UV diagnostics of star formation and AGN backed up by our photoionization models (left) and observational data compiled from the literature (right). From top to bottom: UV lines diagrams of C4C3 versus C34 (top), EW([CIII]) versus [CIII]/He II ratio (middle), and EW(CIV) versus CIV/He II ratio (bottom) are presented. Left: for star-forming galaxy, the SEDs provided by POPSTAR (solid) and BPASS (long-dashed) are shown. For AGN, two models with different power-law indices of α = −2.0 (dotted) and − 1.2 (dashed) are presented. The same color-scheme is adopted as in Fig. 2. Right: blue and red symbols are star-forminggalaxies and AGNs, respectively, compiled from the literature as shown in the legend. The filled blue symbols present galaxies that are supported to be star-forming according to the optical BPT-diagnostic. The open blue symbols are objects that lack either of the optical BPT-lines but show no sign of AGN. If the EWs and/or the fluxes are below the 2σ detections, we apply the 2σ upper-limitsfor the quantities and denote them with an arrow. The black dashed curves are our proposed separating method of a star-forming galaxy from an AGN (Eqs. (1)–(3)). The gray shaded areas show the regions where both AGN and SF models could overlap, due to a low metallicity (C4C3–C34, top) and a high C/O ratio (EW diagrams; middle and bottom).