# The FeII/MgII flux ratio of low-luminosity quasars at z~3

## TL;DR

This study examines the FeII/MgII flux ratio in low-luminosity quasars at z~3 to understand chemical evolution, finding similar ratios across luminosities, suggesting early chemical maturity in these AGNs.

## Contribution

It introduces new measurements of FeII/MgII ratios in lower-luminosity quasars at z~3, expanding the luminosity range studied and addressing potential selection biases in previous research.

## Key findings

- FeII/MgII ratios are similar in low- and high-luminosity quasars at z~3
- Low-luminosity quasars are already chemically mature
- No significant evolution of FeII/MgII ratio up to z~6

## Abstract

The FeII/MgII line flux ratio has been used to investigate the chemical evolution of high redshift active galactic nuclei (AGNs). No strong evolution has been found out to z~6, implying that the type 1a supernova activity has been already occurred in early universe. However, the trend of no evolution can be caused by the sample selection bias since the previous studies have utilized mostly very luminous AGNs, which may be already chemically matured at the observed redshift. As motivated by the previously reported correlation between AGN luminosity and metallicity, we investigate the FeII/MgII flux ratio over a large dynamic range of luminosity, by adding a new sample of 12 quasars at z~3, of which the lower luminosity limit is more than 1 dex smaller than that of the previously studied high-z quasars. Based on the Gemini/GNIRS observations, we find that the seven low-luminosity quasars with a mean bolometric luminosity log L_bol~46.5 \pm 0.2 has an average FeII/MgII ratio of 0.68 \pm 0.11 dex. This ratio is comparable to that of high-luminosity quasars (log L_bol~47.3 \pm 0.3) in our sample (i.e., FeII/MgII ratio of 0.59 \pm 0.15 dex) and that of the previously studied high-luminosity quasars at higher redshifts. One possible scenario is that the low-luminosity quasars in our sample are still relatively luminous and already chemically matured. To search for chemically-young AGNs, and fully understand the chemical evolution based on the FeII/MgII flux ratio, it is required to investigate much lower-luminosity AGNs.