Energetics of the molecular gas in the H2 luminous radio galaxy 3C 326: Evidence for negative AGN feedback

We present a detailed analysis of the gas conditions in the H-2 luminous radio galaxy 3C 326 N at z similar to 0.1, which has a low star-formation rate (SFR similar to 0.07 M-circle dot yr(-1)) in spite of a gas surface density similar to those in starburst galaxies. Its star-formation efficiency is likely a factor similar to 10-50 lower than those of ordinary star-forming galaxies. Combining new IRAM CO emission-line interferometry with existing Spitzer mid-infrared spectroscopy, we find that the luminosity ratio of CO and pure rotational H-2 line emission is factors 10-100 lower than what is usually found. This suggests that most of the molecular gas is warm. The Na D absorption-line profile of 3C 326 N in the optical suggests an outflow with a terminal velocity of similar to-1800 km s(-1) and a mass outflow rate of 30-40 M-circle dot yr(-1), which cannot be explained by star formation. The mechanical power implied by the wind, of order 10(43) erg s(-1), is comparable to the bolometric luminosity of the emission lines of ionized and molecular gas. To explain these observations, we propose a scenario where a small fraction of the mechanical energy of the radio jet is deposited in the interstellar medium of 3C 326 N, which powers the outflow, and the line emission through a mass, momentum and energy exchange between the different gas phases of the ISM. Dissipation times are of order 10(7-8) yrs, similar or greater than the typical jet lifetime. Small ratios of CO and PAH surface brightnesses in another 7 H-2 luminous radio galaxies suggest that a similar form of AGN feedback could be lowering star-formation efficiencies in these galaxies in a similar way. The local demographics of radio-loud AGN suggests that secular gas cooling in massive early-type galaxies of >= 10(11) M-circle dot could generally be regulated through a fundamentally similar form of "maintenance-phase" AGN feedback.