UV Fe II emission model of HE 0413-4031 and its relation to broad-line time delays

TL;DR

This study models UV Fe II emission in quasar HE 0413-4031, revealing a revised redshift, improved spectral decomposition, and insights into the spatial relationship between Fe II and Mg II emitting regions, impacting black hole mass estimates.

Contribution

It provides a new UV Fe II emission model, a revised redshift, and an updated Mg II radius-luminosity relation based on extensive monitoring data.

Findings

Revised redshift ($z=1.39117$) differs from previous estimates.

UV Fe II emission region is slightly more distant than Mg II.

Updated Mg II R-L relation is flatter than other emission lines.

Abstract

Fe II emission is a well-known contributor to the UV spectra of active galactic nuclei and the modeling of this part may affect the results obtained for the MgII$\lambda2800$ emission, which is one of the lines used for black hole mass measurements and cosmological applications. We use the 11-year monitoring of the selected quasar HE 0413-4031 with the South African Large Telescope (SALT) and we supplement this monitoring with the near-IR spectrum taken with the SOAR telescope. A new redshift determination ($z=1.39117 \pm 0.00017$) using [OIII]$\lambda \lambda 4959,5007$ gave a very different value than the previous determination based only on the UV FeII pseudocontinuum ($z=1.3764$). It favors a different decomposition of the spectrum into Mg II and UV Fe II emissions. The line characteristics and the time delay of the Mg II emission ($224^{+21}_{-23}$ days) are not significantly affected. However, in comparison with the previous analysis, the rest-frame UV FeII time delay ($251^{+9}_{-7}$ days) is consistent with the inferred UV FeII line FWHM of $4200\,{\rm km/s}$ that is only slightly smaller than the MgII line FWHM. Hence the FeII-emitting material is more distant than the MgII-emitting gas in HE 0413-4031 by $\sim 0.023$ pc (4700 AU). The inferred velocity shift of both Mg II and UV Fe II lines with respect to the systemic redshift is now rather low, below 300 km s$^{-1}$. In addition, we construct an updated MgII radius-luminosity ($R-L$) relation from 194 sources, which is more than double the previous sample. The MgII $R-L$ relation is flatter than the UV FeII, optical FeII, and H$\beta$ $R-L$ relations. While the new decomposition of the spectrum is satisfactory, we see a need to create better Fe II templates using the newest version of the code CLOUDY.