Impact of gas spin and Lyman–Werner flux on black hole seed formation in cosmological simulations: implications for direct collapse
Aklant K. Bhowmick, Laura Blecha, Paul Torrey, Luke Zoltan Kelley, Mark Vogelsberger, Dylan Nelson, Rainer Weinberger, Lars Hernquist
Abstract
ABSTRACT 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 $M_{\rm seed} \sim 10^4\!-\!10^6\, \mathrm{M}_{\odot }\, h^{-1})$ 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 $8\times 10^{5}~\mathrm{M}_{\odot }\, h^{-1}$ seeds are formed. While lower mass ($1.25\times 10^{4},1\times 10^{5}~\mathrm{M}_{\odot }\, h^{-1}$) seeds do form, they are strongly suppressed (by factors of ∼10–100) compared to the baseline model at gas mass resolutions of $\sim 10^4~\mathrm{M}_{\odot }\, h^{-1}$ (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 ($\gtrsim 10^6~\mathrm{M}_{\odot }\, h^{-1}$) 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.