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 H2 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 ionising 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. The goal of this work is to produce an accurate model of the LW radiation field at $z\geq6$, by post-processing the physics-rich high-resolution FiBY simulation. Our novelties include updated cross sections for H$_2$, H$^-$ and H$^+_2$ chemical species, IGM absorption by neutral Hydrogen and various spectral models for Population III and Population II stars. With our fiducial set of parameters, we show that the mean LW intensity steadily increases by three orders of magnitude from $z\sim23$ to $z\sim6$, while spatial inhomogeneities originate from massive star-forming galaxies that dominate the photon budget up to a distance of $\sim100$ proper kpc. Our model can be easily applied to other simulations or semi-analytical models as an external radiation field that regulates the formation of stars and massive black hole seeds in high-$z$ low-mass halos.