Abundance ratios of neutron-capture elements Y and Ba as a function of metallicity. Grey filled circles are Galactic field stars from the compilation by Venn et al. (2004). The other symbols are as in Fig. 6: Pal 12 (red filled squares, Cohen 2004), Terzan 7 (orange filled stars, Sbordone et al. 2007), Terzan 8 (filled light-blue triangles, Carretta et al. 2014a) and Arp 2 (green empty squares, Mottini et al. 2008).