Pairing of Supermassive Black Holes in unequal-mass mergers

TL;DR

This study uses high-resolution simulations to investigate how supermassive black holes pair during unequal-mass galaxy mergers, revealing the importance of gas dissipation and redshift in the pairing process.

Contribution

It provides the first detailed analysis of SMBH pairing in 1:4 and 1:10 galaxy mergers with star formation and feedback included.

Findings

Gas dissipation is crucial for SMBH pairing in minor mergers.

High redshift mergers facilitate SMBH pairing due to shorter orbital times.

Efficient SMBH pairing in 1:10 mergers occurs mainly at high redshift.

Abstract

Disk galaxies with a spheroidal component are known to host Supermassive Black Holes (SMBHs) in their center. Unequal-mass galaxy mergers have been rarely studied despite the fact that they are the large majority of merging events by number and they are associated with the typical targets of gravitational wave experiments such as LISA. We performed N-body/SPH simulations of disk galaxy mergers with mass ratios 1:4 and 1:10 at redshifts z=0 and z=3. They have the highest resolution achieved so far for merging galaxies, and include star formation and supernova feedback. Gas dissipation is found to be necessary for the pairing of SMBHs in these minor mergers. Still, 1:10 mergers with gas allow an efficient pairing only at high z when orbital times are short enough compared to the Hubble time.