Improved Fe ii Emission-line Models for AGNs Using New Atomic Data Sets

Abstract

Understanding the Fe II emission from active galactic nuclei (AGNs) has been a grand challenge for many decades. The rewards from understanding the AGN spectra would be immense, involving both quasar classification schemes such as “Eigenvector 1” and tracing the chemical evolution of the cosmos. Recently, three large Fe II atomic data sets with radiative and electron collisional rates have become available. We have incorporated these into the spectral synthesis code Cloudy and examined predictions using a new generation of AGN spectral energy distribution (SED), which indicates that the ultraviolet (UV) emission can be quite different depending on the data set utilized. The Smyth et al. data set better reproduces the observed Fe II template of the I ZW 1 Seyfert galaxy in the UV and optical regions, and we adopt these data. We consider both thermal and microturbulent clouds and show that a microturbulence of ≈100 km s−1 reproduces the observed shape and strength of the so-called Fe II “UV bump.” Comparing our predictions to the observed Fe II template, we derive a typical cloud density of 1011 cm−3 and photon flux of 1020 cm−2 s−1, and show that these largely reproduce the observed Fe II emission in the UV and optical. We calculate the I(Fe II)/I(Mg II) emission-line intensity ratio using our best-fitting model and obtain log(I(Fe II)/I(Mg II)) ∼ 0.7, suggesting many AGNs have a roughly solar Fe/Mg abundance ratio. Finally, we vary the Eddington ratio and SED shape as a step in understanding the Eigenvector 1 correlation.

Strong correlation between FeII/MgII ratio and Eddington ratio of type 1 Active galactic nuclei

“…Interestingly, the Fe II/Hβ ratio (Panda et al 2018(Panda et al , 2019a and N V/C IV ratio (Hamann & Ferland 1992Nagao et al 2006) have been suggested to be associated with BLR metallicity. Therefore, the correlation between the Fe II/Mg II ratio and Eddington ratio may suggest that the Fe II/Mg II ratio is also related to BLR metallicity, also supported by theoretical studies (Verner et al 2003;Sarkar et al 2021).…”

“…However, their modeled emission line strengths of Fe II and Mg II are found to be largely inconsistent (∼1 dex) with the observed line strengths (see Figures 4 and 9 of Sameshima et al 2017), which indicates that their result may not be reliable. The inconsistency can be because their adopted atomic 371-level Fe + model (Verner et al 1999) may be not accurate (see e.g., Verner et al 2003;Smyth et al 2019;Sarkar et al 2021) and/or their parameter space may not reflect the nature of the BLR. Therefore, to confirm the relation between gas density and Eddington ratio, further investigations are required.…”

Taming the derivative: diagnostics of the continuum and H$β$ emission in a prototypical Population B active galaxy

“…solid angle that is equal to 4 steradians. We utilize a covering factor of 20% that is found to be viable for a near-Keplerian distribution of the BLR clouds (Baldwin, Ferland, Korista, Hamann, & LaCluyzé, 2004;Korista & Goad, 2000;Panda, 2021;Sarkar et al, 2021).…”

High-ionization emission line ratios from quasar broad line regions: metallicity or density?

“…The adopted SED is the intermediate 𝐿/𝐿 Edd case described by Ferland et al (2020). To be consistent with our previous models we assume a cloud column density of 10 23 cm −2 and use the complete models of Fe emission adopted by Temple et al (2020) and Fe described by Sarkar et al (2021). These data sets are included in the C17.03 download.…”

“…However, while it is undoubtedly true that the metal content of the BLR gas will have an effect on the observed emission-line ratios, the line emission strengths will also depend on the physical conditions of the emitting gas. For example, Sameshima et al (2017) showed that the lower-ionization line ratio Mg /Fe was strongly affected by the gas cloud density, which needs to be accounted for before inferring the Mg/Fe abundance ratio (see also Sarkar et al 2021). In this paper we demonstrate that the full range of high-ionization broad-line ratios observed in high-luminosity, 𝐿 bol 10 46−47 erg s −1 , quasar spectra with redshifts 𝑧 2, can be explained by varying the physical conditions of the emitting gas, in particular the density and the flux of ionizing photons, without needing to invoke changes in metallicity.…”