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 prev iously 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 t ruly physical pairs and seven are apparent pairs, for the remaining pa irs we could only extract the spectra of the A components. The spectra of the physical pairs were classified into four groups according to t he emission-line spectra observed in each component. These classificat ions were made because the sample exhibits a very large range of spect ral properties, ranging from well-evolved stellar populations (older t han 200 Myr) to emission-line-dominated starburst systems (80 Myr or y ounger). In general terms, these spectral types are well correlated wi th the morphological types of the galaxies. However, we find no eviden ce of correlation of the equivalent width of H alpha + [N II] emission lines with the degree of the interaction or with the blue absolute ma gnitude of the components. From the data it is also determined that th e average EW(H alpha + [N II]) for the physical pairs is 37 Angstrom f or the A components and 54 Angstrom for the B components. For the gala xies that form apparent pairs we obtain EW(H alpha + [N II]) = 27 Angs trom, confirming that physical pairs have higher mean star formation r ates than isolated galaxies. This enhancement of the star formation ac tivity 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 obse rved 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 rep orted in studies of similar samples of galaxies.