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 arcmin^2 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 ≳ 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 (>10^(9.75) 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∗ (<10^(9.75) M⊙) galaxies is 0.11^(+0.32)_(-0.42) ± 0.34 (bootstrap and formal uncertainties) and 0.14^(+0.15)_(-0.14) ± 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 ≳ 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 (∝(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.

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