First emergence of cold accretion and supermassive star formation in the early universe

We investigate the first emergence of the so-called cold accretion, the accretion flows deeply penetrating a halo, in the early universe with cosmological N-body/SPH simulations. We study the structure of the accretion flow and its evolution within small haloes with less than or similar to 10(8) M-circle dot with sufficiently high spatial resolutions down to similar to 1 pc scale. While previous studies only follow the evolution for a short period after the primordial cloud collapse, we follow the long-term evolution until the cold accretion first appears, employing the sink particle method. We show that the cold accretion emerges when the halo mass exceeds similar to 2.2 x 10(7) M-circle dot{(1 + z)/15}(-3/2), the minimum halo masses above which the accretion flow penetrates haloes. We further continue simulations to study whether the cold accretion provides the dense shock waves, which have been proposed to give birth to supermassive stars (SMSs). We find that the accretion flow eventually hits a compact disc near the halo centre, creating dense shocks over a wide area of the disc surface. The resulting post-shock gas becomes dense and hot enough with its mass comparable to the Jeans mass M-J similar to 10(4-5) M-circle dot, a sufficient amount to induce the gravitational collapse, leading to the SMS formation.