The ALMA REBELS survey. Obscured star formation in massive Lyman-break galaxies at z = 4–8 revealed by the IRX–β and M⋆ relations

We investigate the degree of dust obscured star formation in 49 massive (log(10)(M-star/M-circle dot) > 9) Lyman-break galaxies (LBGs) at z = 6.5-8 observed as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Survey (REBELS) large program. By creating deep stacks of the photometric data and the REBELS ALMA measurements we determine the average rest-frame ultraviolet (UV), optical, and far-infrared (FIR) properties which reveal a significant fraction (f(obs) = 0.4-0.7) of obscured star formation, consistent with previous studies. From measurements of the rest-frame UV slope, we find that the brightest LBGs at these redshifts show bluer (beta similar or equal to -2.2) colours than expected from an extrapolation of the colour-magnitude relation found at fainter magnitudes. Assuming a modified blackbody spectral energy distribution (SED) in the FIR (with dust temperature of T-d = 46K and beta(d) = 2.0), we find that the REBELS sources are in agreement with the local 'Calzetti-like' starburst Infrared-excess (IRX)-beta relation. By re-analysing the data available for 108 galaxies at z similar or equal to 4-6 from the ALMA Large Program to Investigate C+ at Early Times (ALPINE) using a consistent methodology and assumed FIR SED, we show that from z similar or equal to 4-8, massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX-beta relation. When comparing the IRX-M-star relation derived from the combined ALPINE and REBELS sample to relations established at z < 4, we find a deficit in the IRX, indicating that at z > 4 the proportion of obscured star formation is lower by a factor of greater than or similar to 3 at a given a M-star. Our IRX-beta results are in good agreement with the high-redshift predictions of simulations and semi-analytic models for z similar or equal to 7 galaxies with similar stellar masses and star formation rates.