The ALMA REBELS survey: obscured star formation in massive Lyman-break galaxies at z= 4-8 revealed by the IRX-β and M⋆ relations
Inami, H.; Bowler, R. A. A.; Cullen, F.; McLure, R. J.; Dunlop, J. S.; De Looze, I.; Schneider, R.; Fudamoto, Y.; da Cunha, E.; Aravena, M.; Hodge, J. A.; van der Werf, P. P.; Palla, M.; Nanayakkara, T.; Ferrara, A.; Bouwens, R.; Smit, R.; Stefanon, M.; Schouws, S.; Graziani, L.; Dayal, P.; Stark, D. P.; Algera, H. S. B.; Barrufet, L.; Sommovigo, L.; Mauerhofer, V.
United Kingdom, Japan, Italy, Chile, Switzerland, Netherlands, Australia, Belgium, Spain, United States
Abstract
We investigate the degree of dust obscured star formation in 49 massive (log10(M⋆/M⊙) > 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 (fobs = 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 (β ≃ -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 $T_{\rm d} = 46\, {\rm K}$ and βd = 2.0), we find that the REBELS sources are in agreement with the local 'Calzetti-like' starburst Infrared-excess (IRX)-β relation. By re-analysing the data available for 108 galaxies at z ≃ 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 ≃ 4-8, massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX-β relation. When comparing the IRX-M⋆ 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 ≳ 3 at a given a M⋆. Our IRX-β results are in good agreement with the high-redshift predictions of simulations and semi-analytic models for z ≃ 7 galaxies with similar stellar masses and star formation rates.