Influence of the optical Fe II quasi-continuum on measuring the spectral parameters of active galactic nuclei

TL;DR

This study investigates how the optical Fe II quasi-continuum affects the measurement of spectral parameters in Type 1 AGNs, revealing potential underestimations in line widths and fluxes, and modeling the quasar main sequence.

Contribution

It introduces a synthetic spectral modeling approach to quantify Fe II quasi-continuum effects on AGN spectral measurements and the quasar main sequence.

Findings

Fe II quasi-continuum can cause underestimation of Hβ line width and fluxes.

The Fe II equivalent width is strongly underestimated due to blending.

The model reproduces the quasar main sequence anti-correlation.

Abstract

We explore the influence of optical Fe II quasi-continuum on the measured spectral parameters in the 4150-5500 A range for the spectra of Type 1 active galactic nuclei (AGNs). We assume that the broad line region is composed of two sub-regions: the very broad line region (VBLR) and the intermediate line region (ILR). We constructed a large set of synthetic AGN spectra by taking different portions of the VBLR and ILR contributions, where initially the VBLR and ILR model spectra were constructed on the basis of prototypes of two observed spectra with dominant VBLR (i.e. ILR) emission. To investigate the influence of the optical Fe II quasi-continuum on the AGN measured spectral parameters, we fit the power-law continuum and emission lines in a set of model spectra, as commonly done for observed AGN spectra. We then compared the spectral parameters obtained after the fitting procedure with those of the model. We find that the optical Fe II quasi-continuum can be very strong in the case of spectra with strong and very broad Fe II lines and it is difficult to fully separate it from the power-law continuum. This gives the effect of a slightly underestimated H$\beta$ width and underestimated fluxes of the H$\beta$ and Fe II lines, while the continuum flux is then slightly overestimated. The most affected spectral parameters are the line equivalent widths (EWs), especially EW Fe II, which may be strongly underestimated. We discuss the possible underlying physics in the quasar main sequence, as implied by the results of our spectral modelling. We find that the set of AGN model spectra assuming different ILR and VBLR contributions can aptly reproduce the quasar main sequence, that is, the full width at half maximum (FWHM) H$\beta$ versus Fe II/H$\beta$ anti-correlation, where both parameters in this anti-correlation are strongly dependent on the ILR and VBLR contribution rate.