Growing Massive Black Hole Pairs in Minor Mergers of Disk Galaxies

We perform a suite of high-resolution smoothed particle hydrodynamics simulations to investigate the evolution of massive black hole (MBH) pairs during minor mergers of disk galaxies. Our simulation set includes star formation and accretion onto the MBHs, as well as feedback from both processes. We consider 1:10 merger events occurring around a predicted peak of MBH pair formation at a redshift of $z \sim 3$, in the sensitivity window of the Laser Interferometer Space Antenna. Owing to strong tidal torques acting on its host and orbital circularization inside the disk of the primary galaxy, the companion MBH undergoes distinct episodes of enhanced accretion which cause an increase of the initial 1:10 mass ratio of the MBHs. We also find that the efficiency of MBH pair formation in the nuclei of the remnants correlates with the final mass ratio of the pair itself, so that MBH pairs with larger mass ratios are produced more effectively and promptly. Depending on the initial fraction of cold gas in the galactic disks and the geometry of the encounter, the final mass ratios of the resulting MBH pairs can be as large as 1:2, suggesting that minor galaxy mergers can give rise to MBH pairs with major mass ratios. These findings indicate that the mass ratios of MBH pairs in galactic nuclei do not necessarily trace the mass ratios of their host merging galaxies, but are a consequence of the complex interplay between accretion and merger dynamics.