ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: the infrared excess of UV-selected z = 2–10 galaxies as a function of UV-continuum slope and stellar mass

We make use of deep 1.2 mm continuum observations (12.7 µJy beam-1 rms) of a 1 arcmin² region in the Hubble Ultra Deep Field to probe dust-enshrouded star formation from 330 Lyman-break galaxies spanning the redshift range z = 2–10 (to ~2–3 M? yr-1 at 1s over the entire range). Given the depth and area of ASPECS, we would expect to tentatively detect 35 galaxies, extrapolating the Meurer z ~ 0 IRX–ß relation to z > 2 (assuming dust temperature Td ~ 35 K). However, only six tentative detections are found at z >~ 2 in ASPECS, with just three at >3s. Subdividing our z = 2–10 galaxy samples according to stellar mass, UV luminosity, and UV-continuum slope and stacking the results, we find a significant detection only in the most massive (>109.75 Me) subsample, with an infrared excess (IRX = LIR/LUV) consistent with previous z ~ 2 results. However, the infrared excess we measure from our large selection of sub-L* (<109.75 M?) galaxies is - + 0.11 0.42 0.32 ± 0.34 (bootstrap and formal uncertainties) and - + 0.14 0.14 0.15 ± 0.18 at z = 2–3 and z = 4–10, respectively, lying below even an IRX–ß relation for the Small Magellanic Cloud (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z >~ 2 galaxies. We find that the evolution of the IRX–stellar mass relationship depends on the evolution of the dust temperature. If the dust temperature increases monotonically with redshift (µ +1 z 0.32 ( ) ) such that Td ~ 44–50 K at z 4, current results are suggestive of little evolution in this relationship to z ~ 6. We use these results to revisit recent estimates of the z > 3 star formation rate density.