The UV and optical Fe II emission lines in type 1 AGNs

TL;DR

This study analyzes UV and optical Fe II emission lines in 293 type 1 AGNs, revealing correlations, kinematic connections, and inflow signatures, advancing understanding of the broad line region and emission mechanisms.

Contribution

It provides new insights into the origin, kinematics, and physical conditions of Fe II emitting regions in AGNs, especially the connection between UV and optical lines.

Findings

UV and optical Fe II lines originate from the outer broad line region.

UV Fe II lines show a redshift indicating inflow, unlike optical Fe II.

Correlations suggest the importance of column density and microturbulence.

Abstract

We investigate the spectral properties of the UV ($\lambda\lambda$2650-3050 \AA) and optical ($\lambda\lambda$4000-5500 \AA) Fe II emission features in a sample of 293 type 1 active galactic nuclei (AGNs) from Sloan Digital Sky Survey (SDSS) database. We explore different correlations between their emission line properties, as well as the correlations with the other emission lines from the spectral range. We find several interesting correlations and we can outline the most interesting results as follows. (i) There is a kinematical connection between the UV and optical Fe II lines, indicating that the UV and optical Fe II lines originate from the outer part of the broad line region, so-called intermediate line region; (ii) The unexplained anticorrelations of the optical Fe II (EW Fe II$_{opt}$) versus EW [O III] 5007 \AA\ and EW Fe II$_{opt}$ versus FWHM Hbeta have not been detected for the UV Fe II lines; (iii) The significant averaged redshift in the UV Fe II lines, which is not present in optical Fe II, indicates an inflow in the UV Fe II emitting clouds, and probably their asymmetric distribution. (iv) Also, we confirm the anticorrelation between the intensity ratio of the optical and UV Fe II lines and FWHM of Hbeta, and we find the anticorrelations of this ratio with the widths of Mg II 2800 \AA, optical Fe II and UV Fe II. This indicates a very important role for the column density and microturbulence in the emitting gas. We discuss the starburst activity in high-density regions of young AGNs as a possible explanation of the detected optical Fe II correlations and intensity line ratios of the UV and optical Fe II lines.