Ionization Front Instabilities in Primordial H II Regions

TL;DR

This study uses 3D simulations to show that ionization front instabilities likely occurred in the primordial H II regions of the first stars, affecting early universe reionization and chemical enrichment.

Contribution

It demonstrates that ionization front instabilities can form in metal-free primordial gas, influenced by Population III star spectra and molecular hydrogen, which was previously uncertain.

Findings

Primordial gas exhibits shadow and thin-shell instabilities in simulations.

H2 formation can trigger instabilities even under strong Lyman-Werner flux.

Metals at 0.001-0.01 solar abundance influence instability development.

Abstract

Radiative cooling by metals in shocked gas mediates the formation of ionization front instabilities in the galaxy today that are responsible for a variety of phenomena in the interstellar medium, from the morphologies of nebulae to triggered star formation in molecular clouds. An important question in early reionization and chemical enrichment of the intergalactic medium is whether such instabilities arose in the H II regions of the first stars and primeval galaxies, which were devoid of metals. We present three-dimensional numerical simulations that reveal both shadow and thin-shell instabilities readily formed in primordial gas. We find that the hard UV spectra of Population III stars broadened primordial ionization fronts, causing H2 formation capable of inciting violent thin- shell instabilities in D-type fronts, even in the presence of intense Lyman-Werner flux. The high post- front gas temperatures associated with He ionization sustained and exacerbated shadow instabilities, unaided by molecular hydrogen cooling. Our models indicate that metals eclipsed H2 cooling in I-front instabilities at modest concentrations, from 0.001- 0.01 solar. We conclude that ionization front instabilities were prominent in the H II regions of the first stars and galaxies, influencing the escape of ionizing radiation and metals into the early universe.