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

¹ Institut d'Astrophysique Spatiale, CNRS, Université Paris Sud, 91405 Orsay, France e-mail: nicole.nesvadba@ias.u-psud.fr
² Institut de Radioastronomie Millimétrique (IRAM), St. Martin d'Heres, France
³ LERMA, Observatoire de Paris, CNRS, Paris, France
⁴ GEPI, Observatoire de Paris, CNRS, Université Denis Diderot, Meudon, France
⁵ Spitzer Science Center, California Institute of Technology, Pasadena, USA
⁶ NASA Herschel Science Center, California Institute of Technology, Pasadena, USA
⁷ École Normale Supérieure and Observatoire de Paris, Paris, France

Received: 21 September 2009
Accepted: 17 March 2010

Abstract

We present a detailed analysis of the gas conditions in the H₂ luminous radio galaxy 3C 326 N at z ~ 0.1, which has a low star-formation rate (SFR ~ 0.07  yr^-1) in spite of a gas surface density similar to those in starburst galaxies. Its star-formation efficiency is likely a factor ~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 H₂ 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 ~–1800 km s^-1 and a mass outflow rate of 30–40  yr^-1, which cannot be explained by star formation. The mechanical power implied by the wind, of order 10⁴³ 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 H₂ 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 ≥10¹¹  could generally be regulated through a fundamentally similar form of “maintenance-phase” AGN feedback.