The differences in the Narrow Line Region of nearby QSOs 1 and 2 -- I: higher excitation and contribution of shocks in type 1's

Abstract

We compare the excitation of the Narrow-Line Region (NLR) of type 1 and type 2 QSOs for redshifts $0.4 \le z \le 0.5$ via the analysis of their emission line properties in Sloan Digital Sky Survey (SDSS) near-UV/optical spectra. We fit the continuum and emission lines, using two kinematic components for \oiii$\lambda$5007 and \hb\ (narrow and broad) and a single component for the weaker lines. We find two main differences in the NLR excitation of type 1 and 2 QSOs: (i) QSOs 2 have higher \oiii/\hb\ than QSOs 1 in both narrow and broad components; (ii) QSOs 1 present higher \nev, \neiii\ and \oiii$\lambda4363$ luminosities, higher \nev/\neiii\ and \neiii/\oii\ 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 highest excitation regions obscured in type 2's and cleared up in a ``blow-out phase". Support for the evolutionary scenario is given by the usually higher L\oiii\ in QSOs 2, in the sense that these sources host a more powerful AGN that, in its evolution, clears up the excess dust and gas to reveal a lower-luminosity but more highly excited type 1 AGN.