Illustration of our MIR color selection (see Sect. 2 for details). Top: WISE color–color diagram (left) and color–magnitude diagram (right). The symbols in all panels are the same as in Fig. 1. The small gray dots show the entire WISE photometry within the Spitzer footprint. The small black dots are the sample of bright WISE sources used to adjust the coordinates (see Appendix E for details). The red dashed lines show our adopted quasar locus defined in Eq. (1) after Mateos et al. (2012). The sources falling inside are plotted in dark gray in all panels. Middle and bottom: Similar diagrams, but respectively with Spitzer 3.6, 4.6 µm and 8, 24 µm bands. The dashed vertical line in the bottom plot shows our additional color constraint. Summarizing, our targets for follow-up spectroscopy were selected from among the dark gray points that meet this additional criterion.