Emission line activity in type 2 quasars from the Sloan Digital Sky Survey

TL;DR

This study compares optical emission line ratios of type 2 quasars with models and other galaxy types, finding similarities with Seyfert 2s and radio galaxies, and discusses the adequacy of photoionization models and stellar impact.

Contribution

It provides a detailed comparison of emission line ratios in type 2 quasars with models and other galaxy types, highlighting model successes and limitations.

Findings

Type 2 quasars resemble Seyfert 2s and radio galaxies in emission lines.

Standard AGN photoionization models can reproduce key features but have limitations.

Stellar photoionization may influence emission line properties.

Abstract

We have compared the optical emission line ratios of type 2 quasars from Zakamska et al. with standard AGN photoionization model predictions, Seyfert 2s, HII galaxies, and narrow line FRII radio galaxies. Moderate to high ionization narrow line radio galaxies and Seyfert 2s are indistinguishable from type 2 quasars based on their optical line ratios. The standard AGN photoionization models, widely discussed for other type 2 AGNs, can reproduce successfully the loci and trends of type 2 quasars in some of the main diagnostic diagrams. These models are not exempt of problems and the discrepancies with the data are the same encountered for other type 2 AGNs. As for these, realistic models must take into account a range of cloud properties, as widely demonstrated in the literature. We also study the possible impact of stellar photoionization.