  Neutral hydrogen synthesis mapping of NGC 4532 and DDO 137, a pair of Sm
galaxies on the edge of the Virgo cluster, is used to determine rotation curves
for each of the galaxies and to resolve the structure and kinematics of three
distinct HI concentrations embedded in an extended envelope of diffuse HI
discovered in earlier Arecibo studies of the system. The HI masses of the three
concentrations do not appear to be sufficient for them to be self-gravitating;
however, their HImasses and dynamical masses are very similar to those of faint
Im galaxies in the Virgo cluster. The peak HI column density of each of the
clouds is close to the star formation threshold, but CCD images in B and R
reveal no trace of stars nor star formation. If the system is gravitationally
bound and in Virial equilibrium, we find its total mass to be ten times that
within the outermost HI contours of the individual galaxies and clouds.
Thirty-seven percent of the total HI lies outside the boundaries of the
individual galaxies as determined by the VLA mapping; 28% cannot be ascribed to
either galaxy nor to the discrete concentrations. One possible way to account
for the velocity field and the large quantity of diffuse gas is to assume that
DDO 137, the gas concentrations and other apparent tidal features are due to
gas infall into clumps within the dark matter potential of the group, and that
prior to the interaction the group consisted of an irregular galaxy (NGC 4532)
and a large, star-poor HI cloud like HI 1225+01 (Giovanelli et al. 1991).
