Dust attenuation in 2 < z < 3 star-forming galaxies from deep ALMA observations of the Hubble Ultra Deep Field

We present the results of a new study of the relationship betweeninfrared excess (IRX≡ LIR/LUV), UV spectralslope (β) and stellar mass at redshifts 2 <z <3, based on adeep Atacama Large Millimeter Array (ALMA) 1.3-mm continuum mosaic ofthe Hubble Ultra Deep Field (HUDF). Excluding the most heavily-obscuredsources, we use a stacking analysis to show that z ≃ 2.5star-forming galaxies in the mass range 9.25≤ log (M_{\ast}/M_{⊙}) ≤ 10.75 are fully consistent with the IRX-βrelation expected for a relatively grey attenuation curve, similar tothe commonly adopted Calzetti law. Based on a large, mass complete,sample of 2 ≤ z ≤ 3 star-forming galaxies drawn from multiplesurveys, we proceed to derive a new empirical relationship betweenβ and stellar mass, making it possible to predict UV attenuation(A1600) and IRX as a function of stellar mass, for anyassumed attenuation law. Once again, we find that z ≃ 2.5star-forming galaxies follow A1600-M* andIRX-M* relations consistent with a relatively greyattenuation law, and find no compelling evidence that star-forminggalaxies at this epoch follow a reddening law as steep as the SmallMagellanic Cloud (SMC) extinction curve. In fact, we use a simplesimulation to demonstrate that previous determinations of the IRX-βrelation may have been biased toward low values of IRX at red values ofβ, mimicking the signature expected for an SMC-like dust law. Weshow that this provides a plausible mechanism for reconciling apparentlycontradictory results in the literature and that, based on typicalmeasurement uncertainties, stellar mass provides a cleaner prediction ofUV attenuation than β. Although the situation at lower stellarmasses remains uncertain, we conclude that for 2 <z <3star-forming galaxies with log (M_{\ast }/M_{⊙}) ≥ 9.75, both theIRX-β and IRX-M* relations are well described by aCalzetti-like attenuation law.