Narrow [O III] emission lines as a potential proxy for the evolutionary stage of quasars

TL;DR

This study investigates the correlation between [O III] emission line strength and radio spectral index in quasars, proposing [O III] as a potential indicator of their evolutionary stage based on a large SDSS dataset.

Contribution

It introduces a novel correlation between [O III] emission and radio spectral shape, suggesting [O III] as a proxy for quasar age, based on extensive multi-frequency data analysis.

Findings

Strong correlation between [O III] equivalent width and radio spectral index.

[O III] emission strength varies with quasar evolutionary stage.

Early-stage quasars show weaker [O III], older ones show stronger.

Abstract

Radio spectral shape of quasars can provide insight into the ages of quasars. We have compiled data for 1804 quasars with $z\lesssim1$ from the Sloan Digital Sky Survey (SDSS). Additionally, these quasars were also mapped by the Low-Frequency Array at 144 MHz and the Very Large Array Sky Survey at 3000 MHz. The radio spectral index, designated as $\alpha^{\rm 144}_{\rm 3000}$ (with $S(\nu)\propto\nu^\alpha$), is analyzed between 144 MHz and 3000 MHz as a proxy for the ages of quasars. We measure the [O III] $\lambda$5007 emission line in the SDSS spectra. A strong correlation was found between the equivalent width of the core component of the [O III] $\lambda$5007 emission line and $\alpha^{\rm 144}_{\rm 3000}$. This relationship suggests that the core component of the [O III] $\lambda$5007 emission line could potentially serve as a surrogate for the evolutionary stage of a quasar. The quasars at an early stage of evolutions tend to show weaker [O III] $\lambda$5007 emission, while older quasars exhibit stronger [O III] $\lambda$5007 emission.