Number density of sources in the SFR–M_∗ plane. Shading is independent for each stellar mass bin, i.e. the darkest colour indicates the highest number density of sources in the stellar mass bin and not the highest number density of sources in the entire SFR–M_∗ plane. Short-dashed lines on a white background show the second-order polynomial functions used here to describe the MS of star formation (Magnelli et al. 2014). Dotted lines represent the MS and its redshift evolution as found in Elbaz et al. (2011). The red triple-dot-dashed lines represent the MS and its redshift evolution as found in Rodighiero et al. (2010).