Stacked rest-frame optical spectrum of the eight spectroscopically confirmed z > 3 galaxies with no strong emission lines (i.e., all the galaxies with robust or uncertain redshifts at z > 3, except ZF-COS-20133 and ZF-UDS-8197). The stacked MOSFIRE spectrum is shown in black, normalized to unit flux density at λ ~ 0.48 μm, and the error spectrum is shown in shaded gray at the bottom. We overlay the stacked model spectrum of all the galaxies in red (as obtained in Sect. 4.1), and we indicate the main absorption and emission lines with colored lines (blue: emission, green: absorption, orange: Balmer series) with labels at the top of the figure. The residuals of the spectrum, after subtracting the stacked model and normalizing by the uncertainty, are displayed at the bottom of the figure.