Feedback from the infrared background in the early universe

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 (UV) Lyman-Werner (LW) photons emitted by the first stars (directly photodissociating H2), or by the X-rays produced by accretion on to the black hole (BH) remnants of these stars (heating the gas but catalysing H2 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 H2 formation. Nevertheless, we find that the baryon fraction which must be incorporated into low-mass stars in order to suppress H2 cooling is only a factor of a few higher than for high-mass stars.