Witnessing the birth of a supermassive protostar

The detection of z > 6 quasars reveals the existence of supermassive black holes of a few 10⁹M_⊙. 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 10⁵-10⁶M_⊙. An isothermal direct collapse mandates that haloes 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 haloes employing a detailed chemical model which includes H^- cooling as well as realistic opacities for both the bound-free H^- emission and the Rayleigh scattering of hydrogen atoms. We are able to resolve the collapse up to unprecedentedly high densities of ~10^-3 g cm^-3 and to scales of about 10^-4 au. Our results show that the gas cools down to ~5000 K in the presence of H^- cooling, and induces fragmentation at scales of about 8000 au in one of the two simulated haloes, which may lead to the formation of a binary. In addition, fragmentation also occurs on the au scale in one of the haloes but the clumps are expected to merge on short time-scales. Our results confirm that H^- cooling does not prevent the formation of a supermassive star and the trapping of cooling radiation stabilizes the collapse on small scales.