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

We present 21 cm H I 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 and 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 interstellar medium (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. Because of this, part of the molecular gas is dissociated and becomes neutral or even ionized if the UV continuum produced by the shocks is hard and powerful enough. In the 21 cm H I line new data, we detect only part of the strong, blueshifted H I absorption that was previously observed with ATCA at lower resolution. In particular, the main component detected in the VLBI absorption profile corresponds to the most blueshifted component in the ATCA data, with a central velocity of 2786 km s(-1) and therefore blueshifted similar to 614 km s(-1) 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 similar to 2 x 10(21) atoms cm(-2). Most of the remaining blueshifted components detected in the ATCA H I absorption profile are now undetected, presumably because this absorption occurs against continuum emission that is resolved out in these high-resolution observations. The H I absorption properties observed in IC 5063 appear different from those observed in other Seyfert galaxies, where the H I absorption detected is attributed to undisturbed foreground gas associated with the large-scale galaxy disk. In the case of IC 5063, only a small fraction of the absorption can perhaps be due to this. The reason for this could be that the western jet in IC 5063 passes through a particularly rich ISM. Alternatively, because of the relatively strong radio flux produced by this strong interaction and the high spectral dynamic range of our observations, broad absorption lines of low optical depth as detected in IC 5063 may have remained undetected in other Seyfert galaxies that are typically much weaker radio emitters or for which existing data are of poorer quality.