Merger Histories Explain Black Hole Mass Scatter in L* Galaxies, Predicting Distinct Paths for Milky Way and M31
Preprint simulations tie BH-mass scatter in L* galaxies to merger-driven growth differences, forecasting a quiescent history for the Milky Way and a merger-rich path for M31.
A preprint posted to arXiv on 15 May 2026 (arXiv:2605.16501) uses the ARTEMIS and EAGLE cosmological hydrodynamical simulations to trace how supermassive black hole (BH) growth diverges among L* galaxies. Galaxies are split at fixed stellar mass into LBH (lowest BH-to-stellar-mass ratio) and HBH (highest ratio) samples; both populations look similar at z~2 but separate sharply at later times. LBH systems retain higher gas fractions and continue forming stars, while HBH galaxies quench earlier after rapid early BH growth and stronger feedback. The work attributes the scatter to merger histories: frequent mergers drive gas inflows and BH-BH coalescences that build more massive BHs and produce elliptical morphologies, whereas quiescent histories leave lower-mass BHs and disc galaxies. In HBH galaxies ~90% of final BH mass arrives via gas accretion; in LBH galaxies the split is roughly 60% accretion and 40% mergers. This preprint remains unrefereed and inherits the usual limitations of large-volume simulations—finite resolution (~kpc scale), sub-grid prescriptions for AGN and stellar feedback, and uncertain BH seeding. No direct observational sample size is quoted; instead the runs draw on the full EAGLE and ARTEMIS volumes that each contain several hundred L* analogues. The analysis fills a gap by linking these trends to concrete local-universe predictions: the Milky Way’s comparatively modest 4-million-solar-mass BH and ongoing star formation are consistent with a quieter merger record, while M31’s 100-million-solar-mass BH and earlier quenching point to a more active assembly history. Complementary results from the IllustrisTNG suite (Pillepich et al. 2018) and from Gaia-Enceladus merger reconstructions (Helmi et al. 2018) reinforce the picture that one or two significant mergers after z2 can boost central BH growth by factors of several. The simulations therefore supply a falsifiable forecast: future Gaia DR4 and JWST stellar-population maps should reveal an earlier truncation of star formation in M31’s inner disc than in the Milky Way’s.
HELIX: Merger timing after z~2 sets final BH mass and morphology; the Milky Way avoided late major mergers while M31 did not, explaining their contrasting BH masses and star-formation histories.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2605.16501)
- [2]Related Source(https://arxiv.org/abs/1707.03395)
- [3]Related Source(https://ui.adsabs.harvard.edu/abs/2018Natur.563...85H)