No tension between assembly models of super massive black hole binaries and pulsar observations

• Published in: Nature communications Vol. 9; no. 1; pp. 573 - 7
• Main Authors: Middleton, Hannah, Chen, Siyuan, Del Pozzo, Walter, Sesana, Alberto, Vecchio, Alberto
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 08.02.2018; Nature Publishing Group; Nature Portfolio
• Subjects: 639/33/34; 639/33/34/124; 639/33/34/863; Assembly; Astronomical models; Bayesian analysis; Binary stars; Galactic evolution; Galaxies; Galaxy mergers & collisions; Gravitational waves; Gravity; Humanities and Social Sciences; Mathematical models; multidisciplinary; Pulsars; Science; Science (multidisciplinary); Stalling; Stellar evolution; Stochasticity; Supermassive black holes
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-02916-7

Pulsar timing arrays are presently the only means to search for the gravitational wave stochastic background from super massive black hole binary populations, considered to be within the grasp of current or near-future observations. The stringent upper limit from the Parkes Pulsar Timing Array has been interpreted as excluding (>90% confidence) the current paradigm of binary assembly through galaxy mergers and hardening via stellar interaction, suggesting evolution is accelerated or stalled. Using Bayesian hierarchical modelling we consider implications of this upper limit for a range of astrophysical scenarios, without invoking stalling, nor more exotic physical processes. All scenarios are fully consistent with the upper limit, but (weak) bounds on population parameters can be inferred. Recent upward revisions of the black hole–galaxy bulge mass relation are disfavoured at 1.6 σ against lighter models. Once sensitivity improves by an order of magnitude, a non-detection will disfavour the most optimistic scenarios at 3.9 σ . Pulsar timing arrays enable the search for the isotropic gravitational-wave (GW) background originating from super massive black hole binary populations, but impose a stringent upper limit on the GW characteristic amplitude. Here, the authors use Bayesian hierarchical modelling applied to a range of astrophysical scenarios to revisit the implications of this upper limit.