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

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.