Diagnostic diagrams for the MARTA galaxies analysed in this work. From left to right: Distribution of the combined golden and silver samples on the O₃₂ vs R₂₃, [O III]/Hβ vs [N II]/Hα, and [O III]/Hβ vs [S II]/Hα diagrams, colour-coded by galaxy redshift. The grey contours indicate the region populated by local galaxies from SDSS, with darker shades denoting higher densities. Red contours mark the distribution of galaxies from the MOSDEF survey (Kriek et al. 2015), whereas the orange curve is the fit to KBSS galaxies from Strom et al. (2017); both surveys target primarily z ∼ 2 − 3 systems. The dividing lines between star-forming galaxies, AGNs, and LINER galaxies from Kewley et al. (2001, 2013) and Kauffmann et al. (2003) in both the [N II]-BPT and [S II]-BPT diagrams are also drawn. Median error bars are not shown, as they are smaller than the symbol size and would not be visible in the plot; this is due to the fact that these are among the strongest lines, measured with very high S/Ns.