Fueling the central engine of radio galaxies

Abstract

Context. 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. Aims. 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 H i outflow in this source.Methods. 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 12 CO 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. Results. The CO emission in 3C 236 comes from a spatially resolved ∼1.4 (2.6 kpc-) 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(H 2 ) ∼ 2.1 × 10 9 M . 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 H i profiles indicates that the deep H i absorption in 3C 236 can be accounted for by a rotating H i structure. This restricts the evidence of H i 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 (∼1000 km s −1 ) outflow. Outflow signatures in the CO emitting gas, are nevertheless absent in the warm molecular gas emission traced by infrared H 2 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. Conclusions. 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 10 5 yr ago) is a likely explanation for the early evolutionary status of its molecular disk.

Fueling the central engine of radio galaxies

“…6.2, we study the SFE and the location of 3C 293 in the Kennicut-Schmidt (KS) diagram and discuss the differences with respect to the results obtained by Nesvadba et al (2010). We complete the comparison sample with the young radio sources 4C 12.50, 4C 31.04 (Willett et al 2010), and the reactivated radio source 3C 236 Labiano et al 2013). …”

“…5), suggesting that the outflow might deplete the gas reservoir faster than the star formation, lowering the SFE of 3C 293. Figure 14 compares the canonical KS-law fitted for normal star-forming galaxies (Roussel et al 2007;Kennicutt 1998b) with the SFR surface density (Σ SFR = S FR/area) and cold-H 2 mass surface density (Σ MH2 = M(H 2 )/area) for 3C 293, the young sources 4C 12.50, 4C 31.04 (Willett et al 2010), the re-started source 3C 236 (Labiano et al 2013), and the sample of Nesvadba et al (2010) 6 . Inspection of Fig.…”

“…Our previous study of feedback in radio galaxies (Labiano et al 2013) showed that the young ( 10 5 yr) radio sources 3C 236, 4C 12.50, and 4C 31.04 had an unexpectedly high SFE compared with the evolved ( 10 7 yr) radio galaxies in the sample of Nesvadba et al (2010). Based on the age of these sources, we proposed an evolutionary scenario where the impact of AGN feedback on the ISM of the host galaxy would increase with the age of the radio source.…”

“…Our recent work on 3C 236 (Labiano et al 2013) found that not all radio galaxies show a low star formation efficiency (SFE). 3C 236 is a re-activated radio source that shows an SFE consistent with the KS-law.…”

Exceptional AGN-driven turbulence inhibits star formation in the 3C 326N radio galaxy

“…However, a significant fraction of early-type massive galaxies have a substantial amount of warm neutral and cold molecular gas Crocker et al 2012;Serra et al 2012;Bayet et al 2013), so simple energy arguments may not be enough. To inhibit star formation and efficient AGN fueling, many observational studies of AGN feedback have focused on outflows (Fischer et al 2010;Feruglio et al 2010;Dasyra & Combes 2012;Müller-Sánchez et al 2011;Sturm et al 2011;Combes et al 2013;Labiano et al 2013;Alatalo et al 2014c;Morganti et al 2013a;Mahony et al 2013;McNamara et al 2014). Spectroscopic observations of ionized, neutral, and molecular gas suggest that winds are ubiquitous in galaxies hosting AGN (e.g., Morganti et al 2005b;Lehnert et al 2011;Dasyra & Combes 2011;Alatalo et al 2011;Guillard et al 2012;Cicone et al 2014;Förster Schreiber et al 2014).…”