Radiative feedback and cosmic molecular gas: numerical method

TL;DR

This paper introduces a comprehensive 3D numerical simulation method combining radiative transfer and non-equilibrium chemistry to study cosmic structure formation and the impact of radiative feedback on primordial molecular gas.

Contribution

It presents a novel implementation of multi-frequency radiative transfer with detailed chemistry in GADGET, validated through various tests and applied to early universe simulations.

Findings

Ionization fronts can enhance H2 and HD fractions in shock regions.

Radiative feedback significantly reduces molecular abundances, affecting subsequent gas collapse.

Results highlight the importance of re-ionized or metal-enriched gas for structure formation.

Abstract

We present results from self-consistent 3D numerical simulations of cosmic structure formation with a multi-frequency radiative transfer scheme and non-equilibrium molecular chemistry of 13 primordial species (e-, H, H+, H-, He, He+, He++, H2, H2+, D, D+, HD, HeH+), performed by using the simulation code GADGET. We describe our implementation and show tests for ionized sphere expansion in a static and dynamic density field around a central radiative source, and for cosmological abundance evolution coupled with the cosmic microwave background radiation. As a demonstrative application of radiative feedback on molecular gas, we run also cosmological simulations of early structure formation in a ~1Mpc size box. Our tests agree well with analytical and numerical expectations. Consistently with other works, we find that ionization fronts from central sources can boost H2 fractions in shock-compressed gas. The tight dependence on H2 lead to a corresponding boost of HD fractions, as well. We see a strong lowering of the the typical molecular abundances up to several orders of magnitudes which partially hinders further gas collapse of pristine neutral gas, and clearly suggests the need of re-ionized gas or metal cooling for the formation of the following generation of structures.