Dark Matter and Gas Concentrations in the NGC 4532/DDO 137 System

Abstract

H I 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 H I concentrations embedded in an extended envelope of diffuse H I discovered in earlier Arecibo studies of the system. The H I masses of the three concentrations do not appear to be sufficient for them to be self-gravitating; however, their H I masses and dynamical masses are very similar to those of faint Im galaxies in the Virgo Cluster. The peak H I 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 or star formation. If the system is gravitationally bound and in virial equilibrium, we find its total mass to be 10 times that within the outermost H I contours of the individual galaxies and clouds. Thirty-seven percent of the total H I lies outside the boundaries of the individual galaxies as determined by the VLA mapping; 28% cannot be ascribed to either galaxy or 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 H I cloud like H I 1225+01.