2026A&A...707A..38T

Intermediate-mass black holes (IMBHs) are the link between stellar-mass and supermassive black holes. Gravitational waves have started to unveil a population of IMBHs in the ∼100-300 M_⊙ range. Here, we investigate the formation of IMBHs from non-rotating very massive stars (VMSs, > 100 M_⊙). We calculate new VMS models that account for the transition from optically thin to optically thick winds, and study how this enhanced mass loss affects IMBH formation and the black hole mass function at intermediate and high metallicity (Z = 10^–4-0.02). We show that optically thick winds suppress the formation of IMBHs from direct VMS collapse at metallicities Z > 0.001, one order of magnitude lower than predicted by previous models. Our models indicate that the stellar progenitors of GW231123 must have had a metallicity Z < 0.002, if the primary black hole formed via direct VMS collapse.