Backflows by active galactic nuclei jets: global properties and influence on supermassive black hole accretion

Abstract

Jets from Active Galactic Nuclei (AGN) inflate large cavities in the hot gas environment around galaxies and galaxy clusters. The large-scale gas circulation promoted within such cavities by the jet itself gives rise to backflows that propagate back to the centre of the jet-cocoon system, spanning all the physical scales relevant for the AGN.Using an Adaptive Mesh Refinement code, we study these backflows through a series of numerical experiments, aiming at understanding how their global properties depend on jet parameters. We are able to characterize their mass flux down to a scale of a few kiloparsecs to about 0.5 M yr −1 for as long as 15 or 20 Myr, depending on jet power. We find that backflows are both spatially coherent and temporally intermittent, independently of jet power in the range 10 43−45 erg/s. Using the mass flux thus measured, we model analytically the effect of backflows on the central accretion region, where a Magnetically Arrested Disc lies at the centre of a thin circumnuclear disc. Backflow accretion onto the disc modifies its density profile, producing a flat core and tail.We use this analytic model to predict how accretion beyond the BH magnetopause is modified, and thus how the jet power is temporally modulated. Under the assumption that the magnetic flux stays frozen in the accreting matter, and that the jets are always launched via the Blandford-Znajek (1977) mechanism, we find that backflows are capable of boosting the jet power up to tenfold during relatively short time episodes (a few Myr).

Feedback from reorienting AGN jets

“…We test the reorienting jets model using a total of five numerical simulations. These simulations were run using the hydrodynamical, adaptive mesh refinement (AMR) code FLASH v4.2 (Fryxell et al 2000), adopting a modified setup described in Cielo et al (2017). In our computational setup, FLASH solves the non-relativistic Euler equations for an ideal gas, with specific heat ratio γ = 5/3, initially placed in hydrostatic equilibrium within a gravitational potential well.…”

“…The metallicity of the gas is set to [Fe/H] = −0.1 throughout and over the course of the simulation, the gas is subject to radiative cooling following the same prescriptions as used by Cielo et al (2017). Specifically, we use the cooling function of Sutherland & Dopita (1993), extended to higher plasma temperatures (i.e.…”

“…Our implementation of the jet source terms is essentially the same as in Cielo et al (2017). There, the bipolar jets were introduced as source terms within a rectangular prism consisting of eight central cells (four cells per beam) whose long axis was aligned along one of the axes of the simulation grid.…”

“…Simulations of AGN feedback in galaxy clusters, in which jets are able to counteract cooling in the cores of CCCs, predict a state of dynamical equilibrium involving a rich pattern of inflows and outflows across a range of spatial scales within the central few hundred kiloparsecs (Prasad et al 2015(Prasad et al , 2017(Prasad et al , 2018Yang & Reynolds 2016;Cielo et al 2017;Lau et al 2017;Gaspari et al 2018). Cooling flows, expanding heated gas, gas displaced by inflating cavities, uplifted gas in the wakes of rising cavities, and backflows all contribute to the velocity structure (c.f.…”

“…The latter calls attention to a broader set of related questions regarding the impact of AGN feedback on gas accretion processes; for example, does the AGN simply expel the gas around the SMBH, leading to the shutdown of AGN activity for a period of time, or does it promote other mechanisms of accretion onto the AGN that also contribute to its self-regulation? In a recent study, Cielo et al (2017) found that jet-induced gas circulation (backflows) can funnel as much as 1 M yr −1 to the innermost parsecs (see also Antonuccio-Delogu & Silk 2010).…”