2013A&A...549A..58L

There is growing observational evidence of active galactic nuclei (AGN) feedback on the interstellar medium (ISM) of radio-quiet and radio-loud galaxies. While AGN feedback is expected to be more common at high-redshift objects, studying local universe galaxies helps to better characterize the different manifestations of AGN feedback. Molecular line observations can be used to quantify the mass and energy budget of the gas affected by AGN feedback. We study the emission of molecular gas in 3C 236, a Faranoff-Riley type 2 (FR II) radio source at z∼0.1, and search for the footprints of AGN feedback. The source 3C 236 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. The IRAM Plateau de Bure interferometer (PdBI) was used to study the distribution and kinematics of molecular gas in 3C 236 by imaging with high spatial resolution (0.6'') the emission of the 2-1 line of ¹²CO in the nucleus of the galaxy. We searched for outflow signatures in the CO map. We also derived the star-formation rate (SFR) in 3C 236 using data available from the literature at UV, optical, and IR wavelengths, to determine the star-formation efficiency (SFE) of molecular gas. The CO emission in 3C 236 comes from a spatially resolved ∼1.4''(2.6kpc-) diameter disk characterized by 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)∼2.1x10⁹M_☉. Based on CO we determine a new value for the redshift of the source z_CO=0.09927±0.0002. The similarity between the CO and HI profiles indicates that the deep HI absorption in 3C 236 can be accounted for by a rotating HI structure. This restricts the evidence of HI outflow to only the most extreme velocities. In the light of the new redshift value, the analysis of the ionized gas kinematics reveals a fast (∼1000km/s) outflow. Outflow signatures in the CO emitting gas, are nevertheless absent in the warm molecular gas emission traced by infrared H₂ lines. The SFE in 3C 236 is consistent with the value measured in normal galaxies, which follow the canonical Kennicutt-Schmidt (KS) relation. This result, confirmed to hold in other young radio sources examined in this work, is in stark contrast with the SFE that is lower by a factor 10-50 that has been claimed to characterize evolved powerful radio galaxies. There are no signs of ongoing AGN feedback to the molecular ISM of 3C 236. The recent reactivation of the AGN in 3C 236 (about 10⁵yr ago) is a likely explanation for the early evolutionary status of its molecular disk.