Unravelling the formation of the first supermassive black holes with the SKA pulsar timing array

TL;DR

This paper forecasts the detectability of gravitational waves from merging supermassive black holes at high redshifts using the SKA Pulsar Timing Array, and explores how these detections can inform models of black hole formation and growth.

Contribution

It combines observational constraints and theoretical models to predict the number and properties of high-redshift black hole mergers detectable by SKA PTA, highlighting the potential to constrain early black hole evolution.

Findings

Most massive black hole counterparts will be localizable at z > 6.

SKA PTA can detect binaries with orbital periods of weeks to years.

Detections can constrain black hole seeding and growth mechanisms.

Abstract

Galaxy mergers at high redshifts trigger the activity of their central supermassive black holes, eventually also leading to their coalescence -- and a potential source of low-frequency gravitational waves detectable by the SKA Pulsar Timing Array (PTA). Two key parameters related to the fuelling of black holes are the Eddington ratio of quasar accretion $\eta_{\rm Edd}$, and the radiative efficiency of the accretion process, $\epsilon$ (which affects the so-called active lifetime of the quasar, $t_{\rm QSO}$). We forecast the regime of detectability of gravitational wave events with SKA PTA, finding the associated binaries to have orbital periods on the order of weeks to years, observable through relativistic Doppler velocity boosting and/or optical variability of their light curves. Combining the SKA regime of detectability with the latest observational constraints on high-redshift black hole mass and luminosity functions, and theoretically motivated prescriptions for the merger rates of dark matter haloes, we forecast the number of active counterparts of SKA PTA events expected as a function of primary black hole mass at $z \gtrsim 6$. We find that the quasar counterpart of the most massive black holes will be ${uniquely \ localizable}$ within the SKA PTA error ellipse at $z \gtrsim 6$. We also forecast the number of expected counterparts as a function of the quasars' Eddington ratio and active lifetime. Our results show that SKA PTA detections can place robust constraints on the seeding and growth mechanisms of the first supermassive black holes.