TANGO I: ISM in nearby radio galaxies. Molecular gas

Abstract

Powerful radio-AGN are hosted by massive elliptical galaxies which are usually very poor in molecular gas. Nevertheless gas is needed in the very center to feed the nuclear activity. We aim to study the molecular gas properties (mass, kinematics, distribution, origin) in such objects, and to compare them with results of other known samples. We have performed at the IRAM-30m telescope a survey of the CO(1-0) and CO(2-1) emission in the most powerful radio galaxies of the Local Universe, selected only on the basis of their radio continuum fluxes. The main result of our survey is the very low content in molecular gas of such galaxies compared to spiral or FIR-selected galaxies. The median value of the molecular gas mass, including detections and upper limits, is 2.2x10^8 Msun. If separated into FR-I and FR-II types, a difference in H_2 masses between them is found. The median value of FR-I galaxies is about 1.9x10^8 and higher for FR-II galaxies, about 4.5x10^8 Msun but this is very probably entirely due to a Malmquist bias. Our results contrast with previous surveys, mainly selected through the FIR emission, implying larger observed masses of molecular gas. Moreover, the shape of CO spectra suggest the presence of a central molecular gas disk in 30% of these radio galaxies, a lower rate than in other active galaxy samples.