Temperature Discrepancy with Photoionization Models of the Narrow Line Region

TL;DR

This study compares observed [O III] emission line ratios in active galactic nuclei with photoionization models, revealing higher NLR temperatures than predicted, challenging existing models of AGN narrow line regions.

Contribution

It provides observational evidence of higher NLR temperatures than standard photoionization models predict, questioning current theoretical assumptions.

Findings

Observed NLR temperature is about 13,500 K.

No evidence of collisional deexcitation after considering density and dust effects.

Standard photoionization models underestimate NLR temperatures.

Abstract

Using published work on the Narrow Line Region of Active Nuclei, we make a comparison of the observed [O III] $4363\AA/5007\AA$ ratio observed among quasars, Seyfert 2's and spatially resolved NLR plasma. It is broadly accepted that the span of this ratio among quasars, from 0.015 to 0.2, is the result of collisional deexcitation as corroborated by Baskin and Laor (2005). However, the coincidence of the AGN at towards the lowest [O III] ratios, however, suggests that it represents plasma in the low density regime, where this ratio can be interpreted as the actual NLR temperature. Using the density indicator [Ar IV] $4711\AA/4740\AA$ doublet ratio which was observed by Koski (1978) in Seyfert 2's, we found evidence of relatively low densities ($< 10000/cc$). Even after considering a powerlaw distribution for the densities as well as a nonuniform foreground dust extinction, we find no evidence of collisional deexcitation. The mean NLR OIII temperature we infer for our Seyfert 2 sample is 13,500 K. This is a problem for photoionization models with a standard ionizing spectral energy distribution since they predict significantly lower temperatures.