Dark Matter Spike surrounding Supermassive Black Holes Binary and the Nanohertz Stochastic Gravitational Wave Background

Abstract: The NANOGrav, PPTA, EPTA, CPTA and MPTA collaborations have reported compelling evidence for the existence of the Stochastic Gravitational-Wave Background (SGWB). This inferred background's amplitude and frequency spectrum align closely with the astrophysical predictions for a signal originating from the population of supermassive black hole (SMBH) binaries. Considering these findings, we explore the possibility of detecting dark matter (DM) spikes surrounding SMBHs, which could alter the gravitational-wave waveform and influence the SGWB. We show that the evolution of SMBH binaries, driven by both gravitational radiation and the dynamic friction of the surrounding DM spike, presents observable effects in the nHz frequency domain of the SGWB. We also employ the Bayesian inference method to fit the SGWB spectra from the NANOGrav, EPTA, and PPTA. The model with DM spike improves the fittings to the former two data sets. The spike slope $\gamma_{\rm sp}$ is slightly smaller than 1, which may suggest that the spike is flattened during the inspiral of the SMBHBs.