The First Galaxies: Assembly, Cooling and the Onset of Turbulence

TL;DR

This study uses high-resolution simulations to explore the formation, thermal evolution, and turbulence onset in the first galaxies at redshifts greater than 10, highlighting the transition from primordial to Population II star formation.

Contribution

It provides detailed insights into the assembly, cooling processes, and turbulence development in the first galaxies, incorporating primordial chemistry and complex accretion modes.

Findings

First galaxies form via hot and cold accretion phases.

Partial ionization promotes molecular cooling, enabling gas to reach CMB temperature.

Turbulence onset likely triggers the transition to Population II star formation.

Abstract

We investigate the properties of the first galaxies at z > 10 with highly resolved numerical simulations, starting from cosmological initial conditions and taking into account all relevant primordial chemistry and cooling. A first galaxy is characterized by the onset of atomic hydrogen cooling, once the virial temperature exceeds 10^4 K, and its ability to retain photoheated gas. We follow the complex accretion and star formation history of a 5*10^7 M_sun system by means of a detailed merger tree and derive an upper limit on the number of Population III (Pop III) stars formed prior to its assembly. We investigate the thermal and chemical evolution of infalling gas and find that partial ionization at temperatures > 10^4 K catalyses the formation of H2 and hydrogen deuteride, allowing the gas to cool to the temperature of the cosmic microwave background. Depending on the strength of radiative and chemical feedback, primordial star formation might be dominated by intermediate-mass Pop III stars formed during the assembly of the first galaxies. Accretion on to the nascent galaxy begins with hot accretion, where gas is accreted directly from the intergalactic medium and shock-heated to the virial temperature, but is quickly accompanied by a phase of cold accretion, where the gas cools in filaments before flowing into the parent halo with high velocities. The latter drives supersonic turbulence at the centre of the galaxy and could lead to very efficient chemical mixing. The onset of turbulence in the first galaxies thus likely marks the transition to Pop II star formation.