Sub-arcsecond imaging of the radio continuum and neutral hydrogen in the Medusa merger

We present sub-arcsecond, Multi-Element Radio Linked Interferometer (MERLIN) observations of the decimetre radio continuum structure and neutral hydrogen (H) absorption from the nuclear region of the starburst galaxy NGC 4194 (the Medusa Merger). The continuum structure of the central kiloparsec of the Medusa has been imaged, revealing a pair of compact radio components surrounded by more diffuse, weak radio emission. Using the constraints provided by these observations and those within the literature we conclude that the majority of this radio emission is related to the ongoing star-formation in this merger system. With these observations we also trace deep H absorption across the detected radio continuum structure. The absorbing H gas structure exhibits large variations in column densities. The largest column densities are found toward the south of the nuclear radio continuum, co-spatial with both a nuclear dust lane and peaks in 12CO (1→0) emission. The dynamics of the H absorption, which are consistent with lower resolution 12CO emission observations, trace a shallow north-south velocity gradient of ∼320 km s−1 kpc−1. This gradient is interpreted as part of a rotating gas structure within the nuclear region. The H and CO velocity structure, in conjunction with the observed gas column densities and distribution, is further discussed in the context of the fuelling and gas physics of the ongoing starburst within the centre of this merger.