Chemical abundances along the quasar main sequence

TL;DR

This study analyzes the metallicity of broad line regions in type-1 AGNs along the 4D eigenvector 1 sequence, revealing a systematic increase from sub-solar to super-solar metallicities correlated with the main sequence.

Contribution

It provides a comprehensive analysis of metallicity trends along the quasar main sequence using multi-component emission line diagnostics and photoionization models.

Findings

Metallicity increases along the 4DE1 sequence.

Sub-solar to several times solar metallicities observed.

Extreme metallicities confirmed in high Eddington ratio AGNs.

Abstract

The 4D eigenvector 1 (E1) sequence has emerged as a powerful tool for organizing the observational and physical characteristics of type-1 active galactic nuclei (AGNs). In this study, we present a comprehensive analysis of the metallicity of the broad line region gas, incorporating both new data and previously published findings, to assess the presence of any trend along the sequence. We perform a multi-component analysis on the strongest UV and optical emission lines, compute $\sim 10$ diagnostic ratios, and compare them with the prediction of CLOUDY photoionization simulations, identifying a photoionization solution closest to the data. Our investigation reveals a consistent pattern along the optical plane of the E1. We observe a systematic progression in metallicity, ranging from sub-solar values to metallicity levels several times higher than solar values. These findings underscore the role of metallicity as a fundamental correlate of the 4DE1/main sequence. Extreme values of metallicity, at least several tens solar, are confirmed in low-$z$ AGNs radiating at a high Eddington ratio, although the origin of the extreme enrichment remains open to debate.