Dual black holes in merger remnants. I: linking accretion to dynamics

TL;DR

This study uses high-resolution simulations to explore how massive black hole pairs evolve and accrete in circumnuclear discs, revealing an orbital angular momentum flip and linking accretion variability to orbital dynamics.

Contribution

It demonstrates the orbital angular momentum flip of counter-rotating black holes and links accretion behavior to orbital evolution using unprecedented high-resolution simulations.

Findings

Counter-rotating MBHs experience an angular momentum flip due to gas dynamical friction.

Accretion rates are suppressed and highly variable for retrograde MBHs.

Post-flip, MBHs can accrete near the Eddington rate for several million years.

Abstract

We study the orbital evolution and accretion history of massive black hole (MBH) pairs in rotationally supported circumnuclear discs up to the point where MBHs form binary systems. Our simulations have high resolution in mass and space which, for the first time, makes it feasible to follow the orbital decay of a MBH either counter- or co-rotating with respect to the circumnuclear disc. We show that a moving MBH on an initially counter-rotating orbit experiences an "orbital angular momentum flip" due to the gas-dynamical friction, i.e., it starts to corotate with the disc before a MBH binary forms. We stress that this effect can only be captured in very high resolution simulations. Given the extremely large number of gas particles used, the dynamical range is sufficiently large to resolve the Bondi-Hoyle-Lyttleton radii of individual MBHs. As a consequence, we are able to link the accretion processes to the orbital evolution of the MBH pairs. We predict that the accretion rate is significantly suppressed and extremely variable when the MBH is moving on a retrograde orbit. It is only after the orbital angular momentum flip has taken place that the secondary rapidly "lights up" at which point both MBHs can accrete near the Eddington rate for a few Myr. The separation of the double nucleus is expected to be around ~10 pc at this stage. We show that the accretion rate can be highly variable also when the MBH is co-rotating with the disc (albeit to a lesser extent) provided that its orbit is eccentric. Our results have significant consequences for the expected number of observable double AGNs at separations of <100 pc.