TANGO I: Interstellar medium in nearby radio galaxies

Abstract

Context. Powerful radio-AGN are hosted by massive elliptical galaxies that are usually very poor in molecular gas. Nevertheless, gas is needed at their very center to feed the nuclear activity. Aims. We study the molecular gas properties (i.e., mass, kinematics, distribution, origin) of these objects, and compare them with results for other known samples. Methods. At the IRAM-30m telescope, we performed a survey of the CO(1-0) and CO(2-1) emission from the most powerful radio galaxies of the Local Universe, selected only on the basis of their radio continuum fluxes. Results. The main result of our survey is that the molecular gas content of these galaxies is very low compared to spiral or FIR-selected galaxies. The median value of the molecular gas mass, including detections and upper limits, is 2.2 × 10 8 M . When separated into FR-I and FR-II types, a difference in their H 2 masses is found. The median value of FR-I galaxies is about 1.9 × 10 8 M and higher for FR-II galaxies, at about 4.5 × 10 8 M . Which is probably entirely because of a Malmquist bias. Our results contrast with those of previous surveys, whose targets were mainly selected by means of their FIR emission, implying that we measure higher observed masses of molecular gas. Moreover, the shape of CO spectra suggest that a central molecular gas disk exists in 30% of these radio galaxies, a lower rate than in other active galaxy samples. Conclusions. We find a low level of molecular gas in our sample of radio-selected AGNs, indicating that galaxies do not need much molecular gas to host an AGN. The presence of a molecular gas disk in some galaxies and the wide range of molecular gas masses may be indicative of different origins for the gas, which we can not exclude at present (e.g., minor/major mergers, stellar mass loss, or accretion).

A relation between circumnuclear H I, dust, and optical cores in low-power radio galaxies

“…Systematic studies of the cold, molecular gas component are now starting. However, Flaquer et al (2010) found that radio galaxies tend to have relatively low molecular gas masses. Given the limited sensitivity of current instruments, ALMA will make a major leap in this field.…”

The circumnuclear cold gas environments of the powerful radio galaxies 3C 236 and 4C 31.04

“…Surveys of the subclass of radio-loud early type galaxies yield similar rates of H i and CO detections (e.g. Emonts et al 2010;Ocaña Flaquer et al 2010) suggesting no major difference in cold gas mass content between radio-loud and radioquiet early-type galaxies. Differences between radio-loud and radio-quite objects may instead be related to gas kinematicsbecause in order to fuel a black hole gas needs to be transported to the inner few tens of parsec.…”

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

“…Hence, the H 2 gas that comprises the bulk of the molecular interstellar medium (ISM) is commonly believed to be too cold to be visible. And yet, despite these observational disadvantages, the advent of the Infrared Space Observatory and in particular Spitzer's Infrared Spectrograph (IRS, Houck et al 2004) revealed pure rotational H 2 emission from a rich variety of extragalactic sources, including normal star-forming galaxies (Roussel et al 2007), ultraluminous and luminous infrared galaxies (U/LIRGs Lutz et al 2003;Veilleux et al 2009;PereiraSantaella et al 2010;Stierwalt et al 2014), galaxy mergers (Appleton et al 2006), radio-loud active galactic nuclei (AGNs) (Ocaña Flaquer et al 2010), UV-selected galaxies (O'Dowd et al 2009), quasar hosts (Evans et al 2001), cooling-flow cluster systems (Egami et al 2006), sources with extreme shockdominated energetics (Ogle et al 2010), and even star-forming sources at redshifts 2 ).…”