  We utilise a series of high-resolution cosmological zoom simulations of
galaxy formation to investigate the relationship between the ultraviolet (UV)
slope, beta, and the ratio of the infrared luminosity to UV luminosity (IRX) in
the spectral energy distributions (SEDs) of galaxies. We employ dust radiative
transfer calculations in which the SEDs of the stars in galaxies propagate
through the dusty interstellar medium. Our main goals are to understand the
origin of, and scatter in the IRX-beta relation; to assess the efficacy of
simplified stellar population synthesis screen models in capturing the
essential physics in the IRX-beta relation; and to understand systematic
deviations from the canonical local IRX-beta relations in particular
populations of high-redshift galaxies. Our main results follow. Galaxies that
have young stellar populations with relatively cospatial UV and IR emitting
regions and a Milky Way-like extinction curve fall on or near the standard
Meurer relation. This behaviour is well captured by simplified screen models.
Scatter in the IRX-beta relation is dominated by three major effects: (i) older
stellar populations drive galaxies below the relations defined for local
starbursts due to a reddening of their intrinsic UV SEDs; (ii) complex
geometries in high-z heavily star forming galaxies drive galaxies toward blue
UV slopes owing to optically thin UV sightlines; (iii) shallow extinction
curves drive galaxies downward in the IRX-beta plane due to lowered NUV/FUV
extinction ratios. We use these features of the UV slopes of galaxies to derive
a fitting relation that reasonably collapses the scatter back toward the
canonical local relation. Finally, we use these results to develop an
understanding for the location of two particularly enigmatic populations of
galaxies in the IRX-beta plane: z~2-4 dusty star forming galaxies, and z>5 star
forming galaxies.
