Illustration of the effect of the mix of two lines. These lines have both an intrinsic velocity dispersion σ_corr = 15 km s⁻¹, and with a LSF width σ_LSF = 13 km s⁻¹, on the extracted velocity dispersion with two line separations: half of a single line FWHM (left) and FWHM (right). The mix of the two lines (red and blue dotted curves) with an intensity ratio of 0.2 is shown on the upper panels by the black curve, whereas the single Gaussian fit is shown in dotted grey. The single line and the fitted line velocity dispersions (2 × σ_obs) are shown as blue and grey horizontal segments, respectively. The lower plots illustrate the variation of the relative uncertainty on both the observed (red) and corrected (blue) velocity dispersion when the line ratio varies between 0 and 1.