Median stacked 1D rest-frame emission-line spectra of the Hγ sample (z = 6.3–7), in three stellar mass bins offset for visualisation purposes. The shaded region shows the uncertainty estimated through bootstrap resampling. In the right panel, we zoom into the wavelength range containing Hγ and [O III]λ4364. We fitted a complex of three Gaussians with the same standard deviation to obtain the fluxes of Hγ and [O III]λ4364. An additional Gaussian component was fitted to account for possible contamination by [Fe II]λ4360.