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We investigate the degree of dust obscured star formation in 49 massive(${\rm log}_{10}(M_{\star}/{\rm M}_{\odot})>9$) Lyman-break galaxies (LBGs) at$z = 6.5$-$8$ observed as part of the ALMA Reionization Era Bright EmissionLine Survey (REBELS) large program. By creating deep stacks of the photometricdata and the REBELS ALMA measurements we determine the average rest-frame UV,optical and far-infrared (FIR) properties which reveal a significant fraction($f_{\rm obs} = 0.4$-$0.7$) of obscured star formation, consistent withprevious studies. From measurements of the rest-frame UV slope, we find thatthe brightest LBGs at these redshifts show bluer ($\beta \simeq -2.2$) coloursthan expected from an extrapolation of the colour-magnitude relation found atfainter magnitudes. Assuming a modified blackbody spectral-energy distribution(SED) in the FIR (with dust temperature of $T_{\rm d} = 46\,{\rm K}$ and$\beta_{\rm d} = 2.0$), we find that the REBELS sources are in agreement withthe local ''Calzetti-like'' starburst Infrared-excess (IRX)-$\beta$ relation.By reanalysing the data available for 108 galaxies at $z \simeq 4$-$6$ from theALPINE ALMA large program using a consistent methodology and assumed FIR SED,we show that from $z \simeq 4$-$8$, massive galaxies selected in the rest-frameUV have no appreciable evolution in their derived IRX-$\beta$ relation. Whencomparing the IRX-$M_{\star}$ relation derived from the combined ALPINE andREBELS sample to relations established at $z < 4$, we find a deficit in theIRX, indicating that at $z > 4$ the proportion of obscured star formation islower by a factor of $\gtrsim 3$ at a given a $M_{\star}$. Our IRX-$\beta$results are in good agreement with the high-redshift predictions of simulationsand semi-analytic models for $z \simeq 7$ galaxies with similar stellar massesand SFRs.

The ALMA REBELS survey: obscured star formation in massive Lyman-break galaxies at z = 4-8 revealed by the IRX-$\beta$ and $M_{\star}$ relations