Feedback from the infrared background in the early Universe

Abstract

It is commonly believed that the earliest stages of star formation in the Universe were selfregulated by global radiation backgrounds, either by the ultraviolet (UV) Lyman-Werner (LW) photons emitted by the first stars (directly photodissociating H 2), or by the X-rays produced by accretion on to the black hole (BH) remnants of these stars (heating the gas but catalysing H 2 formation). Recent studies have suggested that a significant fraction of the first stars may have had low masses (a few M (). Such stars do not leave BH remnants and they have softer spectra, with copious infrared (IR) radiation at photon energies ~1 eV. 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 (, contributed >0.3 per cent of the UV background (or their mass fraction exceeded ~80 per cent), 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 photodetachment 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 a few higher than for high-mass stars. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.