Non-steady-state long-term evolution of supermassive black hole binaries surrounded by accretion discs

TL;DR

This study models the long-term, non-steady evolution of supermassive black hole binaries with surrounding accretion discs, revealing that past disc properties influence the entire evolution and that steady state is rarely achieved.

Contribution

It introduces a time-dependent 1D model for SMBH binary evolution with discs, highlighting non-steady-state behavior and longer residence times than previous models.

Findings

Higher residual masses and longer residence times than prior studies.

Evolution is non-steady-state, influenced by initial disc conditions.

Steady state is generally not achieved due to disc inflow timescales.

Abstract

Supermassive black holes (SMBHs) pair and form bound binaries after their host galaxies merge. In a gas-rich merger, accretion discs are expected to form around the binary and its components. These discs control the binary orbital evolution until the system is compact enough for gravitational waves to drive the SMBHs to coalescence. In this work, we implemented a time-dependent 1D model to follow the long-term evolution of the coupled binary + discs system, from a separation of 10^5 down to 20 Schwarzschild radii. We run different models changing the system parameters, including the binary mass ratio q \leq 0.3 and a factor {\gamma} that controls the inflow across the gap created by the secondary. We find that our implementation yields higher residual masses and longer binary residence times than previous studies. Our main conclusion is the non-steady-state nature of the evolution of the system: the properties the disc had when the binary was still at large separations influence its whole evolution. To recover steady state, the binary residence time would have to be much longer than the inflow time-scale of the disc throughout their entire history, which in general is not satisfied.