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Parsec-scale separation supermassive black hole binaries in the centre ofgas-rich galaxy merger remnants could be surrounded by massive circumbinarydiscs (CBDs). Black hole mass and spin evolution during the gas-rich binaryinspiral are crucial in determining the direction and power of relativisticjets that radio observations with LOFAR and SKAO will probe, and for predictinggravitational wave (GW) emission that IPTA and LISA will measure. We present 3Dhydrodynamic simulations capturing gas-rich, self-gravitating CBDs around a$2\times 10^6$M$_{\odot}$ supermassive black hole binary, that probe differentmass ratios, eccentricities and inclinations. We employ a sub-gridShakura-Sunyaev accretion disc to self-consistently model black hole mass andspin evolution together with super-Lagrangian refinement techniques to resolvegas flows, streams and mini-discs within the cavity, which play a fundamentalrole in torquing and feeding the binary. We find that higher mass ratio andeccentric binaries result in larger cavities, while retrograde binaries resultin smaller cavities. All of the simulated binaries are expected to shrink withnet gravitational torques being negative. Unlike previous simulations, we donot find preferential accretion onto the secondary black hole. This impliessmaller chirp masses at coalescence and hence a weaker GW background.Critically this means that spin-alignment is faster than the binary inspiraltimescale even for low mass ratios. However, we find that mini-disc and hencespin alignment is not guaranteed in initially misaligned systems, potentiallyleading to a significant fraction of recoiled remnants displaced from theirhost galaxies if chaotic accretion is the dominant feeding channel.

Dynamics and spin alignment in massive, gravito-turbulent circumbinary discs around supermassive black hole binaries