Dense gas without star formation: The kpc-sized molecular disk in 3C326 N

TL;DR

This study discovers a large, dense molecular gas disk in the galaxy 3C326 N that shows no star formation, highlighting the role of turbulence and shocks in suppressing star formation despite abundant molecular gas.

Contribution

It provides evidence that AGN jets can create large molecular reservoirs without triggering star formation, emphasizing turbulence's role in star formation suppression.

Findings

Large molecular gas disk detected without star formation

Gas is highly turbulent and not gravitationally bound

Jets may create molecular reservoirs but do not necessarily induce star formation

Abstract

We report the discovery of a 3 kpc disk of few 10^9 Ms of dense, warm H_2 in the nearby radio galaxy 3C326 N, which shows no signs of on-going or recent star formation and falls a factor 60 below the Schmidt-Kennicutt law. VLT/SINFONI imaging spectroscopy shows broad (FWHM \sim 500 km/s) ro-vibrational H_2 lines across all of the disk, with irregular profiles and line ratios consistent with shocks. The ratio of turbulent and gravitational energy suggests that the gas is highly turbulent and not gravitationally bound. In absence of the driving by the jet, short turbulent dissipation times suggest the gas should collapse rapidly and form stars, at odds with the recent star-formation history. Motivated by hydrodynamic models of rapid H_2 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 sets 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 not sufficient to trigger star formation.