Metal enrichment by radiation pressure in active galactic nucleus outflows -- theory and observations

TL;DR

This paper investigates how radiation pressure on metal ions can drive outflows in active galactic nuclei, proposing a method to measure metal enrichment and applying it to observed quasars, revealing potential metal-rich outflows.

Contribution

It introduces a theoretical framework for metal-enriched outflows driven by radiation pressure and a novel observational method to estimate metal abundance in UV outflows.

Findings

Identified conditions for metal ions to decouple from the gas in AGN outflows.

Developed a method to constrain metals/H ratio from UV absorption profiles.

Found candidate outflows with higher than solar metal/H ratios in SDSS data.

Abstract

Outflows from active galactic nuclei may be produced by absorption of continuum radiation by UV resonance lines of abundant metal ions, as observed in broad absorption line quasars (BALQs). The radiation pressure exerted on the metal ions is coupled to the rest of the gas through Coulomb collisions of the metal ions. We calculate the photon density and gas density which allow decoupling of the metal ions from the rest of the gas. These conditions may lead to an outflow composed mostly of the metal ions. We derive a method to constrain the metals/H ratio of observed UV outflows, based on the Ly {\alpha} and Si iv {\lambda}{\lambda}1394, 1403 absorption profiles. We apply this method to an SDSS sample of BALQs to derive a handful of candidate outflows with a higher than solar metal/H ratio. This mechanism can produce ultra fast UV outflows, if a shield of the continuum source with a strong absorption edge is present.