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 theAustralian Long Baseline Array, of the Seyfert 2 galaxy IC 5063. This objectappears to be one of the best examples of Seyfert galaxies where shocksproduced by the radio plasma jet influence both the radio as well as thenear-infrared emission. The picture resulting from the new observations of IC5063 confirms and completes the one derived from previous Australia TelescopeCompact Array (ATCA) lower resolution observations. A strong interactionbetween the radio plasma ejected from the nucleus and a molecular cloud of theISM is occurring at the position of the western hot spot, about 0.6 kpc fromthe 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 highspeed. Due to this, part of the molecular gas is dissociated and becomesneutral or even ionised if the UV continuum produced by the shocks is hard andpowerful enough. The main component detected in the VLBI HI absorption profilehas a central velocity of 2786 km/s and is therefore blue-shifted 614 km/s withrespect to the systemic velocity. Its peak optical depth is 5.4 Thecorresponding column density of the detected absorption, for a spin temperatureof 100 K, is N_HI ~ 2x10^21 atoms cm^-2. The HI absorption properties observedin IC 5063 appear different from those observed in other Seyfert galaxies,where the HI absorption detected is attributed to undisturbed foreground gasassociated with the large-scale galaxy disk