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 ≥ 6, by post-processing the physics-rich high-resolution FiBY simulation. Our novelties include updated cross sections for H2, $\rm {H^-}$ and $\rm {H^+_2}$ 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 ∼ 23 to z ∼ 6, while spatial inhomogeneities originate from massive star-forming galaxies that dominate the photon budget up to a distance of ∼100 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.

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).

Thin stellar discs on both galactic and nuclear, sub-kpc scales are believed to be fragile structures that would be easily destroyed in major mergers. In turn, this makes the age-dating of their stellar populations a useful diagnostics for the assembly history of galaxies. We aim at carefully exploring the fragility of such stellar discs in intermediate- and low-mass encounters, using high-resolution N-body simulations of galaxy models with structural and kinematic properties tailored to actually observed galaxies. As a first but challenging step, we create a dynamical model of FCC 170, a nearly edge-on galaxy in the Fornax cluster with multiple galactic components and including both a galactic scale and nuclear stellar disc (NSD), using detailed kinematic data from the Multi Unit Spectroscopic Explorer and a novel method for constructing distribution function-based self-consistent galaxy models. We then create N-body realisations of this model and demonstrate that it remains in equilibrium and preserves its properties over many Gyr, when evolved with a sufficiently high particle number. However, the NSD is more prone to numerical heating, which gradually increases its thickness by up to 22 per cent in 10 Gyr even in our highest-resolution runs. Nevertheless, these N-body models can serve as realistic representations of actual galaxies in merger simulations.