Anisotropy of Nanohertz Gravitational Wave Background and Individual Sources from Supermassive Binary Black Holes Based on Cosmological Simulation

Abstract: Several pulsar timing array (PTA) groups have recently claimed the detection of nanohertz gravitational wave background (GWB), but the origin of this gravitational wave (GW) signal remains unclear. Nanohertz GWs generated by supermassive binary black holes (SMBBHs) are one of the most important GW sources in the PTA band. Utilizing data from cosmological simulation, we construct a comprehensive dataset of SMBBHs within a mock observable universe incorporates the cosmic large-scale structure. We carry out an exhaustive analysis of the distribution characteristics of these merger events, as well as the GWB signals they produce. Specifically, we predict the characteristic amplitude of GWB to be $h_c=4.85\times10^{-16}$ at the frequency of ${\rm yr^{-1}}$, while the energy density of GWB signal exhibit an anisotropic part with $C_1/C_0\approx2.50\times10^{-3}\pm2.04\times10^{-3}$. We study the clustering pattern of the positional distribution of SMBBHs, and found that they show similar behavior with that of galaxies on relatively small scales. Furthermore, for the upcoming Chinese Pulsar Timing Array (CPTA) and Square Kilometre Array (SKA)-PTA, we predict the spatial distribution, numbers and signal-to-noise ratio (SNR) distribution of individual GW sources that may be detected with SNR$>$8, and study the anisotropic properties in the spatial distribution of these individual GW sources. We finally investigated the impact of lensing effects and found that their influence is rather limited.