Blandford-Znajek jets in galaxy formation simulations

Jets launched by active galactic nuclei (AGN) are believed to play a significant role in shaping the properties of galaxies and provide an energetically viable mechanism through which star formation can become
quenched.

There are numerous open questions surrounding the nature of this jet-mode feedback, however, and due to the complex interactions between the different processes at play in these environments, simulations prove a vital tool to explore these questions. In our work we focus on simulations that can resolve galactic-scale processes but, due to the vast dynamic range that would be required, these simulations cannot resolve the jet launching scales. This means that ab-initio AGN jets will not form and the jet launching processes are necessarily encoded in sub-grid models.

Fig 1: The lower panel shows a temperature slice for a jet with axis perpendicular to the plane of the circumnuclear disc and launched into a high pressure CGM. The top left panel shows a profile of the specific energy of the (left) lobe of the jet, split into kinetic and thermal components. The top right panel shows a mass profile of the (right) lobe of the jet split into contributions from disc, jet and CGM material.

Many such sub-grid models assume that the jets have fixed powers and directions but we know that, in reality, the jet power is modulated by the gas accreted by the black hole and that the amount of gas available depends on the heat that the jet itself is able to impart to its surroundings, linking the properties of the jet with those of the galaxy and forming a feedback loop. One of the most promising processes by which these jets are launched is that of the Blandford-Znajek mechanism. In this model, the jet direction is tied to that of the black hole spin and, thus, is not guaranteed to be fixed. It is therefore of particular importance that self-regulation of AGN jets is fully investigated in the context of galaxy-scale simulations.

Motivated by this, we have developed a self-consistent sub-grid model for AGN feedback in the form of a Blandford-Znajek jet. Our model assumes that the black hole is surrounded by a sub-grid, thin accretion
disc which modulates the accretion flow onto the black hole, allowing us to accurately follow both the evolution of the mass and angular momentum of the black hole.

Fig 2: Various slices for a jet that is initially directed into the circumnuclear disc. The three smaller panels on the left show slices of the temperature field, evolving in time with 3 Myr intervals from top to bottom. The main panel shows a slice of the density field after a further 3 Myrs have elapsed. The two inset plots show slices of the temperature field in the centre and the vorticity field in the outflow. In the bottom left of each panel the arrow indicates the direction of the jet and is labelled with the corresponding inclination angle to the vertical.

Our model has been designed such that it can be employed in galaxy scale simulations with the ultimate aim of assessing whether self-consistent Blandford-Znajek jets can reproduce large-scale galaxy and cluster observables. First, however, we apply our model to the central region of a typical radio-loud Seyfert galaxy embedded in a hot circumgalactic medium (CGM) which allows us to resolve the parsec-scale interactions of jets with the cold dense circumnuclear gas found close to the black hole as well as the interactions of the jet with the surrounding hot CGM.

These simulations were carried out on COSMA7 in Durham and Peta4 in Cambridge as part of the DiRAC project dp012.

We found that jets launched into high pressure environments thermalise efficiently due to the formation of recollimation shocks and the vigorous instabilities that these shocks excite increase the efficiency of the mixing of CGM and jet material (see Fig. 1). The beams of more overpressured jets, however, are not as readily disrupted by instabilities so the majority of the momentum flux at the jet base is retained out to the head, where the jet terminates in a reverse shock. All jets entrain a significant amount of cold circumnuclear disc material which, while energetically insignificant, dominates the lobe mass together with the hot, entrained CGM material. The jet power evolves significantly due to effective self-regulation by the black hole, fed by secularly-driven, intermittent mass flows. The direction of jets launched directly into the circumnuclear disc changes considerably due to effective Bardeen-Petterson torquing. Interestingly, these jets obliterate the innermost regions of the disc and drive large-scale, multi-phase, turbulent, bipolar outflows (see Fig. 2).