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Astronomers using simulations of 6,890 high-redshift galaxies (from when the universe was very young) found that approximately 16% show significant spatial misalignment between their ultraviolet light from stars and far-infrared emission from dust clouds, with offsets of 2.5 kilometers or more. These misaligned galaxies have distinctive characteristics including bluer UV slopes, accelerated star formation histories forming half their stellar mass about 100 million years earlier than aligned galaxies, and enhanced stellar metallicities. The offsets appear to result from dust-obscured cores displacing the visible UV emission center and indicate a transition to outward galactic growth at very early cosmic times.
Why it matters
This research helps explain the physical processes governing galaxy formation in the early universe and reveals that some young galaxies grew from the inside out rather than uniformly. Understanding these formation patterns provides critical constraints for models of how the first galaxies assembled and evolved into the structures we observe today.
arXiv:2605.27370v1 Announce Type: new
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 (textsc{Flares}) to compute the spatial offset of ultraviolet (UV) and far-infrared (FIR) centres for a statistical sample (6890) of massive (M$_{star}, gtrsim10^{9} ,{rm M_{odot}}$) high redshift galaxies ($z in [5,10]$). The galaxies are post-processed with the textsc{skirt} radiative transfer code, to obtain the full spectral energy distribution and surface brightness profile. We simulate textit{James Webb Space Telescope (JWST)} Near Infrared Camera (NIRCam; rest-frame 1500 AA , $ approx 0.031 ”$ resolution) and ALMA rest-frame 158 um ($approx$ $0.3”$ 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 $sim16.23%$ of galaxies exhibit spatial offsets of $geq 2.5$ 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 ($-2.5<beta<-1.5$), 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 $sim$0.1 Gyr earlier than galaxies without offsets. These galaxies are enriched earlier than galaxies without an offset and show enhanced stellar metallicities, indicating a transition to an outward growth at higher redshifts ($z geq 6$).