Feedback from the IR Background in the Early Universe

TL;DR

This paper explores how soft-spectrum, low-mass stars in the early universe could have dominated radiative feedback through infrared radiation, affecting H_2 formation and star formation regulation.

Contribution

It demonstrates that low-mass stars with soft spectra can significantly influence early universe feedback via IR radiation, a previously underappreciated mechanism.

Findings

IR radiation from low-mass stars can dominate early feedback.

Feedback mechanisms differ from those of high-mass stars.

A small fraction of low-mass stars can suppress H_2 cooling effectively.

Abstract

It is commonly believed that the earliest stages of star-formation in the Universe were self-regulated by global radiation backgrounds - either by the ultraviolet Lyman-Werner (LW) photons emitted by the first stars (directly photodissociating H_2), or by the X-rays produced by accretion onto the black hole (BH) remnants of these stars (heating the gas but catalyzing H_2 formation). Recent studies have suggested that a significant fraction of the first stars may have had low masses (a few M_sun). Such stars do not leave BH remnants and they have softer spectra, with copious infrared (IR) radiation at photon energies around 1eV. Similar to LW and X-ray photons, these photons have a mean-free path comparable to the Hubble distance, building up an early IR background. Here we show that if soft-spectrum stars, with masses of a few M_sun, contributed more than 1% of the UV background (or their mass fraction exceeded 90%), then their IR radiation dominated radiative feedback in the early Universe. The feedback is different from the UV feedback from high-mass stars, and occurs through the photo-detachment of H^- ions, necessary for efficient H_2 formation. Nevertheless, we find that the baryon fraction which must be incorporated into low-mass stars in order to suppress H_2-cooling is only a factor of few higher than for high-mass stars.