Dense gas without star formation: the kpc-sized turbulent molecular disk in 3C 326 N

International audience; We report the discovery of a 3 kpc disk of few 109 M&sun; of dense, warm H2 in the nearby radio galaxy 3C 326 N, which shows no signs of ongoing or recent star formation and falls a factor of 60 below the Schmidt-Kennicutt law. Our VLT/SINFONI imaging spectroscopy shows broad (FWHM ~ 500 km s-1) ro-vibrational H2 lines across the entire disk, with irregular profiles and line ratios consistent with shocks. The ratio of turbulent to gravitational energy suggests that the gas is highly turbulent and not gravitationally bound. In the absence of the driving by the jet, the short turbulent dissipation times indicate that the gas should collapse rapidly and form stars, at odds with the recent star-formation history. Motivated by hydrodynamic models of rapid H2 formation boosted by turbulent compression, we propose that the molecules formed from diffuse atomic gas in the turbulent jet cocoon. Since the gas is not self-gravitating, it cannot form molecular clouds or stars while the jet is active, and is likely to disperse and become atomic again after the nuclear activity ceases. We speculate that very low star-formation rates are to be expected under such conditions, provided that the large-scale turbulence controls the gas dynamics in molecular clouds. Our results illustrate that jets may create large molecular reservoirs as expected in "positive feedback" scenarios of AGN-triggered star formation, but that this alone is insufficient to trigger star formation.