The IRX–β Relation: Insights from Simulations

We study the relationship between the UV continuum slope and infrared excess (IRX ≡ L_(IR)/L_(FUV)) predicted by performing dust radiative transfer on a suite of hydrodynamical simulations of galaxies. Our suite includes both isolated disk galaxies and mergers intended to be representative of galaxies at both z ~ 0 and z ~ 2-3. Our low-redshift systems often populate a region around the locally calibrated Meurer et al. relation but move above the relation during merger-induced starbursts. Our high-redshift systems are blue and IR luminous and therefore lie above the Meurer et al. relation. The value of β strongly depends on the dust type used in the RT simulation: Milky-Way-type dust leads to significantly more negative (bluer) slopes compared with Small-Magellanic-Cloud-type dust. The effect on β due to variations in the dust composition with galaxy properties or redshift is the dominant model uncertainty. The dispersion in β is anticorrelated with specific star formation rate (sSFR) and tends to be higher for the z ~ 2-3 simulations. In the actively star-forming z ~ 2-3 simulated galaxies, dust attenuation dominates the dispersion in β, whereas in the z ~ 0 simulations, the contributions of star formation history (SFH) variations and dust are similar. For low-sSFR systems at both redshifts, SFH variations dominate the dispersion. Finally, the simulated z ~ 2-3 isolated disks and mergers both occupy a region in the IRX-β plane consistent with observed z ~ 2-3 dusty star-forming galaxies (DSFGs). Thus, contrary to some claims in the literature, the blue colors of high-z DSFGs do not imply that they are short-lived starbursts