Analysis of the Interaction Effects in the Southern Galaxy Pair Tol1238-364 and ESO381-G009

In the context of the connection among galaxy-galaxy interaction, starbursts, and nuclear activity, we present and discuss a quantitative morphological analysis based on BVR images and a detailed spectroscopic investigation of two interacting galaxies, the Seyfert 2 Tol 1238-364 (IC 3639) and its companion ESO 381-G009, forming a triple system with ESO 381-G006. Broadband optical photometry is complemented by Hα imaging, which provides information about the distribution of star-forming regions across the galaxies. Long-slit spectroscopic data obtained at different position angles of the slit are employed to determine the physical conditions of circumnuclear and extranuclear regions. A mixture of thermal and nonthermal ionizing radiation is found in the surroundings of the nucleus of Tol 1238-364, and the energy budget supports the presence of a circumnuclear starburst. Several regions in both the galaxies show anomalous line ratios: additional ionization by shock heating and low ionization of some extranuclear H II regions are suggested as possible explanations. An analysis of the emission-line profiles reveals the presence of a broad Hα component in the nuclear region of Tol 1238-364. Independent estimates of the star formation rates (SFRs) were obtained through flux-calibrated Hα images and far-infrared (FIR) emission in the four IRAS bands. Overall, SFR densities have been compared with the SFR densities derived from Hα emission in the individual regions of the galaxies sampled by long-slit spectra. In both the galaxies an enhancement of the star formation activity with respect to isolated galaxies is revealed. The prevalence of starburst or nuclear activity has been examined through FIR color indices. The interaction scenario is discussed on the basis of the observed galaxy properties.