Black Hole highlight: Modelling black hole spins and jets in the new era of gravitational wave astronomy

Active galactic nuclei (AGN), with powerful nuclear emission that can outshine the entire host galaxy, represent some of the most extreme physical environments in the Universe. At the heart of these systems reside rapidly growing supermassive black holes (SMBH). They act as efficient gravity engines releasing vast amounts of energy through accretion, over most of cosmic time. It is believed that these energy outbursts may change not only the properties of gas in the vicinity of the AGN, but transform the whole host galaxy, expelling material all the way into intergalactic space.

Figure 1: Column density rendering of a self-gravitating gaseous disc surrounding an equal mass, 10⁶ M_☉ sol SMBH binary. Gas streams excited by the time-varying gravitational potential of the binary develop from the cavity edge to the SMBHs. These streams provide fresh material for accretion and affect the binary dynamics through gravitational torques, extracting angular momentum and reducing the SMBH separation. Simulations have been performed on DiRAC supercomputers at the Universities of Cambridge, Leicester and Durham.

There are, none the less, many outstanding questions on the connection between SMBHs and their host galaxies, and numerical simulations have a major role in advancing our knowledge and providing testable theories that future observations may prove or rule out. For example, it is well established that accretion discs play a fundamental role in modulating mass accretion onto SMBHs and driving their spin evolution, with profound implications for relativistic jets and gravitational waves from coalescing SMBH binaries. Their contribution is, however, customarily neglected in large-scale galaxy formation simulations due to the inherent complexity.

Researchers at the Institute of Astronomy (IoA) and at the Kavli Institute for Cosmology (KICC) at the University of Cambridge, have taken a major step forward in solving this issue by developing a novel numerical model within the moving-mesh code AREPO, which self-consistently describes mass feeding through an accretion disc, governing both the evolution of mass and spin of a SMBH. In a recently submitted paper (Fiacconi, Sijacki, Pringle, MNRAS submitted, arXiv:1712.00023), they showed that SMBHs in spiral galaxies, like in our own Galaxy, are expected to be highly spinning, while SMBHs in more massive elliptical galaxies likely have lower spins. IoA/KICC researchers are now exploring the pairing of SMBH spins in binaries (see Fig. 1), which will represent the main cosmological source of gravitational waves for the LISA space observatory (launch 2034).

SMBHs spins are thought to be closely related to the jet phenomena, which are often observed to inflate giant lobes of relativistic plasma in galaxy clusters. Understanding the propagation and interaction of jets with the surroundings by means of hydrodynamical simulations is challenging because of the large dynamic range involved, spanning over 8 orders of magnitude in spatialscales. Building on an earlier implementation of a super-Lagrangian refinement scheme that allows unprecedented spatial resolution close to SMBHs (Curtis & Sijacki, 2015, MNRAS, 454, 3445), IoA/KICC researchers have developed a new jet model for AREPO capable of injecting realistic, highly beamed jets (see Fig. 2; Bourne & Sijacki, 2017, MNRAS, 472, 4707). They found that the inflation of lobes can heat the intra-cluster medium (ICM) through a number of processes, although jets seem to be unable to drive significant turbulence within the ICM in agreement with the gas kinematics observed by the Hitomi satellite in the centre of the Perseus
cluster.

Figure 2: Simulated AGN-driven jet propagating through the ICM. Each panel shows a different gas property in a slice through the jet. Also shown in the middle panel is the Voronoi cell structure, highlighting the wide dynamic range afforded by the super-Lagrangian refinement scheme: smallest cell size is ~10pc while the size of the computational domain is 20Mpc. Simulations have been performed on DiRAC supercomputers at the Universities of Cambridge, Leicester and Durham.

The next immediate aim is to combine the new models of accretion and jet feedback, as jet power and direction are expected to be directly related to the BH spin. With the upcoming Hitomi replacement, X-ray Astronomy Recovery Mission (XARM, launch 2021), and future Advanced Telescope for High Energy Astrophysics (Athena, launch 2028) on the horizon, IoA/KICC researchers’ simulations on DiRAC supercomputers will provide testable predictions and help interpret these new observations.