Connecting low-redshift LISA massive black hole mergers to the nHz stochastic gravitational wave background

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
e-mail: david.izquierdovillalba@unimib.it
² INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³ Department of Astronomy, MongManWai Building, Tsinghua University, Beijing 100084, PR China
⁴ Donostia International Physics Centre (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastian, Spain
⁵ IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain
⁶ INAF/Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁷ INFN, Sezione Roma 1, Dipartimento di Fisica, “Sapienza” Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy

Received: 19 January 2024
Accepted: 27 March 2024

Abstract

Pulsar Timing Array (PTA) experiments worldwide recently reported evidence of a nHz stochastic gravitational wave background (sGWB) compatible with the existence of slowly inspiralling massive black hole (MBH) binaries (MBHBs). The shape of the signal contains valuable information about the evolution of z < 1 MBHs above 10⁸ M_⊙, suggesting a faster dynamical evolution of MBHBs towards the gravitational-wave-driven inspiral or a larger MBH growth than usually assumed. In this work, we investigate if the nHz sGWB could also provide constraints on the population of merging lower-mass MBHBs (< 10⁷ M_⊙) detectable by LISA. To this end, we use the L-Galaxies semi-analytical model applied to the Millennium suite of simulations. We generate a population of MBHs compatible simultaneously with current electromagnetic and nHz sGWB constraints by including the possibility that, in favourable environments, MBHs can accrete gas beyond the Eddington limit. The predictions of this new model for the sGWB show that the global (integrated up to high-z) LISA detection rate is not significantly affected when compared to a fiducial model whose nHz sGWB signal is ∼2 times smaller. In both cases, the global rate yields ∼12 yr⁻¹ and is dominated by systems of 10^5 − 6 M_⊙. The main differences are limited to low-z (z < 3), high-mass (> 10⁶ M_⊙) LISA MBHBs. The model compatible with the latest PTA results predicts up to ∼1.6 times more detections, with a rate of ∼1 yr⁻¹. We find that these LISA MBHB systems have 50% probability of shining with bolometric luminosities > 10⁴³ erg s⁻¹. Hence, in case PTA results are confirmed and given the current MBH modelling, our findings suggest there will be higher chances to perform multimessenger studies with LISA MBHB than previously expected.