The spin evolution of massive black holes across cosmic time
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Full Text E-thesis
Date
2025
Authors
Whitaker, Emily
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Publisher
University College Cork
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Abstract
Massive black hole spin governs radiative efficiency, jet power, and gravitational-wave signatures, making it a key variable in galaxy-black hole co-evolution.
Cosmological simulations, however, cannot resolve disc-scale physics and typically omit spin evolution. Yet, how unresolved accretion geometry--especially during off-centre phases--and unconstrained early growth rates shape cosmic spin histories remains poorly quantified, and observed spin samples are luminosity biased. Here we develop and apply a post-processing spin model that combines the accretion-driven prescriptions Dubois et al. (2014) and Ricarte et al. (2023) to the Romulus25 simulation, evolving both spin magnitude and direction through thin-disk and hot/magnetically arrested disk (MAD) regimes, including spin-dependent radiative and jet efficiencies. We take advantage of Romulus uniquely following realistic massive black hole (MBH) orbits that often wander off-centre to bracket unresolved physics with three off-centre accretion prescriptions (assumed central, chaotic, gas adhesion), and two seed masses (10^6 M_sol and 10^5 M_sol), yielding six models. We find that (i) high spins are built primarily at early times (z≳3) via prolonged, coherent thin-disk accretion; (ii) declining gas supply and increasing hot-mode duty cycles drive a broad low-spin distribution by z=0; (iii) mergers, occurring between largely aligned progenitors, produce moderate remnant spins and play a secondary role in setting the ensemble distribution; (iv) lowering the seed mass and therefore increasing early effective Eddington ratios increases spin-up, but this imprint fades once black holes grow beyond ~10^(7-8) M_sol; and (v) imposing an observational luminosity cut better reproduces the high spins seen in current X-ray samples, revealing strong selection bias relative to the full population. These results imply that present high-spin measurements do not reflect the cosmic mean, but rather a luminosity-weighted subset with sustained coherent accretion. Strong spin alignment and moderate remnant spins predict low recoil velocities and a Laser Interferometer Space Antenna (LISA) population dominated by weakly precessing binaries. More generally, our off-centre experiments set quantitative bounds on how wandering suppresses spin growth, highlighting the need to model displaced accretion explicitly.
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Keywords
Black hole physics , AGN , Black hole spin , Galaxy evolution , Black hole evolution , Astronomy
Citation
Whitaker, E. 2025. The spin evolution of massive black holes across cosmic time. MSc Thesis, University College Cork.