New theoretical Fe II templates for bright quasars

TL;DR

This paper introduces new theoretical Fe II emission templates for bright quasars using advanced atomic data and photoionization models, highlighting persistent discrepancies with observations and discussing potential physical causes.

Contribution

The study develops a comprehensive set of Fe II templates based on updated atomic data and explores their limitations in matching observed quasar spectra.

Findings

The shielded side of clouds produces insufficient Fe II emission.

Predicted Fe II UV/optical ratio exceeds observed values.

Microturbulence affects apparent velocity shifts but not ratios.

Abstract

We present a set of new theoretical Fe II templates for bright quasars covering a wavelength range of 1000-10000 \AA\, based on the recent atomic database available in the C23.00 version of the photoionization code CLOUDY. We compute a grid of models for a range of incident photon flux, gas density, and multiple microturbulence velocities. We examine the equivalent widths (EWs) and the ratios of Fe II emission over various wavebands and compare them with observations. Our key results are: (1) The flux generated from the shielded side of the cloud is insufficient to describe the measured Fe II emission. (2) Despite using the newest atomic data we still confirm the long-standing problem that the predicted Fe II UV/optical ratio is significantly larger than that observed in the AGN spectra. (3) The Fe II UV/optical ratio is not significantly affected by the variations in the microturbulence and the metallicity. (4) The microturbulence can create an additional apparent velocity shift of up to 1000 km/s in the spectra. (5) There is no Fe II template based on a single set of physical parameters that can fit the observed UV to optical Fe II emission spectra. We shortly discuss the most likely effects responsible for the Fe II UV/optical mismatch problem: the assumption of the constant density clouds and the heating mechanism for Fe II emitting clouds.