Misaligned accretion on to supermassive black hole binaries

TL;DR

This study uses high-resolution simulations to analyze how gas clouds form accretion discs around supermassive black hole binaries, revealing differences in disc longevity and fragmentation based on orbit orientation and cooling rates, with implications for binary evolution.

Contribution

It provides new insights into the dynamics of prograde and retrograde circumbinary discs, especially regarding their fragmentation, lifetime, and role in binary black hole evolution.

Findings

Prograde discs are shorter-lived than retrograde ones when they fragment.

Fragmenting discs of all masses have a narrower radial extent in prograde configurations.

Retrograde discs' longer lifetime can significantly influence binary evolution.

Abstract

We present the results of high-resolution numerical simulations of gas clouds falling onto binary supermassive black holes to form circumbinary accretion discs, with both prograde and retrograde cloud orbits. We explore a range of clouds masses and cooling rates. We find that for low mass discs that cool fast enough to fragment, prograde discs are significantly shorter-lived than similar discs orbiting retrograde with respect to the binary. For fragmenting discs of all masses, we also find that prograde discs fragment across a narrower radial region. If the cooling is slow enough that the disc enters a self-regulating gravitoturbulent regime, we find that alignment between the disc and binary planes occurs on a timescale primarily dictated by the disc thickness. We estimate realistic cooling times for such discs, and find that in the majority of cases we expect fragmentation to occur. The longer lifetime of low-mass fragmenting retrograde discs allows them to drive significant binary evolution, and may provide a mechanism for solving the 'last parsec problem'.