996 September

      ROTATION CURVES AND VELOCITY MEASURES FOR SPIRAL GALAXIES IN PAIRS

                                WILLAM C. KEEL
    Department of Physics and Astronomy, Box 870324, University of Alabama,
              Tuscaloosa, AL 35487-0324; keel@bildad.astr.ua.edu


                                   ABSTRACT

        Rotation curves have been obtained for 76 spiral galaxies in pairs,
     including a geometrically selected subset from the Karachentsev
     catalog and a set of Seyfert galaxies with close companions.  Derived
     parameters of the rotation curves and the galaxies' light
     distributions are also presented. The rotation curves are classified
     broadly by shape, with special emphasis on kinematic disturbances and
     regions of solid-body behavior that may lead to bar development.
     Broadband images of the galaxies allow assessments of their degree of
     symmetry or disturbance.
        These velocity slices afford an empirical basis for evaluating the
     accuracy of radial velocity measures for spiral galaxies in pairs, and
     the dynamically important radial velocity differences.  Specifically,
     the disagreement among several plausible ways of estimating the
     central velocity from these rotation data is used to estimate how
     closely any of these might approximate the nuclear or center-of-mass
     values.  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.

     Subject headings: galaxies: distances and redshifts -- galaxies:
                       kinematics and dynamics -- galaxies: Seyfert --
                       galaxies: spiral