Interstellar Chemistry: Radiation, Dust and Metals

TL;DR

This paper reviews the chemical processes in primordial systems influenced by radiation, dust, and metals, highlighting how radiation type, metallicity, and cosmic rays affect molecular survival and cooling in early universe environments.

Contribution

It provides a comprehensive overview of how radiation, dust, and metallicity influence chemical reactions and molecular cooling in primordial systems, emphasizing differences between UV and X-ray effects.

Findings

X-ray photons allow molecules to survive better under irradiation.

Metallicity strongly affects the presence of cool molecular phases.

High cosmic ray fluxes convert CO into neutral carbon.

Abstract

An overview is given of the chemical processes that occur in primordial systems under the influence of radiation, metal abundances and dust surface reactions. It is found that radiative feedback effects differ for UV and X-ray photons at any metallicity, with molecules surviving quite well under irradiation by X-rays. Starburst and AGN will therefore enjoy quite different cooling abilities for their dense molecular gas. The presence of a cool molecular phase is strongly dependent on metallicity. Strong irradiation by cosmic rays (>100x the Milky Way value) forces a large fraction of the CO gas into neutral carbon. Dust is important for H2 and HD formation, already at metallicities of 10^-4-10^-3 solar, for electron abundances below 10^-3.