Dynamical Effects of Interactions and the Tully-Fisher Relation for Hickson Compact Groups

We investigate the properties of the B-band Tully-Fisher (T-F) relation for 25 compact group galaxies, using V_max derived from two-dimensional velocity maps. Our main result is that the majority of the Hickson Compact Group galaxies lie on the T-F relation. However, about 20% of the galaxies, including the lowest-mass systems, have higher B luminosities for a given mass, or alternatively, a mass that is too low for their luminosities. We favor a scenario in which outliers have been brightened because of either enhanced star formation or merging. Alternatively, the T-F outliers may have undergone truncation of their dark halo due to interactions. It is possible that in some cases both effects contribute. The fact that the B-band T-F relation is similar for compact group and field galaxies tells us that these galaxies show common mass-to-size relations and that the halos of compact group galaxies have not been significantly stripped inside R₂₅. We find that 75% of the compact group galaxies studied (22 out of 29) have highly peculiar velocity fields. Nevertheless, a careful choice of inclination, position angle, and center, obtained from the velocity field, and an average of the velocities over a large sector of the galaxy enabled the determination of fairly well-behaved rotation curves for the galaxies. However, two of the compact group galaxies HCG 91a and HCG 96a, which are the most massive members in M51-like pairs, have very asymmetric rotation curves, with one arm rising and the other one falling, indicating most probably a recent perturbation by the small close companions.