Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618/4625

Asymmetry is a common trait in spiral galaxies and is particularly frequent among Magellanic spirals. To explore how morphological and kinematic asymmetry are affected by companion galaxies, we analyze neutral hydrogen observations of the interacting Magellanic spirals NGC 4618 and 4625. The analysis of the H I distribution revealed that about 10% of the total H I mass of NGC 4618 resides in a looping tidal structure that appears to wrap all the way around the galaxy. Through calculations based on derived H I profiles, we show that NGC 4618 and 4625 are no more asymmetric than non-interacting Magellanic spirals analyzed by Wilcots & Prescott (2004). We also derive rotation curves for the approaching and receding sides of each galaxy. By fitting the mean curves with an isothermal halo model, we calculate dynamical masses of 4.7 times 10^{9} M_sun and 9.8 times 10^{9} M_sun out to 6.7 kpc, for NGC 4618 and 4625 respectively. While the rotation curves had systematically higher velocities on the receding side of each galaxy, the effect was no more pronounced than in studies of non-interacting spirals (Swaters et. al. 1999). The degree of interaction-driven asymmetry in both galaxies is indistinguishable from the intrinsic degree of asymmetry of lopsided galaxies.