Coevolution of Supermassive Black Holes and their Host Galaxies with Galaxy Mergers

Chi-Hong Lin, Ke-Jung Chen, Chorng-Yuan Hwang

Published 2022-09-21Version 1

Understanding the formation of the supermassive black holes (SMBHs) present in the centers of galaxies is a key topic in modern astrophysics. Observations have detected the SMBHs with mass $M$ of $10^{9}\, \rm M_\odot$ in the high redshifts galaxies with z$\sim7$. However, how SMBHs grew to such huge masses within the first billion years after the big bang remains elusive. One possible explanation is that SMBHs grew in a short period through the frequent mergers of galaxies, which provides sustainable gas to maintain the rapid growth. In this study, we present the hydrodynamics simulations of the SMBHs' growth with their host galaxies using the GIZMO code. In contrast to previous simulations, we developed a molecular cloud model by separating molecular-gas particles from the atomic-gas particles and then evolving them independently. During major mergers, we showed that the effect of the mass segregation of the atomic and molecular gas particles can enhance the dynamical friction of molecular particles. Consequently, molecular gas is substantially accreted onto the galactic centers that grows SMBHs from $10^{6}\, \rm M_\odot$ to $10^{9}\, \rm M_\odot$ within 300 Myr, explaining the rapid growth of SMBHs, and this accretion also triggers a violent starburst at the galactic center. Furthermore, We examined the impact of minor mergers on the bulge of a Milky-Way-like galaxy and found that the size and mass of the bulge can increase from 0.92 kpc to 1.9 kpc and from $4.7\times 10^{10}\, \rm M_\odot$ to $7\times 10^{10}\, \rm M_\odot$.