Multi-wavelength properties of z ≳ 6 Laser Interferometer Space Antenna detectable events

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, PI, Italy
² Dipartimento di Fisica, Sapienza, Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
³ Dipartimento di Fisica dell’Università di Trieste, Sez. di Astronomia, Via Tiepolo 11, I-34131 Trieste, Italy
⁴ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34131 Trieste, Italy
⁵ INFN, Instituto Nazionale di Fisica Nucleare, Via Valerio 2, I-34127 Trieste, Italy
⁶ ICSC – Italian Research Center on High Performance Computing, Big Data and Quantum Computing, Via Magnanelli 2, 40033 Casalecchio di Reno, Italy
⁷ INAF – Osservatorio Astronomico di Bologna, Via Piero Gobetti 93/3, I-40129 Bologna, Italy

^⋆ Corresponding author: srija.chakraborty@sns.it

Received: 8 November 2024
Accepted: 22 March 2025

Abstract

We investigated the intrinsic and observational properties of z ≳ 6 galaxies that host the coalescence of massive black holes (MBHs; M_BH ∼ 10⁵⁻⁶ M_⊙) giving rise to gravitational waves (GWs) detectable with the Laser Interferometer Space Antenna (LISA). We adopted a zoom-in cosmological hydrodynamical simulation of galaxy formation and black hole (BH) co-evolution, based on the GADGET-3 code, zoomed-in on an M_h ∼ 10¹² M_⊙ dark matter halo at z = 6, which hosts a fast accreting (Ṁ ∼ 35 M_⊙ yr⁻¹) super-massive black hole (SMBH; M_BH ∼ 10⁹ M_⊙) and a star-forming galaxy (SFR ∼ 100 M_⊙ yr⁻¹). Tracing the SMBH’s formation backwards in time, we identified the merger events associated with its formation and selected those that are detectable with LISA. Among these LISA-detectable events (LDEs), we selected those–based on their intrinsic properties (Ṁ, SFR, gas metallicity, and dust mass)–that were expected to be bright in one or more electromagnetic (EM) bands, such as rest-frame X-ray, UV and far-infrared (FIR). After considering the effect of delay due to dynamical friction in the MBH coalescence, we further restricted our selection to those LDEs that are still occurring at z ≳ 6. We find that ∼20–30% of the LDEs and their host galaxies are also detectable with EM telescopes. We post-processed these events with dust radiative transfer calculations to make accurate predictions about their spectral energy distributions (SEDs) and continuum maps in the JWST to ALMA wavelength range. We compared the spectra arising from galaxies hosting the merging MBHs with those arising from the active galactic nuclei (AGN) powered by single accreting BHs. We find that identifying an LDE from the continuum SEDs is impossible because of the absence of specific imprints from the merging MBHs. Finally, we computed the profile of the H_α line arising from LDEs, taking into account both the contribution from their star-forming regions and the accreting MBHs. We find that the presence of two accreting MBHs would be difficult to infer even if both MBHs accrete at super-Eddington rates (λ_EDD ∼ 5 − 10). We conclude that the combined detection of GW and EM signals from z ≳ 6 MBHs is challenging (if not impossible) not only because of the poor sky-localization (∼10 deg²) provided by LISA, but also because the loudest GW emitters (M_BH ∼ 10⁵⁻⁶ M_⊙) are not massive enough to leave significant signatures (e.g. extended wings) in the emission lines arising from the broad line region.