Probing the mass relation between supermassive black holes and dark matter halos at high redshifts by gravitational wave experiments

Abstract: Numerous observations have shown that almost all galaxies in our Universe host supermassive black holes (SMBHs), but there is still much debate about their formation and evolutionary processes. Recently, gravitational waves (GWs) have been expected to be a new and important informative observation, in particular, in the low-frequency region by making use of the Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays (PTAs). As an evolutionary process of the SMBHs, we revisit a dark matter (DM) halo-SMBH coevolution model based on the halo merger tree employing an ansatz for the mass relation between the DM halos and the SMBHs at $z=6$. In this model, the mass of SMBHs grows through their mergers associated with the halo mergers, and hence the evolutionary information must be stored in the GWs emitted at the mergers. We investigate the stochastic gravitational background from the coalescing SMBH binaries, which the PTAs can detect, and also the GW bursts emitted at the mergers, which can be detected by the mHz band observations such as LISA. We also discuss the possibility of probing the mass relation between the DM halos and the SMBHs at high redshift by future GW observations.