Comparison of observed line emissions with predictions from photoionization models. Left: [O III]λ5007/Mg IIλ2796, [O II]/Mgiiλ2796, [Ne III]/Mg IIλ2796, [Ne III]/[O II], and C III]/Mg IIλ2796 emission-line ratios predicted from the star-forming galaxy models of Gutkin et al. (2016), described in Sect. 3.1, for different values of the volume-averaged ionization parameter log 〈U〉 (x-axis) and metallicity Z (color-coded as indicated in the top panel). Dotted gray lines mark the minimum and maximum value of the line ratios measured from the MUSE spectra of Mg II emitters. Right (from top to bottom): observed line ratios as function of redshift for the Mg II emitters, as defined in Sect. 2.3 (empty black circles) and for the sample of Guseva et al. (2013; empty gray diamonds). Red arrows in the top right corner of each panel indicate the effect of attenuation by dust for A_v = 1 and 3 mag and a Calzetti et al. (2000) attenuation curve. Data measurements from Guseva et al. (2013) have been corrected for dust attenuation, whereas the MUSE fluxes have not been.