Ratio of FWHM to σ (line shape) of Hα lines fitted with the double-Gaussian model as a function of FWHM. The Gaussian limit (FWHM ∼ 2.35σ) is shown as a horizontal grey solid line. The error bars denote 1σ uncertainties. For comparison, we plot the theoretical line width ratios of rotational line broadened Lorentzian profiles for Hα (black dashed lines and markers) and Hβ (grey dashed lines and markers) which were taken from Kollatschny & Zetzl (2011) and Kollatschny & Zetzl (2013). Different markers pertain to different FWHMs of the Lorentzian profiles.