Investigating the co-evolution of massive black holes in dual active galactic nuclei and their host galaxies via galaxy merger simulations

Abstract: Major galaxy mergers can trigger nuclear activities and are responsible for high-luminosity quasi-stellar objects /active galactic nuclei (QSOs/AGNs). In certain circumstances, such mergers may cause dual active galactic nuclei (dAGN) phenomenon. This study investigates dAGN triggering and evolution of massive black holes (MBHs) during the merging processes using hydrodynamic code GADGET-2 to simulate several gas-rich major mergers at redshift $z=2$ and $3$, respectively. Results reveal that gas-rich major mergers can trigger significant nuclear activities after the second and third pericentric passages and the formation of dAGN with significant time duration ($\sim 10 - 390$ Myr). During the merging processes, galactic bulge evolves with time because of the rapid star formation in each (or both) galactic centers and initial mixing of stars in galactic disks due to violent relaxation. MBHs grow substantially due to accretion and finally merge into a bigger black hole. The growth of galactic bulges and corresponding increases of its velocity dispersions predate the growth of MBHs in the dAGN stages. The MBHs in these stages deviate below the relation between MBH mass and bulge mass (or velocity dispersion), and they revert to the relation after the final mergers due to the significant accretion that occurs mostly at a separation less than a few kpc. Then, the two MBHs merge with each other.