Witnessing the birth of a supermassive protostar

The detection of $\rm z>6$ quasars reveals the existence of supermassive black holes of a few $\rm 10^9~M_{\odot}$. One of the potential pathways to explain their formation in the infant universe is the so-called direct collapse model which provides massive seeds of $\rm 10^5-10^6~M_{\odot}$. An isothermal direct collapse mandates that halos should be of a primordial composition and the formation of molecular hydrogen remains suppressed in the presence of a strong Lyman Werner flux. In this study, we perform high resolution cosmological simulations for two massive primordial halos employing a detailed chemical model which includes $\rm H^-$ cooling as well as realistic opacities for both the bound-free $\rm H^-$ emission and the Rayleigh scattering of hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high densities of $\rm \sim 10^{-3}~g/cm^3$ and to scales of about $\rm 10^{-4}$ AU. Our results show that the gas cools down to $\rm \sim $ 5000 K in the presence of $\rm H^-$ cooling, and induces fragmentation at scales of about 8000 AU in one of the two simulated halos, which may lead to the formation of a binary. In addition, fragmentation also occurs on the AU scale in one of the halos but the clumps are expected to merge on short time scales. Our results confirm that $\rm H^-$ cooling does not prevent the formation of a supermassive star and the trapping of cooling radiation stabilises the collapse on small scales.