The initial conditions of star formation: cosmic rays as the fundamental regulators

TL;DR

Cosmic rays significantly influence the initial conditions of star formation by controlling the thermal, ionization, and chemical states of dense molecular gas, especially in extreme environments like starburst galaxies, leading to variations in the stellar initial mass function.

Contribution

This paper presents a new framework showing how cosmic rays regulate star formation initial conditions and cause a bimodal initial mass function in different galactic environments.

Findings

CRs control the thermal and chemical state of dense gas regions.

Extreme CR energy densities lead to a top-heavy stellar IMF.

Galaxies' star formation history influences the integrated galactic IMF.

Abstract

Cosmic rays (CRs) control the thermal, ionization and chemical state of the dense H_2 gas regions that otherwise remain shielded from far-UV and optical stellar radiation propagating through the dusty ISM of galaxies. It is in such CR-dominated regions (CRDRs) rather than Photon-dominated regions (PDRs) of H_2 clouds where the star formation initial conditions are set, making CRs the ultimate star-formation feedback factor in galaxies, able to operate even in their most deeply dust-enshrouded environments. CR-controlled star formation initial conditions naturally set the stage for a near-invariant stellar Initial Mass Function (IMF) in galaxies as long as their average CR energy density U_{CR} permeating their molecular ISM remains within a factor of ~10 of its Galactic value. Nevertheless, in the extreme environments of the compact starbursts found in merging galaxies, where U_{CR}\sim(few)x10^{3}U_{CR,Gal}, CRs dramatically alter the initial conditions of star formation. In the resulting extreme CRDRs H_2 cloud fragmentation will produce far fewer low mass (<8 M_{sol}) stars, yielding a top-heavy stellar IMF. This will be a generic feature of CR-controlled star-formation initial conditions, lending a physical base for a bimodal IMF during galaxy formation, with a top-heavy one for compact merger-induced starbursts, and an ordinary IMF preserved for star formation in isolated gas-rich disks. In this scheme the integrated galactic IMFs (IGIMF) are expected to be strong functions of the star formation history of galaxies.