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

TL;DR

This study compares the Narrow-Line Regions of type 1 and type 2 QSOs, revealing higher excitation and shock contributions in type 1s, supporting unified and evolutionary models of active galactic nuclei.

Contribution

It provides new evidence of excitation and temperature differences in NLRs, supporting the unified and evolutionary scenarios for QSOs.

Findings

Type 2 QSOs have higher \\oiii/\\hb ratios.

Type 1 QSOs show higher \\nev, \\neiii luminosities and temperatures.

Differences support more highly excited, shock-influenced gas in type 1 QSOs.

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.