tokens: does not depend on stellar mass The mass–IRX relation has been widely examined over a wide redshift range (Reddy et al. 2010 ; Heinis et al. 2014 ; Whitaker et al. 2014 ; Álvarez-Márquez et al. 2016 ), finding that it evolves little out to z = 3 at the low-mass end of M * 10 in contrast to a rapid evolution at the high-mass end (Whitaker et al. 2014 ; Leslie et al. 2018 ). Some authors concluded from this that galaxy stellar mass is the most fundamental parameter determining the dust obscuration in SFGs (Garn Best 2010 ; Zahid et al. 2014b ). Generally speaking, more massive SFGs have a higher SFR and a larger amount of dust in the ISM formed in the past and tend to have a higher IRX. However, we find that four free parameters L IR , R e , Z , and b / a are sufficient to fit IRX, and residuals from the universal relation show no dependence on stellar mass. The inclusion of stellar mass as a free parameter in our fitting gives , suggestive of no correlation within the uncertainties. Studying the Balmer decrement in a sample of ∼1000 nearby SFGs with integral-field spectroscopy, Li et al. ( 2019 ) come to the same conclusion that trends of (spatially resolved) extinction with various local and global galaxy quantities can be reproduced by a model that lacks an intrinsic dependence on galaxy stellar mass. Moreover, as shown in Fig. 6 , the high- z SFGs of different stellar masses all follow the universal IRX relation derived from the nearby SFG population, further confirming that exclusion of stellar mass in our fitting works for distant SFGs as well. The bolometric luminosity (UV+IR) of a star-forming galaxy is vastly dominated by young stars and can thus be seen as a good SFR indicator (Kennicutt Evans 2012 ). The dust obscuration tracer IRX is intimately related to the star formation process as well. Dust in the birth clouds around young stars and in the surrounding ISM absorbs the UV radiation and re-emits into the IR. The overall dust obscuration of the galaxy, in general, depends on the dust column density averaged over the galactic scale. The latter relies on the gas density and gas-phase metallicity. The gas density on its turn is what sets the star formation surface density (i.e. the K–S law). While none of these quantities are directly linked with old stellar populations, SFR, metallicity, and galaxy size are all globally correlated with stellar mass. The observed mass–IRX relation can hence be considered the natural product of the universal relation of dust obscuration together with the scaling relations of stellar mass. We thus conclude that stellar mass is not a causal parameter that directly controls the dust obscuration in an SFG. 6.3 The term <inline-formula><tex-math id="TM0041" notation="LaTeX"></tex-math></inline-formula> and implications on the dust-star geometry One can wonder why IRX depends more strongly on rather than on Σ IR (= ). The latter has been shown to tightly correlate with dust temperature among SFGs that are
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