The dust attenuation of star-forming galaxies at z~3 and beyond: new insights from ALMA observations

We present results on the dust attenuation of galaxies at redshift ~3-6 by studying the relationship between the UV spectral slope (ßUV) and the infrared excess (IRX; LIR/LUV) using ALMA far-infrared continuum observations. Our study is based on a sample of 67 massive, star-forming galaxies with a median mass of M*~10^10.7M? spanning a redshift range z=2.6-3.7 (median z=3.2) that were observed with ALMA at ?rest=300µm. Both the individual ALMA detections (41 sources) and stacks including all galaxies show the IRX-ßUV relationship at z~3 is mostly consistent with that of local starburst galaxies on average. However, we find evidence for a large dispersion around the mean relationship by up to ±0.5 dex. Nevertheless, the locally calibrated dust correction factors based on the IRX-ßUV relation are on average applicable to main-sequence z~3 galaxies. This does not appear to be the case at even higher redshifts, however. Using public ALMA observations of z~4-6 galaxies we find evidence for a significant evolution in the IRX-ßUV and the IRX-M* relations beyond z~3 toward lower IRX values. We discuss several caveats that could affect these results, including the assumed dust temperature. ALMA observations of larger z>3 galaxy samples will be required to confirm this intriguing redshift evolution.