$\mathrm{H_2}$ cooling and gravitational collapse of supersonically induced gas objects

TL;DR

This study uses cosmological simulations to explore the formation and collapse of gas objects induced by supersonic motions in the early universe, highlighting a pathway for primordial star formation outside dark matter halos.

Contribution

It demonstrates the formation of SIGOs via streaming motions and shows their potential to form stars without dark matter halos, including detailed cooling and collapse processes.

Findings

SIGOs form in high streaming velocity regions.

Some SIGOs collapse and become Jeans unstable.

Potential sites for primordial star or star cluster formation.

Abstract

We study the formation and gravitational collapse of supersonically induced gas objects (SIGOs) in the early universe. We run cosmological hydrodynamics simulations of SIGOs, including relative streaming motions between baryons and dark matter. Our simulations also follow nonequilibrium chemistry and molecular hydrogen cooling in primordial gas clouds. A number of SIGOs are formed in the run with fast-streaming motions of 2 times the rms of the cosmological velocity fluctuations. We identify a particular gas cloud that condensates by H$_2$ cooling without being hosted by a dark matter halo. The SIGO remains outside the virial radius of its closest halo, and it becomes Jeans unstable when the central gas-particle density reaches $\sim 100~{\rm cm}^{-3}$ with a temperature of $\sim$ 200 K. The corresponding Jeans mass is $\sim 10^5 M_{\odot}$, and thus the formation of primordial stars or a star cluster is expected in the SIGO.