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 greater than or similar to 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 M-seed similar to 10(4) - 10(6) m(circle dot) n(-1)) in haloes with a total mass >3000 x M-seed and a dense, metal-poor gas mass >5 x M-seed. 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 similar to 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 (J(crit)). Even for J(crit) values as low as 50J(21), no 8 x 10(5) M-circle dot h(-1) seeds are formed. While lower mass (1.25 x 10(4), 1 x 10(5) M-circle dot h(-1)) seeds do form, they are strongly suppressed (by factors of similar to 10-100) compared to the baseline model at gas mass resolutions of similar to 10(4) M-circle dot 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 (greater than or similar to 10(6) M-circle dot h(-1)) by z = 7. Our results hint that producing the bulk of the z greater than or similar to 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.