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

We investigate the first emergence of the so-called cold accretion—supersonic accretion flows that penetrate deeply into a halo—in the early universe using cosmological N-body/SPH simulations. We resolve small halos of order 10^8 M⊙ with spatial resolution down to ~1 pc, and, unlike previous studies that followed only the short-term evolution after primordial cloud collapse, we follow the long-term evolution with a sink particle method until cold accretion first appears. We show that cold accretion emerges once the halo mass exceeds ~2.2 × 10^7 M⊙ × [(1+z)/15]^(-3/2), the minimum halo mass above which accretion flows can penetrate halos. Continuing the simulations, we assess whether cold accretion generates dense shocks capable of forming supermassive stars (SMSs). We find that the accretion flow ultimately impacts a compact disc near the halo center, creating shocks over a broad area of the disc surface. The post-shock gas becomes dense and hot (with mass comparable to the Jeans mass, ~10^4–10^5 M⊙), sufficient to induce gravitational collapse leading to SMS formation.

Masaki Kiyuna, Takashi Hosokawa, Sunmyon Chon