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The relation between infrared excess (IRX) and UV spectral slope ($\beta_{\rmUV}$) is an empirical probe of dust properties of galaxies. The shape, scatter,and redshift evolution of this relation are not well understood, however,leading to uncertainties in estimating the dust content and star formationrates (SFRs) of galaxies at high redshift. In this study, we explore the natureand properties of the IRX-$\beta_{\rm UV}$ relation with a sample of $z=2-6$galaxies ($M_*\approx 10^9-10^{12}\,M_\odot$) extracted from high-resolutioncosmological simulations (MassiveFIRE) of the Feedback in RealisticEnvironments (FIRE) project. The galaxies in our sample show an IRX-$\beta_{\rmUV}$ relation that is in good agreement with the observed relation in nearbygalaxies. IRX is tightly coupled to the UV optical depth, and is mainlydetermined by the dust-to-star geometry instead of total dust mass, while$\beta_{\rm UV}$ is set both by stellar properties, UV optical depth, and thedust extinction law. Overall, much of the scatter in the IRX-$\beta_{\rm UV}$relation of our sample is found to be driven by variations of the intrinsic UVspectral slope. We further assess how the IRX-$\beta_{\rm UV}$ relation dependson viewing direction, dust-to-metal ratio, birth-cloud structures, and the dustextinction law and we present a simple model that encapsulates most of thefound dependencies. Consequently, we argue that the reported `deficit' of theinfrared/sub-millimetre bright objects at $z>5$ does not necessarily imply anon-standard dust extinction law at those epochs.

The IRX-$\beta$ relation of high-redshift galaxies