| Abstract | A link or "bridge" of neutral atomic hydrogen connecting the galaxies NGC 4631 and NGC 4656 is described. The galaxies are separated by O~5. At a distance of 4 Mpc, the minimum separation of their centers is 38 kpc. The systemic radial velocities of the two galaxies agree to within `~-`3O km/s. The hydrogen content for the entire system is 6.1 X 1O~ Mo, of which 0.7 X 1O~ Mo is associated with the bridge. Ten of the observed drift curves for NGC 4631 can be satisfactorily reproduced by a set of drift curves generated from a model galaxy that is convolved with the observing antenna beam. Such a pro- cedure yields a rotation curve as well as the distribution and random motions of the hydrogen. The best fitting model has a random motion term of 30km/s. These data yield an estimate of the average optical depth for this nearly edge-on system. Several of the drift curves, corresponding to velocities of the east end of the galaxy, are oniy poorly reproduced by the model. They are better matched if an expansion term is included in the analysis. However, any adopted model is not necessarily a unique representation of the data. The adopted rotation curve indicates a total mass for NGC 4631 of 6.8 X 1010 Mo; the neutral hydrogen is 6 per cent of the total mass. No satisfactory model for NGC 4656 could be found, although a systemic radial velocity of about 600 km/s is indicated. A rough estimate of the total mass of this system is 1.5 X 1O'°/sin2 i Mo. The frac- tional hydrogen content is 0.09 sin2 i |
