Cosmic rays and the primordial gas

TL;DR

This study investigates how cosmic rays can enhance the formation of molecular hydrogen in primordial gas, thereby aiding cooling and early structure formation in the universe.

Contribution

The paper introduces a new computational model to analyze the impact of cosmic rays on the thermal and chemical evolution of primordial gas, highlighting their role in early universe cooling processes.

Findings

Cosmic rays can generate sufficient free electrons for molecular hydrogen formation.

Enhanced cooling of primordial gas occurs under specific cosmic ray flux and density conditions.

Cosmic rays potentially influence early star formation by facilitating gas cooling.

Abstract

One of the most outstanding problems in the gravitational collapse scenario of early structure formation is the cooling of primordial gas to allow for small mass objects to form. As the neutral primordial gas is a poor radiator at temperatures (T\le10^4\unit{K}), molecular hydrogen is needed for further cooling down to temperatures (T\sim100\unit{K}). The formation of molecular hydrogen is catalyzed by the presence of free electrons, which could be provided by the ionization due to an early population of cosmic rays. In order to investigate this possibility we developed a code to study the effects of ionizing cosmic rays on the thermal and chemical evolution of primordial gas. We found that cosmic rays can provide enough free electrons needed for the formation of molecular hydrogen, and therefore can increase the cooling ability of such primordial gas under following conditions: A dissociating photon flux with (F<10^{-18}\unit{erg cm^{-2} Hz^{-1} s^{-1}}), initial temperature of the gas (\sim10^{3}\unit{K}), total gas number densities (n\ge1\unit{cm^{-3}}), Cosmic ray sources with (\dot{\epsilon}_{CR}>10^{-33}\unit{erg cm^{-3} s^{-1}}).