A3COSMOS: the dust attenuation of star-forming galaxies at z=2.5-4.0 from the COSMOS-ALMA archive

Abstract

We present an analysis of the dust attenuation of star-forming galaxies at z = 2.5-4.0 through the relationship between the UV spectral slope (β), stellar mass (M*), and the infrared excess (IRX = LIR/LUV) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A3COSMOS team, which includes an unprecedented sample of ∼1500 galaxies at z ∼ 3 as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 μJybeam−1 (1σ). The detection rate is highly mass dependent, decreasing drastically below log (M*/M⊙) = 10.5. The detected galaxies show that the IRX-β relationship of massive (log M*/M⊙ > 10) main-sequence galaxies at z = 2.5-4.0 is consistent with that of local galaxies, while starbursts are generally offset by ∼0.5dex to larger IRX values. At the low-mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX-M* relation at log(M∗/M⊙)>9 exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at log M*/M⊙ < 10. However, our results are consistent with earlier measurements at z ∼ 5.5, indicating a potential redshift evolution between z ∼ 2 and z ∼ 6. Deeper observations targeting low-mass galaxies will be required to confirm this finding.