Modelling the cosmological Lyman-Werner background radiation field in the Early Universe

Abstract

The Lyman-Werner (LW) radiation field is a key ingredient in the chemo-thermal evolution of gas in the Early Universe, as it dissociates H 2 molecules, the primary cooling channel in an environment devoid of metals and dust. Despite its important role, it is still not implemented in cosmological simulations on a regular basis, in contrast to the general UV background. This is in part due to uncertainty in the source modelling, their spectra and abundance, as well as the detailed physics involved in the propagation of the photons and their interactions with the molecules. To overcome these difficulties, we present here a model (with the relative fit) for the mean LW intensity during the first billion years after the Big Bang, obtained by post-processing the high-resolution FiBY simulations with an approximated radiative transfer method that employs accurate cross sections for H 2 , as well as for Hand H + 2 , the chemical species associated with its formation. Absorption by neutral Hydrogen in the IGM and various spectral models for Population III and Population II stars are also included. Our model can be easily applied to other simulations or semi-analytical models as an external homogeneous source of radiation that regulates the star formation in low-mass halos at high-z. We also show how to account for spatial inhomogeneities in the LW radiation field, originating from massive star-forming galaxies that dominate the photon budget up to distances of ∼ 100 proper kpc. Such inhomogeneities have a strong impact on the H 2 abundance and the feasibility of scenarios such as the formation of Direct Collapse Black Holes (DCBHs).

Imprints of early Universe galaxy formation on the 21-cm signal

“…[13] and the Lyman-Werner background from Ref. [14]. We also consider the baryon streaming velocity that can prevent the baryons from falling into the gravitational potential well of dark matter haloes [15,16].…”