The accretion mechanism in low-power radio galaxies^*

¹ INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy e-mail: [balmaverde;capetti]@oato.inaf.it
² Università di Torino, via P. Giuria 1, 10125 Torino, Italy e-mail: baldi@oato.inaf.it

Received: 19 March 2008
Accepted: 23 April 2008

Abstract

We study a sample of 44 low-luminosity radio-loud AGN, which represent a range of nuclear radio-power spanning 5 orders of magnitude, to unveil the accretion mechanism in these galaxies. We estimate the accretion rate of gas associated with their hot coronae by analyzing archival Chandra data, to derive the deprojected density and temperature profiles in a spherical approximation. Measuring the jet power from the nuclear radio-luminosity, we find that the accretion power correlates linearly with the jet power, with an efficiency of conversion from rest mass into jet power of ~0.012. These results strengthen and extend the validity of the results obtained by Allen and collaborators for 9 radio galaxies, indicating that hot gas accretion is the dominant process in FR I radio galaxies across their full range of radio-luminosity.
We find that the different levels of nuclear activity are driven by global differences in the structure of the galactic hot coronae. A linear relation links the jet power with the host X-ray surface brightness. This implies that a substantial change in the jet power must be accompanied by a global change in its ISM properties, driven for example by a major merger. This correlation provides a simple widely applicable method to estimate the jet-power of a given object by observing the intensity of its host X-ray emission. 
To maintain the mass flow in the jet, the fraction of gas that crosses the Bondi radius reaching the accretion disk must be ≳0.002. This implies that the radiative efficiency of the disk must be 0.005, an indication that accretion in these objects occurs not only at a lower rate, but also at lower efficiency than in standard accretion disks.