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

TL;DR

This paper investigates how dark matter spikes around supermassive black hole binaries influence the nanohertz gravitational wave background, potentially explaining observed signals and offering new insights into dark matter distribution.

Contribution

It introduces a model considering dark matter spikes' effects on SMBH binary evolution and demonstrates improved data fitting with Bayesian inference.

Findings

Dark matter spikes affect the gravitational wave spectrum.

Model with DM spike fits NANOGrav and EPTA data better.

Spike slope suggests flattening during inspiral.

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.