A high-resolution study of the interstellar medium in the luminous IRAS galaxy Arp 193

We present high-resolution multi-element radio-linked interferometer network (MERLIN) and Very Large Array (VLA) observations of the H I absorption towards the starburst nucleus of Arp 193 (IC 883, UGC 08387), a luminous IRAS merger. We compare the distribution of atomic gas with that of radio continuum emission, high-resolution CO data and near-infrared Hubble Space Telescope (HST) Near-Infrared Camera and Multi-object Spectrometer (NICMOS) data. We find, that for a constant spin temperature and CO flux to H-2 mass conversion factor, X-CO, the interstellar medium (ISM) becomes progressively more H-2 rich towards the centre of the source. We also find that the projected velocity distribution of the atomic and molecular gas components are not the same. We discuss possible reasons for the observed differences and find that the principle difference is probably in the spatial distribution of the molecular and atomic ISM, although the addition of real dynamical differences provides a better explanation of the data. Evidence for free-free absorption towards the centre of the starburst nucleus is found, implying a large mass of ionized gas, and therefore young stars. Arp 193 is one of the very few IRAS luminous objects in which such a detailed study is possible.