H II regions in double and interacting galaxies

The distributions of H ii regions in 14 systems of double and interacting galaxies are examined to detect any possible distortion of the gas that may have been caused by tidal interaction between the galaxies. Most systems show no significant effect; but some, especially NGC 4038--4039, show a conspicuously anomalous H ii region distribution that is probably caused by tidal effects. Remarkably, other systems, such as NGC 2805--2820 and NGC 5457--5474, show a dramatic anti-effect, in the sense that the H ii regions are considerably distorted on the side of the galaxy opposite its companion. Arp's hypothesis that the H ii regions are more numerous and more conspicuous on the side of a galaxy facing its companion is examined statistically. No such systematic tendency is found, and it is concluded that this criterion is probably not a good one by which to demonstrate NGC 7320's interaction with its apparent neighbors in Stefan's Quintet/Quartet