The accretion mechanism in low-power radio galaxies

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 η 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.

Accretion and nuclear activity in Virgo early-type galaxies

“…In agreement with previous studies (e.g. Best et al 2005;Mauch & Sadler 2007), in the Virgo cluster the detection of a radio source is also highly favored in more massive galaxies. The nine VCC/CoreG have similar masses, all within the range 10 11 −10 12 M .…”

A rare example of low surface-brightness radio lobes in a gas-rich early-type galaxy: the story of NGC 3998

“…Eracleous et al (2010) reported an Eddington ratio of 4 × 10 −4 , indicating inefficient SMBH accretion well below the Eddington luminosity, consistent with the classification of NGC 3998 as a Low Excitation Radio Galaxy (LERG; Best & Heckman 2012). The inefficient accretion characteristic of LERGs may be driven by gas cooling from the hot galactic halo supplying fuel to the SMBH via Bondi accretion (Croton et al 2006;Allen et al 2006;Hardcastle et al 2007;Balmaverde et al 2008), or perhaps by chaotic accretion of turbulent clouds (e.g., Gaspari et al 2015, and references therein).…”

“…Hence, they will fade away on relatively short timescales, therefore explaining the rarity of low surface brightness radio lobes in galaxies like NGC 3998. 6 We emphasize that this is only a rough estimate since the intrinsic scatter in both the P jet − P radio and P jet − P acc relation are both ∼0.7 dex (Balmaverde et al 2008;Cavagnolo et al 2010;Nemmen & Tchekhovskoy 2015).…”

“…Previous studies have reported jet efficiencies of a few percent based on studies of the correlation between radio jet power and the Bondi mass accretion rate (ṁ B , Bondi 1952) for samples of nearby, inefficiently-accreting radio AGNs (Allen et al 2006;Balmaverde et al 2008;Russell et al 2013;Nemmen & Tchekhovskoy 2015).…”

The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

“…Bondi accretion has been suggested by several studies (Croton et al 2006;Allen et al 2006;Hardcastle et al 2007;Balmaverde et al 2008). Taking the core luminosity measured by Liuzzo et al (2010) and following the reasoning of Balmaverde et al (2008), connecting the radio core luminosity to the radio jet power and relating that to the Bondi accretion power (P B = 0.1Ṁc 2 ), we estimate that the accretion rate of 4C 35.06 is on the order of 0.2 M yr −1 . If part of the detected H I is associated with infalling gas (something possible given the width of the profile that also spans to redshifted velocities) it could provide the required fueling material.…”