Observations of quasars at $ z > 6$ suggest the presence of black holes with a few times $\rm 10^9 ~M_{ \odot}$. Numerous models have been proposed to explain their existence including the direct collapse which provides massive seeds of $\rm 10^5~M_{ \odot}$. The isothermal direct collapse requires a strong Lyman-Werner flux to quench $\rm H_2$ 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 $\rm > 10^7~M_{ \odot}$ with $\rm Z/Z_{ \odot}= 10^{ -4}-10^{ -6}$ illuminated by an intense Lyman Werner flux of $\rm J_{ 21}=10^5$. Our results show that initially the collapse proceeds isothermally with $\rm T \sim 8000$ K but dust cooling becomes effective at densities of $\rm 10^{ 8}-10^{ 12} ~cm^{ -3}$ and brings the gas temperature down to a few 100-1000 K for $\rm Z/Z_{ \odot} \geq 10^{ -6}$. No gravitationally bound clumps are found in $\rm Z/Z_{ \odot} \leq 10^{ -5}$ cases by the end of our simulations in contrast to the case with $\rm Z/Z_{ \odot} = 10^{ -4}$. Large inflow rates of $\rm \geq 0.1~M_{ \odot}/yr$ are observed for $\rm Z/Z_{ \odot} \leq 10^{ -5}$ similar to a zero-metallicity case while for $\rm Z/Z_{ \odot} = 10^{ -4}$ the inflow rate starts to decline earlier due to the dust cooling and fragmentation. For given large inflow rates a central star of $\rm \sim 10^4~M_{ \odot}$ may form for $\rm Z/Z_{ \odot} \leq 10^{ -5}$.
Categories: astro-ph.GA astro-ph.CO