Extreme CII emission in type 2 quasars at z~2.5: a signature of kappa-distributed electron energies?

TL;DR

This study examines unusual carbon emission line ratios in high-redshift type 2 quasars and suggests that non-thermal electron energy distributions, specifically kappa distributions, may explain the observed discrepancies.

Contribution

It introduces the idea that kappa-distributed electron energies can account for emission line anomalies in quasar narrow line regions, expanding beyond traditional Maxwell-Boltzmann assumptions.

Findings

Observed CII line ratios exceed predictions of standard models.

Kappa electron energy distributions provide a more natural explanation.

A grid of photoionization models with kappa distributions is presented.

Abstract

We investigate the flux ratio between the 1335 A and 2326 A lines of singly ionized carbon in the extended narrow line regions of type 2 quasars at z~2.5. We find the observed CII 1335 / CII] 2326 flux ratio, which is not sensitive to the C/H abundance ratio, to be often several times higher than predicted by the canonical AGN photoionization models that use solar metallicity and a Maxwell-Boltzmann electron energy distribution. We study several potential solutions for this discrepancy: low gas metallicity, shock ionization, continuum fluorescence, and kappa-distributed electron energies. Although we cannot definitively distinguish between several of the proposed solutions, we argue that a kappa distribution gives the more natural explanation. We also provide a grid of AGN photoionization models using kappa-distributed electron energies.