Fueling the central engine of radio galaxies. II. The footprints of AGN
  feedback on the ISM of 3C 236
  Aims: We study the emission of molecular gas in 3C236, a FR II radio source
at z~0.1, and search for the footprints of AGN feedback. 3C236 shows signs of a
reactivation of its AGN triggered by a recent minor merger episode.
Observations have also previously identified an extreme HI outflow in this
source. Methods: The IRAM PdBI has been used to study the distribution and
kinematics of molecular gas in 3C236 by imaging with high spatial resolution
the emission of the 12CO(2-1) line in the nucleus of the galaxy. We have
searched for outflow signatures in the CO map. We have also derived the SFR in
3C236 using data available from the literature at UV, optical and IR
wavelengths, to determine the star-formation efficiency of molecular gas.
Results: The CO emission in 3C236 comes from a spatially resolved 2.6 kpc disk
with a regular rotating pattern. Within the limits imposed by the sensitivity
and velocity coverage of the CO data, we do not detect any outflow signatures
in the cold molecular gas. The disk has a cold gas mass M(H2)~2.1x10^9 Msun. We
determine a new value for the redshift of the source zCO=0.09927. The
similarity between the CO and HI profiles indicates that the deep HI absorption
in 3C236 can be accounted for by a rotating HI structure, restricting the
evidence of HI outflow to the most extreme velocities. In the light of the new
redshift, the analysis of the ionized gas kinematics reveals a 1000 km/s
outflow. As for the CO emitting gas, outflow signatures are nevertheless absent
in the warm molecular gas emission traced by infrared H2 lines. The
star-formation efficiency in 3C236 is consistent with the value measured in
normal galaxies, which follow the canonical KS-law. This result, confirmed to
hold in other young radio sources examined in this work, is in stark contrast
with the factor of 10-50 lower SFE that seems to characterize evolved powerful
radio galaxies.
