Radiative efficiency, variability and Bondi accretion on to massive black holes: the transition from radio AGN to quasars in brightest cluster galaxies

Abstract

We examine unresolved nuclear X-ray sources in 57 brightest cluster galaxies to study the relationship between nuclear X-ray emission and accretion onto supermassive black holes. The majority of the clusters in our sample have prominent X-ray cavities embedded in the surrounding hot atmospheres, which we use to estimate mean jet power and average accretion rate onto the supermassive black holes over the past several hundred Myr. We find that roughly half of the sample have detectable nuclear X-ray emission. The nuclear X-ray luminosity is correlated with average accretion rate determined using X-ray cavities, which is consistent with the hypothesis that nuclear X-ray emission traces ongoing accretion. The results imply that jets in systems that have experienced recent AGN outbursts, in the last ∼ 10 7 yr, are 'on' at least half of the time. Nuclear X-ray sources become more luminous with respect to the mechanical jet power as the mean accretion rate rises. We show that nuclear radiation exceeds the jet power when the mean accretion rate rises above a few percent of the Eddington rate, or a power output of ∼ 10 45 erg s −1 , where the AGN apparently transitions to a quasar. The nuclear X-ray emission from three objects (A2052, Hydra A, M84) varies by factors of 2 − 10 on timescales of 6 months to 10 years. If variability at this level is a common phenomenon, it can account for much of the scatter in the relationship between mean accretion rate and nuclear X-ray luminosity. We find no significant change in the spectral energy distribution as a function of luminosity in the variable objects. The relationship between accretion and nuclear X-ray luminosity is consistent with emission from either a jet, an ADAF, or a combination of the two, although other origins are possible. We also consider the longstanding problem of whether jets are powered by the accretion of cold circumnuclear gas or nearly spherical inflows of hot keV gas. For a subset of 13 nearby systems in our sample, we re-examine the relationship between the jet power and the Bondi accretion rate. The results indicate weaker evidence for a trend between Bondi accretion and jet power, primarily due to the uncertainty in the cavity volumes. We suggest that cold gas fuelling could be a likely source of accretion power in these objects however we cannot rule out Bondi accretion, which could play a significant role in low power jets.

AGN jet-driven stochastic cold accretion in cluster cores

“…More importantly, the scatter and the lack of obvious correlation in the observational data from Russell et al (2013) in Figure 9 is similar to that for the Bondi powercavity power scatter seen in our fiducial simulation. Notice that the variation in Bondi power is smaller than the variation in cavity power in the simulations.…”

“…6). The observational data point in Figure 9 with the highest Bondi power is for M87, for which Russell et al (2013) use a SMBH mass of 6.6 × 10 9 M⊙, much larger than our fiducial choice of 10 9 M⊙; recalling thatṀB ∝ M 2 BH brings this point in agreement with the simulation results. The highest cavity power scales with the halo mass and our halo mass is on the higher side.…”

“…Russell et al (2013) recently refined the determination of Bondi and cavities powers from Allen et al's sample and included more data points, and found that the correlation is much weaker. Figure 9 shows the Bondi-power cavity-power relationship obtained for our fiducial NFW run where, as we have discussed, the jet is in fact triggered by cold gas condensation, not Bondi accretion.…”

“…Emulating what Russell et al 2013 do with the observational data, we also use two different best-fit models for the hot gas density applied from rin to 10 kpc: a power-law [PL] model (red squares in Fig. 9); and a β model (blue stars) with the hot gas density going like n0[1 + (r/r0) 2 ] −3β/2 .…”

STAR-FORMING BRIGHTEST CLUSTER GALAXIES AT 0.25 < z < 1.25: A TRANSITIONING FUEL SUPPLY

“…If the high fraction of systems identified here as star forming are instead AGN, it implies a high accretion rate onto the supermassive black hole at early times (e.g., Russell et al 2013). Of the hundreds of known BCGs at low-z, only a handful of systems appear to harbor rapidly accreting, radiatively efficient AGN (e.g., Russell et al 2010;O'Sullivan et al 2012;Ueda et al 2013;Kirk et al 2015;Reynolds et al 2014;Walker et al 2014).…”