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We utilise a series of high-resolution cosmological zoom simulations ofgalaxy formation to investigate the relationship between the ultraviolet (UV)slope, beta, and the ratio of the infrared luminosity to UV luminosity (IRX) inthe spectral energy distributions (SEDs) of galaxies. We employ dust radiativetransfer calculations in which the SEDs of the stars in galaxies propagatethrough the dusty interstellar medium. Our main goals are to understand theorigin of, and scatter in the IRX-beta relation; to assess the efficacy ofsimplified stellar population synthesis screen models in capturing theessential physics in the IRX-beta relation; and to understand systematicdeviations from the canonical local IRX-beta relations in particularpopulations of high-redshift galaxies. Our main results follow. Galaxies thathave young stellar populations with relatively cospatial UV and IR emittingregions and a Milky Way-like extinction curve fall on or near the standardMeurer relation. This behaviour is well captured by simplified screen models.Scatter in the IRX-beta relation is dominated by three major effects: (i) olderstellar populations drive galaxies below the relations defined for localstarbursts due to a reddening of their intrinsic UV SEDs; (ii) complexgeometries in high-z heavily star forming galaxies drive galaxies toward blueUV slopes owing to optically thin UV sightlines; (iii) shallow extinctioncurves drive galaxies downward in the IRX-beta plane due to lowered NUV/FUVextinction ratios. We use these features of the UV slopes of galaxies to derivea fitting relation that reasonably collapses the scatter back toward thecanonical local relation. Finally, we use these results to develop anunderstanding for the location of two particularly enigmatic populations ofgalaxies in the IRX-beta plane: z~2-4 dusty star forming galaxies, and z>5 starforming galaxies.

The IRX-Beta Dust Attenuation Relation in Cosmological Galaxy Formation Simulations