[O iii] 5007 Å Emission Line Width as a Surrogate for σ ∗ in Type 1 AGNs?

Abstract

We present a study of the relation between the [O iii ] 5007 Å emission line width ( σ _[O _III _] ) and stellar velocity dispersion ( σ _* ), utilizing a sample of 740 type 1 active galactic nuclei (AGNs) with high-quality spectra at redshift z < 1.0. We find the broad correlation between the core component of the [O iii ] emission line width ( ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ ) and σ _* with a scatter of 0.11 dex for the low redshift ( z < 0.1) sample; for redshift (0.3 < z < 1.0) AGNs, the scatter is larger, being 0.16 dex. We also find that the Eddington ratio ( L _bol / L _Edd ) may play an important role in the discrepancies between ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ and σ _* . As the L _bol / L _Edd increases, ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ tends to be larger than σ _* . By classifying our local sample with different minor-to-major axis ratios, we find that σ _* is larger than ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ for those edge-on spiral galaxies. In addition, we also find that the effects of outflow strength properties such as maximum outflow velocity ( ${V}_{\max }$ ) and the broader component of the [O iii ] emission line width and line shift ( σ _[O _III _,out] and V _[O _III _,out] ) may play a major role in the discrepancies between ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ and σ _* . The discrepancies between ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ and σ _* are larger when ${V}_{\max }$ , V _[O _III _,out] , and σ _[O _III _,out] increase. Our results show that the outflow strengths may have significant effects on the differences between narrow-line region gas and stellar kinematics in AGNs. We suggest that caution should be taken when using ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ as a surrogate for σ _* . In addition, the substitute of ${\sigma }_{[{\rm{O}}{\rm\small{III}},\mathrm{core}]}$ for σ _* could be used only for low luminosity AGNs.

Keywords

• Active galaxies
• Emission line galaxies
• Galaxy kinematics