MIXED PAIRS OF GALAXIES

Abstract

According to many schemes for galaxy formation, galaxy morphology should depend strongly on the environment in which a galaxy resides and, especially, in which it was formed. Therefore, in pairs of galaxies one expects to find members of similar morphological type. However, morphological studies of paired galaxies show two different categories: pairs formed by galaxies with similar morphology (EE and SS) and pairs of mixed morphology (ES), where E represents early--type galaxies and S represents late--types. Actually a significant percentage of pairs in reasonably complete samples of binary galaxies (e.g. Catalog of Isolated Pairs of Galaxies by Karachentsev) are of mixed type. Moreover, the distribution of radial velocity differences between components of mixed pairs is similar to that for concordant type (especially SS) pairs. This is further support for the contention that most are physical binaries. We used imaging, spectroscopy and stellar population synthesis to study the interaction effects in 5 mixed pairs of galaxies (AM0327-285, AM1806-852, AM1907-504, AM2016-330, AM2055-492). The present pair sample originated from a sample of 126 candidate mixed pairs that were found during a visual search on the ESO sky surveys. An isolation criterion was employed that required the distance to the nearest neighbor (with major axis diameter ≥ the diameters of the pair components) to be at least five times the pair component separation. Many of these pairs are also found in the Catalogue of Southern Peculiar Galaxies and Associations by Arp & Madore. We find that one of them, AM1907--504, is unlikely to be a physical pair because of the large velocity difference. The other 4 pairs are most likely physically bound with Delta-V ≤ 600 km/s. They can also be classified as true ES pairs since the photometric profiles for the early-type components obey an r1/4 law. We find diverse signs of interaction in our pair sample ranging from direct collisions to small distortions. In principle, this evidence is less ambiguous in mixed pairs because we are dealing with a single gas rich component. A strong but indirect form of evidence for interaction between the galaxies involves the detection of a young stellar component in the early-type members. We used Bica's stellar synthesis approach in order to detect the young stellar population in these pairs. We find that 10% to 37% of their fluxes arises from a young age population (< 5 x 108 years). We suggest that a transfer of gas appears to be the most likely way for the E components to acquire fuel for star formation activity. The only other alternative would be to invoke an unusual star formation history in ellipticals that inhabit mixed pairs.