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tokens: /yr) L and D n 4000. 8 The goal is to check how these properties vary according to the employed classification: GMM, BPT and WHAN. To that end, we portray their statistical properties as boxplots in Figs 9 11 , as well as their summary statistics in Table 3 , which shows the values for the median, first (Q1) and third (Q3) quartiles, and the interquartile range (IQR Q3Q1). Figure 9. Metallicity distributions portrayed as boxplots for GMM, BPT and WHAN diagram classes, from top to bottom. For the GMM we can see the GCs displayed in the following order: GC4, GC1, GC3, GC2 following increasing log[N ii ]/H, i.e. the x -axis of the BPT diagram. The order of the remaining groups is given by the KL distance to the GMM components. The width of boxes is proportional to the square root of the number of galaxies within each bin and the whiskers extend to the most extreme data point, which is within the 50per cent interquartile range (IQR). To better illustrate the overall distribution of the sample, for the boxplots of the BPT diagram we have omitted part of the outlier zone, zooming into the interquartile distance. Figure 10. Average stellar age distributions portrayed as boxplots for GMM, BPT and WHAN diagram classes, from top to bottom. The GCs and remaining groups are ordered as in Fig. 9 . Figure 11. D n 4000 distributions portrayed as boxplots for GMM, BPT and WHAN diagram classes, from top to bottom. The GCs and remaining groups are ordered as in Fig. 9 . Table 3. Summary statistics for the Z /Z L , log( t /yr) L and D n 4000 galaxy properties. Shown are the median, IQR and the first and third quartile for the properties of the GMM, BPT and WHAN groups. On the boxplots, the groups are vertically aligned based on their proximity in terms of the KL distance, and the fiducial order roughly follows increasing values of log[N ii ]/Hin the BPT diagram (i.e. from left to right: SF, composite, AGN). As we can see from the boxplots, by aligning these groups in this way, a positive monotonic relationship between the classes and the median values of Z /Z L , log( t /yr) L and D n 4000 distributions is revealed. The trend is a consequence of the AGN host galaxies having different characteristics from their inactive counterparts, preferentially populating the so-called green valley and red sequence of the colourmass diagram (e.g. Schawinski etal. 2010 ). Thus, as we move towards the right-hand side of the BPT diagram, one is mostly looking at early-type galaxies, that are characterized by older and more metallic stellar populations, and higher values of D n 4000 (e.g. Poggianti Barbaro 1997 ; Schawinski etal. 2010 ; De Souza etal. 2016 ). Notably, the GMM solutions automatically find groups that share meaningful physical properties, beyond the features used in the clustering algorithm. An inspection of Table 3 confirms that the statistical properties are quite similar between the GMM groups and their respective counterparts on the BPT and WHAN diagrams.