On the diversity and complexity of absorption line profiles produced by outflows in Active Galactic Nuclei

TL;DR

This study investigates the origins of the diversity and complexity in absorption line profiles in AGN outflows, revealing that non-monotonic wind properties lead to complex profiles and that absorption lines provide incomplete wind information.

Contribution

The paper introduces a systematic method to analyze absorption line profile complexities caused by accretion disk winds, highlighting the impact of non-monotonic wind properties on observed profiles.

Findings

Complex absorption profiles can arise from non-monotonic wind properties.

Absorption lines often do not represent the entire wind structure.

Mass outflow rate estimates can be significantly inaccurate.

Abstract

Understanding the origin of AGN absorption line profiles and their diversity could help to explain the physical structure of the accretion flow, and also to assess the impact of accretion on the evolution of the AGN host galaxies. Here we present our first attempt to systematically address the issue of the origin of the complexities observed in absorption profiles. Using a simple method, we compute absorption line profiles against a continuum point source for several simulations of accretion disk winds. We investigate the geometrical, ionization, and dynamical effects on the absorption line shapes. We find that significant complexity and diversity of the absorption line profile shapes can be produced by the non-monotonic distribution of the wind velocity, density, and ionization state. Non-monotonic distributions of such quantities are present even in steady-state, smooth disk winds, and naturally lead to the formation of multiple and detached absorption troughs. These results demonstrate that the part of a wind where an absorption line is formed is not representative of the entire wind. Thus, the information contained in the absorption line is incomplete if not even insufficient to well estimate gross properties of the wind such as the total mass and energy fluxes. In addition, the highly dynamical nature of certain portions of disk winds can have important effects on the estimates of the wind properties. For example, the mass outflow rates can be off up to two orders of magnitude with respect to estimates based on a spherically symmetric, homogeneous, constant velocity wind.