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

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.

💡 Research Summary

The paper presents a suite of high‑resolution smoothed‑particle hydrodynamics (SPH) simulations designed to follow the coupled orbital decay and gas accretion of massive black‑hole (MBH) pairs embedded in a rotationally supported circumnuclear disc (CND) up to the formation of a bound binary. By employing more than ten million gas particles, the authors achieve a dynamical range that resolves the Bondi‑Hoyle‑Lyttleton (BHL) radius of each MBH, a prerequisite for a self‑consistent treatment of accretion. Two MBHs of ~10⁶ M⊙ are placed at an initial separation of 100 pc; the primary is fixed on a prograde circular orbit, while the secondary is initialized either co‑rotating or counter‑rotating with respect to the disc. The disc mass (∼10⁹ M⊙) and temperature profile are chosen to mimic observed nuclear discs, and the gas is allowed to cool but not to form stars or experience feedback from the accreting MBHs.

The key dynamical finding is that a secondary MBH on a retrograde orbit experiences a rapid “orbital angular‑momentum flip” caused by gas dynamical friction. Over a timescale of a few 10⁵ yr the torque from the dense gaseous wake reverses the sign of the MBH’s specific angular momentum, forcing it to co‑rotate with the disc before a bound binary forms. This flip cannot be captured in lower‑resolution studies because the BHL sphere must be resolved to correctly model the wake structure and the resulting torque.

Accretion behavior is tightly linked to the orbital state. While the secondary is retrograde, its relative velocity with respect to the gas is high, the effective Bondi radius is small, and the accretion rate remains well below 0.01 Eddington on average. Moreover, the rate is highly stochastic, showing brief spikes up to ~0.1 Eddington but lacking sustained growth. After the angular‑momentum flip, the secondary’s relative velocity drops, the BHL radius expands, and the surrounding gas density rises sharply. Consequently, both MBHs can accrete at near‑Eddington rates for a few Myr, producing a luminous double‑AGN phase. At the moment of flip the binary separation is ≈10 pc, the regime where current high‑resolution interferometers (e.g., ALMA, VLBI) are most sensitive to dual nuclei.

Even when the secondary is initially co‑rotating, a non‑zero eccentricity (e ≳ 0.3) induces periodic variations in the accretion rate. As the MBH passes through pericentre, it encounters denser gas and the accretion spikes; at apocentre the rate drops. The amplitude of this variability is smaller than in the retrograde case but still potentially observable as flickering AGN.

The authors translate these findings into observational predictions. The probability of detecting a double AGN is maximized when the separation lies between ~10 and 100 pc, precisely the stage after the angular‑momentum flip and before the binary hardens further. At smaller separations the gas reservoir is depleted and accretion declines, while at larger separations the secondary may still be retrograde and thus dim. Consequently, the apparent paucity of close (<100 pc) dual AGN could be partially explained by the suppression of accretion on retrograde orbits.

Finally, the paper discusses limitations and future directions. The present runs neglect radiative feedback, star formation, magnetic fields, and a realistic treatment of the surrounding stellar bulge, all of which could modify the friction torque and the gas supply. Incorporating these processes will be essential to assess how robust the angular‑momentum flip is under more realistic conditions and to predict the exact timing of the gravitational‑wave driven inspiral phase. Nonetheless, the study establishes a clear causal link between orbital dynamics and accretion physics in merging galactic nuclei, providing a valuable framework for interpreting upcoming high‑resolution observations of dual AGN and for informing models of massive‑black‑hole growth in the cosmological context.