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

Abstract

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 1σ 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–β relation to z ≥ 2 (assuming dust temperature T d ~ 35 K). However, only six tentative detections are found at z gsim 2 in ASPECS, with just three at >3σ. 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 M ⊙) subsample, with an infrared excess (IRX = L IR/L UV) consistent with previous z ~ 2 results. However, the infrared excess we measure from our large selection of sub-L lowast (<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–β relation for the Small Magellanic Cloud (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z gsim 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 ($\propto {(1+z)}^{0.32}$) such that T d ~ 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