Evidence for Fluorescent Fe II Emission from Extended Low Ionization Outflows in Obscured Quasars

TL;DR

This study presents evidence of fluorescent Fe II emission from extended outflows in obscured quasars, revealing insights into quasar structure and evolution through spectral analysis of infrared-selected objects.

Contribution

It provides the first observational evidence linking extended outflows with fluorescent Fe II emission in obscured quasars, highlighting a potential evolutionary stage.

Findings

Fe II emission arises from resonant scattering in extended outflows.

Obscured quasars show strong UV Fe II lines with weak optical Fe II.

Estimated outflow covering factor ranges from 5% to 20%.

Abstract

Recent studies have shown that outflows in at least some broad absorption line (BAL) quasars are extended well beyond the putative dusty torus. Such outflows should be detectable in obscured quasars. We present four WISE selected infrared red quasars with very strong and peculiar ultraviolet Fe ii emission lines: strong UV Fe II UV arising from transitions to ground/low excitation levels, and very weak Fe II at wavelengths longer than 2800 {\AA}. The spectra of these quasars display strong resonant emission lines, such as C IV, Al III and Mg II but sometimes, a lack of non-resonant lines such as C III], S III and He II. We interpret the Fe II lines as resonantly scattered light from the extended outflows that are viewed nearly edge-on, so that the accretion disk and broad line region are obscured by the dusty torus, while the extended outflows are not. We show that dust free gas exposed to strong radiation longward of 912 {\AA} produces Fe II emission very similar to that observed. The gas is too cool to collisionally excite Fe II lines, accounting for the lack of optical emission. The spectral energy distribution from the UV to the mid-infrared can be modeled as emission from a clumpy dusty torus, with UV emission being reflected/scattered light either by the dusty torus or the outflow. Within this scenario, we estimate a minimum covering factor of the outflows from a few to 20% for the Fe II scattering region, suggesting that Fe II BAL quasars are at a special stage of quasar evolution.