A Resolved Map of the Infrared Excess in a Lyman Break Galaxy at z = 3

We have observed the dust continuum of 10 z = 3.1 Lyman break galaxies with the Atacama Large Millimeter/submillimeter Array at ˜450 mas resolution in Band 7. We detect and resolve the 870 μm emission in one of the targets with a flux density of S 870 = 192 ± 57 μJy, and measure a stacked 3σ signal of S 870 = 67 ± 23 μJy for the remaining nine. The total infrared luminosities are L 8-1000 = (8.4 ± 2.3) × 1010 L ⊙ for the detection and L 8-1000 = (2.9 ± 0.9) × 1010 L ⊙ for the stack. With Hubble Space Telescope Advanced Camera for Surveys I-band imaging we map the rest-frame UV emission on the same scale as the dust, effectively resolving the “infrared excess” (IRX = L FIR/L UV) in a normal galaxy at z = 3. Integrated over the galaxy we measure IRX = 0.56 ± 0.15, and the galaxy-averaged UV slope is β = -1.25 ± 0.03. This puts the galaxy a factor of ˜10 below the IRX-β relation for local starburst nuclei of Meurer et al. However, IRX varies by more than a factor of 3 across the galaxy, and we conclude that the complex relative morphology of the dust relative to UV emission is largely responsible for the scatter in the IRX-β relation at high-z. A naive application of a Meurer-like dust correction based on the UV slope would dramatically overestimate the total star formation rate, and our results support growing evidence that when integrated over the galaxy, the typical conditions in high-z star-forming galaxies are not analogous to those in the local starburst nuclei used to establish the Meurer relation.