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

Abstract

Cosmic rays (CRs) control the thermal, ionization and chemical state of the dense H2 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 H2 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 UCR 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 UCR ~(few) × 103 UCR,Gal, CRs dramatically alter the initial conditions of star formation. In the resulting extreme CRDRs H2 cloud fragmentation will produce far fewer low mass (<8M⊙) 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. Finally the large, CR-induced, ionization fractions expected for (far-UV)-shielded H2 gas in the CRDRs of compact starbursts will lengthen the ambipolar diffusion (AD) timescales so much as to render the alternative AD-regulated rather (Jeans mass)-driven star formation scenario as utterly unrealistic for the ISM in such galaxies.