Abstract

Supermassive black holes (SMBHs), which reside in the centres of massive galaxies, grow over cosmic time either via accretion of material or through mergers. For the latter, the SMB Hs must be brought close enough together for gravitational wave (GW) emission to take effect and lead to their coalescence. When galaxies merge, their central SMB Hs sink to the centre of the new halos as dynamical friction shrinks their orbits, where they can eventually form binary SMB Hs. Such binaries can be surrounded by gaseous circumbinary discs (CBDs) that impact the binary evolution, potentially providing a channel to shrink the binary orbit via dynamical interactions and accretion, and allow GWs to kick in. I will present 3D hydrodynamic simulations using the moving-mesh code Arepo that model gas-rich CBDs around binary SMBH systems covering a range of binary parameters including mass ratio, eccentricity and inclination. Not only do the simulations able to accurately capture the CBD , but thanks to novel refinement techniques gas streams that penetrate the disc cavity and minidiscs that form around the individual SMB Hs are resolved. These streams and cavities play an important role in torquing the binary and hence in shrinking its orbit. The evolution of the SMBH spin is also an important quantity to track for many reasons including its impact on the direction and power of radio jets, as well as for predicting the recoil velocities of newly merged BHs, which can leave an imprint on the resulting GW signal and impact the event rate in future LISA observations. As such, the simulations employ a sub-grid Shakura-Sunyaev accretion disc model that allows us to track not only the SMBH mass evolution but also the spin evolution that arises due to the accretion of material from the CBD . Therefore, we can track key timescales including binary inspiral times and spin alignment timescales.