Fig. 2: [O III]λ5007/Hβ–[Ne III]λ3869/[O II]λ3727 narrow line ratio diagram.

The fiducial result for CANUCS-LRD-z8.6 is shown as a yellow star. The lighter yellow square and circle are the results for the line ratios of CANUCS-LRD-z8.6 from Test 1 and Test 2, respectively, derived by adding a broad Hγ component (see text and Supplementary Methods). Line ratios are dust-corrected. The maximum correction for [Ne III]λ3869/([Ne III]λ3869+Hζ+Hη) due to the blend of [Ne III]λ3869, He Iλ3889, Hη and Hζ is  ≈ 0.2 dex (see Section “Line blending and contamination correction”). Errorbars represent standard deviation. Left: the [O III]λ5007/Hβ ratio as a function of the [Ne III]λ3869/[O II]λ3727 ratio (“OHNO” diagnostic) for our target and other observations as described in the legend and in  Supplementary Methods (Section “Electron temperature and metallicity”). Contours are reported as bars in the legend. Right: overlaid to our results (yellow markers as in the legend of the left panel) for the “OHNO” diagnostic are the AGN photoionization models of ref. ¹¹ at hydrogen densities \(\log n[{{{{\rm{cm}}}}}^{-3}]=2.0\) (gray scale grid) and \(\log n[{{{{\rm{cm}}}}}^{-3}]=4.0\) (coloured-scale grid). The grid shows the variation of the ionization parameter and metallicity (color scales on the right-hand side of the figure, same limits are adopted for gray scale). To guide the eye, lighter colors represent lower metallicity and higher ionization. Y-axis is the same as the right panel. Source data are provided on Zenodo (see “Data Availability” section).