First light and reionization epoch simulations (FLARES) – XIX. Supermassive black hole mergers in the early Universe and their environmental dependence

The upcoming space-based gravitational wave (GW) observatory, Laser Interferometer Space Antenna (LISA), is expected to detect GW signals from supermassive black hole (SMBH) mergers occurring at high redshifts. However, understanding the origin and growth of SMBHs in the early Universe remains an open problem in astrophysics. In this work, we utilize the First Light And Reionization Epoch Simulations (FLARES), a suite of cosmological hydrodynamical zoom-in simulations, to study SMBH mergers at 5≲ z ≲ 10 across a wide range of environments. Most mergers in FLARES involve secondary SMBHs near the seed mass (m_seed ≈ 1.5 × 10⁵ M_⨀) while primary SMBHs span up to 10⁹ M_⨀, resulting in mass ratios from q ∼ 10−⁴ to 1, with a peak at q ∼ 1. The number of mergers increases rapidly towards lower redshifts, and the comoving total number density scales with overdensity as n_merger = 10−^3.81(1 + δ) ^4.78. Denser regions host more massive mergers, with higher merger redshifts and lower mass ratios. Within the FLARES redshift range, LISA is expected to detect mergers with 10⁵ ≲ M_tot /M_⨀ ≲ 10⁸ and q ⪰ q 10−², corresponding to a detection rate of 0.030 yr−1 for events with signal-to-noise ratio SNR ≥ 10. Ourstudy demonstrates the sensitivity of GW predictions at high redshifts to SMBH seed models and merger time delays, highlighting the need for improved modelling in future cosmological simulations to maximize LISA’s scientific return.