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

TL;DR

This study uses cosmological simulations to identify when cold accretion begins in early universe halos and demonstrates how it can lead to the formation of supermassive stars through dense shock-induced collapse.

Contribution

It provides the first long-term simulation evidence of cold accretion emergence and its role in supermassive star formation in early universe halos.

Findings

Cold accretion appears in halos above ~2.2×10^7 M_sun at z~15

Accretion flows create dense shocks on a wide disc surface

Shocks produce gas masses sufficient for gravitational collapse into supermassive stars

Abstract

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 halos with $\lesssim 10^8~{\rm M}_\odot$ with sufficiently high spatial resolutions down to $\sim 1 \ {\rm pc}$ 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 $\sim 2.2\times 10^7 \ {\rm M}_\odot\left\{\left(1+z\right)/15 \right\}^{-3/2}$, the ${\it minimum}$ halo masses above which the accretion flow penetrates halos. 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 $M_{\rm J}\sim 10^{4-5} \ {\rm M}_\odot$, a sufficient amount to induce the gravitational collapse, leading to the SMS formation.