The SCUBA-2 Cosmology Legacy Survey: the submillimetre properties of Lyman-break galaxies at z = 3-5

We present detections at 850 μm of the Lyman-break galaxy (LBG) population at z ≈ 3, 4, and 5 using data from the Submillimetre Common User Bolometer Array 2 Cosmology Legacy Survey in the United Kingdom Infrared Deep Sky Survey `Ultra Deep Survey' field. We employ stacking to probe beneath the survey limit, measuring the average 850 μm flux density of LBGs at z ≈ 3, 4, and 5 with typical ultraviolet luminosities of L1700 ≈ 1029 erg s-1 Hz-1. We measure 850 μm flux densities of (0.25 ± 0.03), (0.41 ± 0.06), and (0.88 ± 0.23) mJy, respectively, finding that they contribute at most 20 per cent to the cosmic far-infrared (IR) background at 850 μm. Fitting an appropriate range of spectral energy distributions to the z ˜ 3, 4, and 5 LBG stacked 24-850 μm fluxes, we derive IR luminosities of L8-1000 μm ≈ 3.2, 5.5, and 11.0 × 1011 L⊙ [and star formation rates (SFRs) of ≈50-200 M⊙ yr-1], respectively. We find that the evolution in the IR luminosity density of LBGs is broadly consistent with model predictions for the expected contribution of luminous-to-ultraluminous IR galaxies at these epochs. We observe a positive correlation between stellar mass and IR luminosity and confirm that, for a fixed mass, the reddest LBGs (UV slope β → 0) are redder due to dust extinction, with SFR(IR)/SFR(UV) increasing by about an order of magnitude over -2 <β <0 with SFR(IR)/SFR(UV) ˜ 20 for the reddest LBGs. Furthermore, the most massive LBGs tend to have higher obscured-to-unobscured ratios, hinting at a variation in the obscuration properties across the mass range.