Optical spectroscopic properties of a sample of interacting galaxies

We present spectroscopic observations of 83 galaxies from a sample of 49 pairs of optically selected interacting galaxies, most of them previously unobserved. These pairs consist of a main galaxy (component A) and a companion (component B) that has about half or less the diameter of component A. From our spectra we determine that 27 galaxies form truly physical pairs and seven are apparent pairs, for the remaining pairs we could only extract the spectra of the A components. The spectra of the physical pairs were classified into four groups according to the emission-line spectra observed in each component. These classifications were made because the sample exhibits a very large range of spectral properties, ranging from well-evolved stellar populations (older than 200 Myr) to emission-line-dominated starburst systems (80 Myr or younger). In general terms, these spectral types are well correlated with the morphological types of the galaxies. However, we find no evidence of correlation of the equivalent width of H alpha + [N II] emission lines with the degree of the interaction or with the blue absolute magnitude of the components. From the data it is also determined that the average EW(H alpha + [N II]) for the physical pairs is 37 Angstrom for the A components and 54 Angstrom for the B components. For the galaxies that form apparent pairs we obtain EW(H alpha + [N II]) = 27 Angstrom, confirming that physical pairs have higher mean star formation rates than isolated galaxies. This enhancement of the star formation activity is more likely to take place in both galaxies, but the strength of the activity seems to be higher in the B components. The mean observed values of EW(H alpha + [N II]) are comparable with those observed in a sample of strongly interacting or merging galaxies. On the other hand, we do not find the excess of Seyfert-type nuclei previously reported in studies of similar samples of galaxies.