Distributions of the parameters that we analysed in this work (see Sect. 3.1). The green vertical lines correspond to the cuts that we applied to select the sub-samples (see Sect. 3.3). We note that we left out one source with EW(Ly α) = 269 Å from the EW(Lyα) histogram for clarity. We also colour coded the distribution of the Lyα velocity offset according to the line used to measure zsys: magenta indicates zsys from the CIII(1907.07 Å) line and blueindicates the zsys from the interstellar absorption lines. In each panel we show the average error on the relevant parameter in the upper right. We note that for the EW(Ly α) we show the error for EW(Lyα) = 50 Å; for the Lyαext we plot for reference an error of 0.6 kpc, which corresponds to a 10% error of the value used to divide the samples.
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