ABSTRACT

VIA H I observations of the interacting pair IC 2163/NGC 2207 are presented. The velocity and structural anomalies of IC 2163 agree with predictions of N-body galaxy encounter simulations if IC 2163 recently underwent a strong, prograde, in-plane encounter with NGC 2207. The velocity disturbances in NGC 2207 suggest that the main tidal force on NGC 2207 was perpendicular to the disk of NGC 2207.

INTRODUCTION

The spiral galaxies IC 2163 and NGC 2207, shown in Figure 1, are involved in a close tidal encounter. IC 2163 has an ocular shape (an eye-shaped central oval with a sharp apex at each end), intense star formation along the eyelid regions, and a double-parallel arm structure on the side opposite its companion, NGC 2207.

The optical morphology of IC 2163 is consistent with N-body simulations by Elmegreen et al (1991) if IC 2163 underwent a strong, in-plane, prograde encounter with NGC 2207 during the last half-rotation. One component of the double arm is the usual tidal tail; the other component is produced by rapid streaming of tidally perturbed stars and gas from the companion (western) side of the galaxy. The simulations predict a velocity difference of 50 to 100 kms-1 between the two components of the double arm and large streaming motions along the oval.

Using the VLA, we made H I observations of this galaxy pair to study the early stages of post-encounter evolution and to look for the velocity anomalies predicted by the numerical simulations.