Spectra emerging from starbursts with neutral column densities ranging from the LyC optically thin regime (N_HI ≤ 10¹⁷ cm^-2) to N_HI = 10^19.6 cm^-2, as an illustration of the line broadening with increasing optical depth. The models are dust free, with an expansion velocity v_exp = 50 km s^-1, and a Doppler parameter b = 20 km s^-1. The intrinsic spectrum is a flat continuum plus a Gaussian emission line of FWHM = 100 km s^-1 and EW = 100 Å. These spectra are not convolved to account for a finite instrumental resolution, but they would correspond to extremely high-resolution spectra (R ≥ 15 000 or Δv ≤ 20 km s^-1). In this plot and in the following theoretical spectra, the notation I/I₀ indicates that the profiles are normalised to the flux level in the continuum, I₀, which is the mean number of escaping photons per frequency bin over an arbitrary frequency range far from line centre, here xr = [−3000 km s^-1; −2000 km s^-1].