Impact of gas spin and Lyman–Werner flux on black hole seed formation in cosmological simulations: implications for direct collapse

Direct collapse black holes (BHs) are promising candidates for producing massive z ≳ 6 quasars, but their formation requires fine-tuned conditions. In this work, we use cosmological zoom simulations to study systematically the impact of requiring: (1) low gas angular momentum (spin), and (2) a minimum incident Lyman–Werner (LW) flux in order to form BH seeds. We probe the formation of seeds (with initial masses of Mseed ~ 10⁴\!-\!10⁶\, M\, h⁻¹) in haloes with a total mass >3000 × Mseed and a dense, metal-poor gas mass >5 × Mseed. Within this framework, we find that the seed-forming haloes have a prior history of star formation and metal enrichment, but they also contain pockets of dense, metal-poor gas. When seeding is further restricted to haloes with low gas spins, the number of seeds formed is suppressed by factors of ∼6 compared to the baseline model, regardless of the seed mass. Seed formation is much more strongly impacted if the dense, metal-poor gas is required to have a critical LW flux (Jcrit). Even for Jcrit values as low as 50J21, no 8× 10⁵~M\, h⁻¹ seeds are formed. While lower mass (1.25× 10⁴,1× 10⁵~M\, h⁻¹) seeds do form, they are strongly suppressed (by factors of ∼10–100) compared to the baseline model at gas mass resolutions of ~ 10⁴~M\, h⁻¹ (with even stronger suppression at higher resolutions). As a result, BH merger rates are also similarly suppressed. Since early BH growth is dominated by mergers in our models, none of the seeds are able to grow to the supermassive regime (10⁶~M\, h⁻¹) by z = 7. Our results hint that producing the bulk of the z ≳ 6 supermassive BH population may require alternate seeding scenarios that do not depend on the LW flux, early BH growth dominated by rapid or super-Eddington accretion, or a combination of these possibilities.