A strong jet/cloud interaction in the Seyfert galaxy IC 5063: VLBI observations

We present 21-cm HI line and 13-cm continuum observations, obtained with the Australian Long Baseline Array, of the Seyfert 2 galaxy IC 5063. This object appears to be one of the best examples of Seyfert galaxies where shocks produced by the radio plasma jet influence both the radio as well as the near-infrared emission. The picture resulting from the new observations of IC 5063 confirms and completes the one derived from previous Australia Telescope Compact Array (ATCA) lower resolution observations. A strong interaction between the radio plasma ejected from the nucleus and a molecular cloud of the ISM is occurring at the position of the western hot spot, about 0.6 kpc from the active nucleus. Because of this interaction, the gas is swept up, forming, around the radio lobe, a cocoon-like structure where the gas is moving at high speed. Due to this, part of the molecular gas is dissociated and becomes neutral or even ionised if the UV continuum produced by the shocks is hard and powerful enough. The main component detected in the VLBI HI absorption profile has a central velocity of 2786 km/s and is therefore blue-shifted 614 km/s with respect to the systemic velocity. Its peak optical depth is 5.4 %. The corresponding column density of the detected absorption, for a spin temperature of 100 K, is N_HI ~ 2x10^21 atoms cm^-2. The HI absorption properties observed in IC 5063 appear different from those observed in other Seyfert galaxies, where the HI absorption detected is attributed to undisturbed foreground gas associated with the large-scale galaxy disk.