The effects of interactions on the structure and morphology of elliptical/lenticular galaxies in pairs

We present a structural and photometric analysis of 42 elliptical/lenticular galaxies in E/S0 + S pairs observed in the BVRI color bands. The aim of the analysis is to empirically determine the effects of interactions on the galaxies' morphology, structure, and stellar populations as seen from the CAS parameters (light concentration C, asymmetry A, and clumpiness S). We further compare these values to a control sample of 67 mostly isolated noninteracting E/S0 galaxies. We find that the paired E/S0 galaxies occupy more scattered loci in CAS space than noninteracting E/S0s and that the structural effects of interactions on E/S0s are minor, in contrast to disk galaxies involved in interactions. This suggests that observational methods for recognizing interactions, such the CAS methodology of Conselice, would not detect E/S0s involved in interactions (related to early phases of the so-called dry mergers), and that the majority of interacting galaxies identified at high redshift must be gas-dominated systems. However, we find statistical differences in the asymmetry index when comparing isolated and interacting E/S0s. On average, paired E/S0 galaxies have A-values 2: 96 +/- 0: 72 times larger than those of noninteracting E/S0s. For the subset of presumably strongly interacting E/S0s, A and S can be several times larger than the typical values of the isolated E/S0s. We show that the asymmetries are consistent with several internal and external morphological distortions. We conclude that the subsample of interacting E/S0s should be dense, gas-poor galaxies in systems spanning a wide range of interaction stages, with typical merging timescales greater than or similar to 0.1-0.5 Gyr. We use the observed phenomenology of this subsample to predict the approximate loci of dry premergers in the CAS parameter space.