NGC 2442: Tidal Encounters and the Evolution of Spiral Galaxies

Abstract

Using imaging Fabry-Perot data, we study the star-forming properties and kinematics of the nearby barred spiral galaxy NGC 2442. The Hα emission is very localized along the strong spiral arms of the galaxy, and shows a marked asymmetry between the sharp, well-defined northern tidal arm and the weaker southern arm. The Hα velocity field appears highly distorted, with a rapidly rotating nuclear component. We find evidence for strong noncircular motions along the northern arm, coincident with the pronounced dust lane and regions of intense star formation. The strong asymmetries, the disturbed velocity field, and the presence of a perturbed companion suggest that we are witnessing a strong kinematic response to a close interaction, which has redistributed the star formation activity throughout the disk of NGC 2442. Dynamical modeling of the NGC 2442 system supports this hypothesis and suggests that the regions of strongest star formation are coincident with strong shocks occurring along the tidally perturbed northern arm. Despite this strong redistribution of the gas on small scales, this galaxy does not show a significant departure from the Tully-Fisher relation, nor does it appear to be experiencing any strong starburst. Moreover, our models predict that in a few times 108 yr, NGC 2442 will have recovered from this first tidal encounter and will experience another passage—and ultimately a merger—in a few gigayears. This merger may provoke stronger, permanent changes in the structural properties of the galaxy, depending on the detailed response of the disk. Given the environment of many disk galaxies, this tidal encounter cycle seems likely to be a normal phase of disk galaxy evolution.