2025MNRAS.542.2525H

We compare the excitation of the narrow-line region (NLR) of type 1 and type 2 quasi-stellar objects (QSOs) for redshifts $0.4 \le z \le 0.5$ via the analysis of their emission line properties in Sloan Digital Sky Survey near-ultraviolet/optical spectra. We fit the continuum and emission lines, using two kinematic components for [O III] $\lambda$5007 and ${\rm H} \, \beta$ (narrow and broad) and a single component for the weaker lines. We find two main differences in the NLR excitation of type 1 and type 2 QSOs: (i) QSOs 2 have higher [O III]/${\rm H} \, \beta$ than QSOs 1 in both narrow and broad components; (ii) QSOs 1 present higher [Ne V], [Ne III], and [O III] $\lambda 4363$ luminosities, higher [Ne V]/[Ne III] and [Ne III]/[O II] ratios, and higher temperatures than QSOs 2. These differences support more highly excited regions, higher temperature gas, and prevalence of shocks in type 1 relative to type 2 QSOs. We suggest two possible scenarios: (i) type 1 QSOs are seen more pole-on, allowing the observation of more highly excited gas closer to the nucleus, supporting the unified model scenario; (ii) evolution from type 2 to type 1 QSOs, with the highest excitation regions obscured in type 2s and cleared up in a 'blow-out phase'. Support for the evolutionary scenario is given by the usually higher L[O III] in QSOs 2, in the sense that these sources host a more powerful active galactic nucleus (AGN) that, in its evolution, clears up the excess dust and gas to reveal a lower luminosity but more highly excited type 1 AGN.