The Local Nanohertz Gravitational-Wave Landscape From Supermassive Black Hole Binaries

TL;DR

This paper estimates the population and GW emission of supermassive black hole binaries in local galaxies, predicting detectable signals in pulsar timing array data and their contribution to GW background anisotropy.

Contribution

It provides the first population estimate of local SMBHBs emitting nanohertz GWs and assesses their potential detectability with current PTA data.

Findings

Approximately 91 SMBHB sources emitting GWs in the PTA band.

Around 7 binaries may never merge, stalling at final parsec.

Local SMBHBs could contribute up to 20% to GW background anisotropy.

Abstract

Supermassive black hole binaries (SMBHBs) in the 10 million to 10 billion $M_\odot$ range form in galaxy mergers, and live in galactic nuclei with large and poorly constrained concentrations of gas and stars. There are currently no observations of merging SMBHBs--- it is in fact possible that they stall at their final parsec of separation and never merge. While LIGO has detected high frequency GWs, SMBHBs emit GWs in the nanohertz to millihertz band. This is inaccessible to ground-based interferometers, but possible with Pulsar Timing Arrays (PTAs). Using data from local galaxies in the 2 Micron All-Sky Survey, together with galaxy merger rates from Illustris, we find that there are on average $91\pm7$ sources emitting GWs in the PTA band, and $7\pm2$ binaries which will never merge. Local unresolved SMBHBs can contribute to GW background anisotropy at a level of $\sim20\%$, and if the GW background can be successfully isolated, GWs from at least one local SMBHB can be detected in 10 years.