Inference on inner galaxy structure via gravitational waves from supermassive binaries
Nature Astronomy, vol.10, no.4, pp.554-563, 2026 (SCI-Expanded, Scopus)
- Publication Type: Article / Article
- Volume: 10 Issue: 4
- Publication Date: 2026
- Doi Number: 10.1038/s41550-026-02782-0
- Journal Name: Nature Astronomy
- Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, INSPEC
- Page Numbers: pp.554-563
- Middle East Technical University Affiliated: Yes
Abstract
The detection of a stochastic gravitational wave background by pulsar-timing arrays indicates the presence of a population of supermassive black hole binaries. Although the observed spectrum generally matches predictions for orbital evolution driven by gravitational-wave emission in circular orbits, there is a preference for a spectral turnover at the lowest observed frequencies, which may point to substantial hardening during a transition from early environmental influences to later stages dominated by emission. In the vicinity of these binaries, the ejection of stars or dark matter particles through gravitational three-body slingshots efficiently extracts orbital energy, leading to a low-frequency turnover in the spectrum. Here we model how the gravitational-wave spectrum depends on the initial inner galactic profile before scouring by binary ejections while accounting for a range of initial binary eccentricities. By analysing the NANOGrav 15-year data, we find that a parsec-scale galactic-centre density of around 106 M⊙ pc−3 is favoured across most of the parameter space, thus shedding light on the environmental effects that shape black hole evolution and the combined matter density near galaxy centres.