Optical to mid-IR stacked SED of the unidentified radio sources with secure radio positions in ELAIS-N1 (571 sources; black points). The stack for the subset of the “FIR-bright” sources (50 sources) is shown as red squares. Overlaid ontop are typical star-forming galaxy templates, computed using models of Bruzual & Charlot (2003) at z = 2, 3 and 4 (blue, orange and green solid lines, respectively), scaled to the median SFR detectable for these sources at the depth of the LOFAR data in ELAIS-N1. The dashed lines show two-component templates of galaxies with an old stellar population undergoing a recent burst of star-formation (see text) at z = 3 and 4 (red and purple, respectively).