The structural properties of isolated galaxies, spiral-spiral pairs, and mergers:: The robustness of galaxy morphology during secular evolution

We present a structural analysis of nearby galaxies in spiral-spiral pairs in optical BVRI bands and compare them with the structures of isolated spiral galaxies and galaxies in ongoing mergers. We use these comparisons to determine how galaxy structure changes during galaxy interactions and mergers. We analyze light concentration (C), asymmetry (A), and clumpiness (S) parameters, and use the projections of CAS parameter space to compare these samples. We find that the CAS parameters of paired galaxies are correlated with the projected separations of the pair. For the widest and closest pairs, the CAS parameters tend to be similar to those of isolated and ongoing major mergers (e.g., ultraluminous infrared galaxies), respectively. Our results imply that galaxy morphology is a robust property that only changes significantly during a strong interaction or major merger. The typical timescale for this change in our paired sample, based on dynamical friction arguments, is short, tau approximate to 0.1-0.5 Gyr. We find average enhancement factors for the spiral-pair asymmetries and clumpiness values of similar to2.2 and 1.5. The S parameter, which is related to star formation (SF) activity, has a moderate level of enhancement, suggesting that this activity in modem spirals depends more on internal processes than on external conditions. We further test the statistical criterion for picking up interacting galaxies in an automated way by using the A-S projection plane. The diversity of our spiral-pair sample in the CAS space suggests that structural/SF/morphological properties of interacting galaxies change abruptly only when the interaction becomes very strong and the criteria given previously by Conselice for finding galaxies involved in major mergers are effective.