An Empirical Temperature Relation for Chemical Abundances in AGN

Abstract

We present for the very first time the empirical temperature structure of the narrow line region (NLR) clouds of active galactic nuclei (AGNs) based on a sample of [N II] and [O III] auroral line measurements. The scaling of Te([N II]) with Te([O III]), expressed as the t2-t3 relation, is key to deriving accurate chemical abundances, in AGNs and other line-emitting nebulae, particularly when only one auroral line is observed. Here, we provide the first robust empirical description of t2-t3 for AGNs: t2(10^4 K)=1.285*t3(10^4 K)+0.799. We find substantial changes in 12+log(O/H) when compared to other t2-t3 calibrations when using the direct, or Te, method with only the [O III] 4363 auroral line. We find that adopting our t2-t3 most accurately recovers the fully direct method 12+log(O/H). To interpret the physical mechanisms behind the AGN t2-t3, we compare the observed distribution of [N II] 6584/5755 and [O III] 5007/4363 flux ratios to a variety of ionization model predictions. We find evidence for power law density gradients, pressure stratification, and (sometimes significant) contributions from astrophysical shocks driving much of the dispersion on these flux ratios while metal cooling dominates the primary shape of the distribution. With more accurate t2-t3 and 12+log(O/H), the properties of the interstellar medium photoionized by AGN can be better characterized and understood across a wide range of cosmic time and may bring better context for novel observations at z>6 with JWST, with implications for understanding chemical enrichment in the earliest stages of galaxy evolution.