Effects of Galaxy Interaction on the Tully-Fisher Relation: CO vs HI Linewidths

We investigate effects of galaxy-galaxy interaction on the Tully-Fisher relation. The HI linewidth in interacting galaxies is significantly broader than the CO linewidth, and the HI to CO linewidth ratio is proportional to the strength of interaction. This provides different distances to galaxies measured by the Tully-Fisher relation with the CO and HI linewidths. Distances derived from the HI linewidths are 62% larger than those derived from the CO linewidths for strongly interacting galaxies, and 25% larger for weakly interacting galaxies. We argue that the CO-line Tully-Fisher relation will be more reliable to measure the distances of interacting galaxies as well as galaxies in rich clusters.