📝 SummarXiv

Abstract

We investigate the impact of gravitational-wave (GW) recoil on the growth of supermassive black holes (SMBHs) in the early Universe. Forming ~10^9 Msun SMBHs by z ~ 6 is challenging and may require hierarchical mergers of smaller seed black holes. We extend a semi-analytic seed model (Sassano et al. 2021) by explicitly incorporating GW recoil physics. Our model includes: (1) recoil velocity formulae calibrated to numerical relativity for spinning, unequal-mass BH binaries (Campanelli et al. 2007; Lousto et al. 2012); (2) assignment of spin magnitudes and orientations based on seed type (Pop III remnant, stellar cluster, or direct-collapse); and (3) a retention probability scheme comparing the recoil speed to the host halo escape velocity. We find that including GW recoil reduces final SMBH masses by ~20-30% by z = 6 and creates a population of off-nuclear ("wandering") BHs amounting to a few percent of the total. Observable consequences include spatial offsets ~0.1 arcsec and line-of-sight velocity shifts ~10^2-10^3 km/s in a few percent of high-z quasars. All code is publicly available at https://github.com/SMALLSCALEDEV/Black-hole-Recoil-Effects