Mapping the neutral atomic hydrogen gas outflow in the restarted radio
  galaxy 3C 236
  The energetic feedback that is generated by radio jets in active galactic
nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic
hydrogen (HI) gas that can be studied in absorption at high spatial resolution.
We have used the Very Large Array (VLA) and a global
very-long-baseline-interferometry (VLBI) array to locate and study in detail
the HI outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT)
in the re-started radio galaxy 3C 236. We confirm, from the VLA data, the
presence of a blue-shifted wing of the HI with a width of
$\sim1000\mathrm{\,km\,s^{-1}}$. This HI outflow is partially recovered by the
VLBI observation. In particular, we detect four clouds with masses of
$0.28\text{-}1.5\times 10^4M_\odot$ with VLBI that do not follow the regular
rotation of most of the HI. Three of these clouds are located, in projection,
against the nuclear region on scales of $\lesssim 40\mathrm{\,pc}$, while the
fourth is co-spatial to the south-east lobe at a projected distance of
$\sim270\mathrm{\,pc}$. Their velocities are between $150$ and
$640\mathrm{\,km\,s^{-1}}$ blue-shifted with respect to the velocity of the
disk-related HI. These findings suggest that the outflow is at least partly
formed by clouds, as predicted by some numerical simulations and originates
already in the inner (few tens of pc) region of the radio galaxy. Our results
indicate that all of the outflow could consist of many clouds with perhaps
comparable properties as the ones detected, distributed also at larger radii
from the nucleus where the lower brightness of the lobe does not allow us to
detect them. However, we cannot rule out the presence of a diffuse component of
the outflow. The fact that 3C 236 is a low excitation radio galaxy, makes it
less likely that the optical AGN is able to produce strong radiative winds
leaving the radio jet as the main driver for the HI outflow.
