tokens: ( 2015 ) galaxies are compatible with our simulations] then the slope of the reddening law required to explain the data is much steeper than the SMC extinction law. For example, the position of the stack of undetected galaxies would require n −20, much steeper than any attenuation, or even extinction law, ever observed. Given this discrepancy, it is worth considering the potential systematics that could be affecting the Capak et al. ( 2015 ) and Bouwens et al. ( 2016 ) results. One possibility is an underestimation of the dust temperature, which is taken to be 35 K in both cases. It has been suggested that dust temperature may increase with redshift due to an increase in the intensity of the radiation field heating the dust (e.g. Béthermin et al. 2015 ), 1 and typically an upper limit of 45 K has been assumed (Schaerer et al. 2015 ). In this case, L IR will be underestimated by a factor ≈2 and the resulting IRX underestimated by ≈0.3 dex. The magnitude of this shift is illustrated by the arrow in the bottom right-hand corner of Fig. 16 . It would bring the Capak et al. ( 2015 ) and Bouwens et al. ( 2016 ) data into better agreement with our model, however it is not enough to fully account for the discrepancy. There are other potential systematic issues affecting the determination of the UV continuum slopes, and estimated upper limits on L IR , which we will discuss in detail in a future work (McLure et al. in preparation). 6.2.5 SMC-like attenuation at lower redshifts? In Fig. 16 , we have not included data at lower redshifts ( z ∼ 2–3) which also support steeper, SMC-like, attenuation laws. In particular, Reddy et al. ( 2010 ) find that galaxies with young mass-weighted ages and typically lower masses (age ≲ 100 Myr; M/M ⊙ 10 10 ) have a smaller IRX, at a given β, relative to older galaxies at the same redshift. Finally, similar conclusions have also been reached using small numbers of individual lensed galaxies (e.g. Baker et al. 2001 ; Siana et al. 2009 ). In Reddy et al. ( 2010 ), these young galaxies represented ≈13 per cent of their full star-forming galaxy sample, and in our simulated sample ≈6 per cent of galaxies have ages 100 Myr. However, since our method involves comparing to the observed LF, it can only be applied to the full galaxy population complete down to a given UV magnitude, and cannot be applied to populations split by age. Therefore, we cannot rule out the possibility that certain populations (e.g. the youngest galaxies), or any given individual galaxy, will follow a steeper attenuation curve. We also note that, since the absolute attenuations decrease towards fainter, lower mass galaxies, the uncertainties on the form of the attenuation law will correspondingly increase, as the difference in absolute attenuation between curves becomes more difficult to distinguish. Therefore, our conclusions are most robust at high masses. Nevertheless, our current analysis suggests that, on average, a Calzetti-like attenuation curve
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