Alex Rawlings

Detections of Recoiling Supermassive Black Holes from Simulations

Fri, 06 Jun 2025 00:00:00 +0000

The coalescence of two spinning supermassive black holes (SMBHs) results in the merged SMBH being kicked out from the centre of the galaxy surrounded by a cluster of stars — a black hole recoil cluster (BRC). We assess the detectability of this phenomenon for large-scale extragalactic surveys such as Euclid, from which targeted follow-up kinematic observations can be done to confirm the presence of an off-centred SMBH.

An increased velocity dispersion coinciding with the BRC, detectable with upcoming Extremely Large Telescope instruments such as MICADO, offer our best chances of confirming these objects. Using Gaussian process regression and transformation sampling, we predict that there should be approximately 8000 such BRCs detectable out to redshift 0.6.

The full details of the study can be found in the article Caught in the act: detections of recoiling supermassive black holes from simulations.

Identifying Supermassive Black Hole Recoil

Tue, 22 Oct 2024 00:00:00 +0000

When two supermassive black holes (SMBHs) coalesce during a merger of galaxies, general relativistic effects impart a ‘recoil velocity’ to the remnant SMBH, which can expel the SMBH to large, kiloparsec-scale distances. We run numerical simulations of merging massive elliptical galaxies, testing a number of recoil velocities, and use Bayesian hierarchical inference to determine how the stellar mass reacts to the recoiling SMBH.

We additionally detect a kinematic signature, brought about by non angular momentum-conserving stellar orbits, which can uniquely constrain how much stellar mass was lost from the centre before the recoil velocity occurred, allowing us to infer the corresponding recoil velocity.

The full details of the study can be found in the article Identifying supermassive black hole recoil in elliptical galaxies.

Binary Eccentricity is Stochastic

Sun, 03 Dec 2023 00:00:00 +0000

Computer simulations of galaxy mergers are affected by the discretisation of the phase space: the exact positions and velocities of particles will influence the orbital dynamics of the supermassive black holes in the system. Increasing the number of particles used to represent the galaxy merger reduces the uncertainty in the impact parameter of the two SMBHs, but this does not necessarily translate to a convergence in the SMBH eccentricity.

The full details of the study can be found in the article Reviving stochasticity: uncertainty in SMBH binary eccentricity is unavoidable.

KETJU - resolving small-scale supermassive black hole dynamics in GADGET-4

Tue, 18 Jul 2023 00:00:00 +0000

We present in this work the new public version of the KETJU supermassive black hole (SMBH) dynamics module, as implemented into GADGET-4. KETJU adds a small region around each SMBH where the dynamics of the SMBHs and stellar particles are integrated using an algorithmically regularized integrator instead of the leapfrog integrator with gravitational softening used by GADGET-4. This enables modelling SMBHs as point particles even during close interactions with stellar particles or other SMBHs, effectively removing the spatial resolution limitation caused by gravitational softening in galaxy evolution simulations. KETJU also includes post-Newtonian (PN) corrections, which allows following the dynamics of SMBH binaries to sub-parsec scales. Systems with multiple SMBHs are also supported, with the code also including the leading non-linear cross terms that appear in the PN equations for such systems.

The paper demonstrates tests of the code showing that it correctly captures, at sufficient mass resolution, the sinking driven by dynamical friction and binary hardening driven by stellar scattering. Also presented is an example application demonstrating how the code can be applied to study the dynamics of SMBHs in mergers of multiple galaxies and the effect they have on the properties of the surrounding galaxy.

The full details of the study can be found in the article KETJU - resolving small-scale supermassive black hole dynamics in GADGET-4.

Spin–Ellipticity Tracks

Wed, 15 Jan 2020 00:00:00 +0000

The advent of integral field unit spectroscopy allows us to combine both morphological and kinematic data of galaxies. By studying how the ellipticity and spin parameter vary with radius, different galaxy types can be revealed that would not necessarily be distinguishable with just photometry or kinematics alone.

The full details of the study can be found in the article The SAMI Galaxy Survey: rules of behaviour for spin-ellipticity radial tracks in galaxies.