First Light and Reionization Epoch Simulations (FLARES) XXII: UV-dust spatial offsets at the Epoch of Reionisation

Abstract Recent observations have revealed intriguing offsets between the UV and FIR emission in high redshift galaxies. In this study, we use the First Light And Reionisation Epoch Simulations (Flares) to compute the spatial offset of ultraviolet (UV) and far-infrared (FIR) centres for a statistical sample (6890) of massive (M⋆ ≳ 109 M⊙) high redshift galaxies (z ∈ [5, 10]). The galaxies are post-processed with the skirt radiative transfer code, to obtain the full spectral energy distribution and surface brightness profile. We simulate James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam; rest-frame 1500 Å, ≈0.031″ resolution) and ALMA rest-frame 158 μm (≈ 0.3″ angular resolution) observations of the galaxies and then calculate the distance between the UV-FIR centres to analyse which physical processes drive the observed UV - FIR spatial offset. We find that ~16.23% of galaxies exhibit spatial offsets of ≥2.5 kpc between their UV and FIR emission peaks. We establish that the spatial offsets do not correlate with stellar mass, UV/FIR luminosity, and size. Offsets also do not correlate with AGN feedback or with large-scale environment or merger history. Galaxies with significant offsets preferentially have bluer UV slopes (−2.5 < β < −1.5), consistent with recent star formation and dust-attenuated cores displacing the observed UV centroid. They show an accelerated star formation history, forming half their z = 5 stellar mass ~0.1 Gyr earlier than galaxies without offsets. These galaxies are enriched earlier than galaxies without an offset and show enhanced stellar metallicities, indicating a transition to an outward growth at higher redshifts (z ≥ 6).