Depletion time and star formation rate for our targets. The RS NIR-dark galaxies are reported as colored stars, following the same color-code as in Fig. 6. The colored triangles are other populations of SMGs, namely those collected by Bothwell et al. (2017), Cañameras et al. (2018), and Walter et al. (2011), reported in blue, orange, and green, respectively. We also report some confirmed QSOs from the same studies as reversed red triangles. The shaded gray area reports the depletion time expected from main-sequence galaxies at z ∼ 3.5 following the relation τ_D = 1.5(1 + z)^α found by Saintonge et al. (2013), with α spanning from −1.0 (Davé et al. 2012) to −1.5 (Magnelli et al. 2013), rescaled to a Chabrier (2003) IMF.