The parsec-scale HI outflows in powerful radio galaxies
  Massive outflows of neutral atomic hydrogen (HI) have been observed in
absorption in a number of radio galaxies and are considered a signature of AGN
feedback. These outflows on kpc-scales have not been investigated in great
detail as they require high-angular resolution observations to be spatially
resolved. In some radio AGN, they are likely the result of the radio jets
interacting with the interstellar medium. We have used a global VLBI array to
map the HI outflow in a small sample of young and restarted radio galaxies
which we previously observed with the VLA and the WSRT at lower resolution.
Here, we report on our findings for 4C52.37 and 3C293 and we discuss the sample
including the previously published sources 4C12.50 and 3C236. For 4C52.37, we
present the first-ever HI VLBI observation which recovered the majority of the
outflowing HI gas in form of clouds towards the central 100pc of the AGN. The
clouds are blue-shifted by up to 600km/s with respect to the systemic velocity.
3C293 is largely resolved out in our VLBI observation, but we detect, towards
the VLBI core, some outflowing HI gas blueshifted with respect to the systemic
velocity by up to 300km/s. We also find indications that the HI outflow is
extended. Overall, we find that the fraction of HI gas recovered by our VLBI
observation varies significantly within our sample. In all cases we find
evidence for a clumpy structure of both the outflowing and the quiescent gas,
consistent with predictions from numerical simulations. The outflows include at
least a component of relatively compact clouds (10^4-10^5Msun) often observed
already at a few tens of pc (in projection) from the core. We also find
indications that the HI outflow might have a diffuse component, especially in
larger sources. Our results support the interpretation that we observe these
AGNs at different stages in the evolution of the jet-ISM interaction.
