Formation of an embryonic supermassive star in the first galaxy

Abstract

Abstract We studied the gravitational collapse of a warm (∼8000 K) primordial-gas cloud as a candidate progenitor for a supermassive star (SMS; ≳ 105 M⊙) using a three-dimensional hydrodynamical simulation including all the relevant cooling processes of both H2 and H, which can potentially induce cloud fragmentation. This is the first simulation of this kind to resolve protostar formation. We find that from a weakly turbulent initial condition, the cloud undergoes runaway collapse without a major episode of fragmentation. Although the H2 fraction jumps by a large factor via the three-body reaction at ∼10−13 g cm−3, its cooling remains inefficient due to the optical thickness, and the temperature remains ≳ 3000 K. When the central core of the cloud becomes opaque to continuum radiation at ∼10−8 g cm−3, a hydrostatic protostar with ≃0.2 M⊙ is formed. The protostar grows to the mass ≃1 M⊙ and the radius ≃2 au within ∼1 yr via rapid accretion of dense filamentary flows. With high accretion rate, ∼2 M⊙ yr−1, the protostar is expected to turn into an SMS within its lifetime, eventually collapsing to a seed for the supermassive black hole observed in the early Universe at z ∼ 7.