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 (βUV) 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 10.7 M⊙ spanning a redshift range z = 2.6−3.7 (median z = 3.2) that were observed with ALMA at λrest = 300 μm. Both the individual ALMA detections (41 sources) and stacks including all galaxies show the IRX–βUV 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–βUV 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–βUV 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.