The Effects of Interactions on the Structure and Morphology of Elliptical/Lenticular Galaxies in Pairs

Abstract

We present a structural and photometric analysis of 42 elliptical/lenticular galaxies in (E/S0 + S) pairs observed in the BV RI color bands. The aim of the analysis is to empirically determine the effects of interactions on their morphology, structure and stellar populations as seen from the light concentration (C), asymmetry (A), and clumpiness (S) parameters. We further compare these values to a control sample of 67 mostly isolated non-interacting E/S0 galaxies. We find that the paired E/S0 galaxies occupy a more scattered loci in CAS space than non-interacting E/S0’s, and that the effects of interactions on E/S0’s are minor, in contrast to disk galaxies involved in interactions. This suggests that observational methods for recognizing interactions at high redshift, such the CAS methodology of Conselice (2003), would hardly detect E/S0’s 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. We however find statistical differences in the asymmetry index when comparing isolated and interacting E/S0s. In the mean, paired E/S0 galaxies have A values 2.96 ± 0.72 times larger than the ones of noninteracting E/S0’s. For the subset of presumably strongly interacting E/S0’s, A and S can be several times larger than the typical values of the isolated E/S0’s. We show that the asymmetries are consistent with several internal and external morphological distortions. We conclude that the subset of interacting E/S0s should be dense, gas poor galaxies in systems spaning a wide range of interaction stages, with typical merging timescales > ∼0.1 0.5 Gyr. We use the observed phenomenology of this subsample to predict the approximate loci of ‘dry pre-mergers’ in the CAS parameter space. Subject headings: Galaxies: elliptical – Galaxies: lenticular – Galaxies: structure – Galaxies: photometry – Galaxies: interactions – Galaxies: morphology