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 μ Jy, beam-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 ∼ 0.5 dex 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.
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