IMPACT OF DUST COOLING ON DIRECT-COLLAPSE BLACK HOLE FORMATION

Observations of quasars at $z>6$ suggest the presence of black holes with a few times ${10}^9{M}_{\odot }$. Numerous models have been proposed to explain their existence, including a direct collapse, which provides massive seeds of ${10}^5{M}_{\odot }$. The isothermal direct collapse requires a strong Lyman–Werner (LW) flux to quench $H_2$ formation in massive primordial halos. In this study, we explore the impact of trace amounts of metals and dust enrichment. We perform three-dimensional cosmological simulations for two halos of $>{10}^7{M}_{\odot }$ with $Z/Z_{\odot }={10}^{-4}\text{--}{10}^{-6}$ illuminated by an intense LW flux of $J_{21}={10}^5$. Our results show that initially the collapse proceeds isothermally with $T\sim 8000$ K, but dust cooling becomes effective at densities of ${10}^8\text{--}{10}^{12}{cm}^{-3}$ and brings the gas temperature down to a few 100–1000 K for $Z/Z_{\odot }⩾{10}^{-6}$. No gravitationally bound clumps are found in the $Z/Z_{\odot }⩽{10}^{-5}$ cases by the end of our simulations, in contrast to the case with $Z/Z_{\odot }={10}^{-4}$. Large inflow rates of $⩾0.1M_{\odot }{yr}^{-1}$ are observed for $Z/Z_{\odot }⩽{10}^{-5}$, similar to a zero-metallicity case, while for $Z/Z_{\odot }={10}^{-4}$ the inflow rate starts to decline earlier because of dust cooling and fragmentation. For given large inflow rates, a central star of $\sim {10}^4{M}_{\odot }$ may form for $Z/Z_{\odot }⩽{10}^{-5}$.