The HI content of the advanced merger NGC 4441

Abstract

NGC 4441 is a candidate for a merger between a spiral and an elliptical galaxy (S+E merger), because it shows typical tidal structures such as an optical tail and two shells. With a far-infrared luminosity of $\sim 5\cdot 10^9 L_{\odot}$ this galaxy belongs to the class of moderate luminosity mergers, in which the merging process induces (if at all) only a moderate starburst. The study of the atomic gas content allows us to investigate the merger history and the impact on the star formation. In particular, it is not clear from simulations whether an S+E merger leads to a gas concentration in the nucleus, resulting in a starburst, or whether the gas is spread out and therefore too diffuse for new star forming regions. We used the Westerbork Radio Synthesis Telescope to observe the properties of the HI. By using this interferometer, we are able to study the large-scale HI distribution and kinematics with high spatial and velocity resolution. We found two HI tails extending out to more than 40 kpc. In a central disc, the gas shows a fairly regular rotation pattern indicating that the gas started to settle after the merger. The total HI mass adds up to $1.5 \cdot 10^9 M_{\odot}$. By comparing the high resolution HI maps with deep optical images, differences between the stellar and gaseous tidal features are apparent, which indicates an S+E merger origin.