To compare our findings to observations , we extract the following characteristics from the computed spectra : ( i ) the ratio of the integrated redward over the integrated blueward flux ( L red /L blue ) -which is a measure of the asymmetry of the line shape ; ( ii ) the flux at ' draught ' between the peaks ( F valley ) in comparison to the flux at the maximum point ( F peak ) , which , in practice , quantifies the ' deepness ' of the draught of the double peaked spectra ; and ( iii ) the position of the red emission peak ( v red ) . We show the evolution of these measures in Fig . 3 from t = 40 to 100 Myr ( in intervals of 10 Myr ) with the last data point marked with a square . We compare this to observations of nearby , star - forming galaxies ( the ' Green Peas ' ; data taken from Yang et al . 2016 , also Henry et al . 2015 , and to z ∼ 2 − 3 Lyman - α selected galaxies ( Erb et al . 2014 ) . In addition , we also display other simulation results presented in Walch et al . ( 2015 ) , Gatto et al . ( 2017 ) , and Peters et al . ( 2017 ) ( shown as black circles in Fig . 3 ) . These hydrodynamical simulations include supernovae feedback with random SN positions and cover SN rates from 5 to 45 Myr −1 with none including CRs ( but partially include stellar wind and UV feedback ) . We show them only to support the case that the spectral shape we obtain for the ' noCR ' simulation is not an outlier -but in fact typical for hydrodynamical simulations containing thermal feedback . Fig . 3 shows that all the simulations without CRs produce Lyα spectra with significant flux at line center ( F valley 0.4F peak ; see also the left panel of Fig . 1 ) . This flux is reduced significantly from t 30 Myr in the CR simulations as the outflow is established . Also the asymmetry of the line is increased leading to L red /L blue ∼ 2 . Both measures are more in line with what is found in observations where mostly F valley /F peak 0.1 and L red /L blue 2.5 ( see also Steidel et al . 2010;Kulas et al . 2012 ) . Determining F valley observationally requires a high spectral resolution since the convolution of the true spectrum with the instrument 's kernel leads to a spreading of the peak flux , and thus , the true value of F valley can be even lower than measured . This effect makes our comparison of F valley /F peak conservative and the true discrepancy between the ' noCR ' simulation and the observations might be even larger .