Quasar Absorption Lines from Radiative Shocks: Implications for Multiphase Outflows and Feedback

TL;DR

This paper proposes that quasar absorption lines originate from radiative shocks created when quasar blast waves impact interstellar clouds, explaining observed properties and supporting AGN-driven feedback in galaxy evolution.

Contribution

It introduces a radiative shock model for quasar absorbers, linking observed absorption features to in situ cloud formation and quantifying outflow energetics.

Findings

Radiative shocks can produce compact, high-velocity absorbers consistent with observations.

Estimated outflow kinetic luminosities are 2-5% of AGN luminosity, supporting feedback models.

The model explains multiphase outflow structures across different astrophysical systems.

Abstract

Photoionization modeling of certain low-ionization broad absorption lines in quasars implies very compact (Delta R~0.01 pc), galaxy-scale (R kpc) absorbers blueshifted by several 1000 km s^-1. While these are likely signatures of quasar outflows, the lifetimes of such compact absorbers are too short for them to be direct ejecta from a nuclear wind. Instead, I argue that the absorbing clouds must be transient and created in situ. Following arguments detailed by Faucher-Giguere, Quataert, & Murray (2011), I show that a model in which the cool absorbers form in radiative shocks arising when a quasar blast wave impacts an interstellar cloud along the line of sight successfully explains the key observed properties. Using this radiative shock model, the outflow kinetic luminosities for three luminous quasars are estimated to be Edot,k~2-5% L_AGN (with corresponding momentum fluxes Pdot~2-15 L_AGN/c), consistent with feedback models of the M-sigma relation. These energetics are similar to those recently inferred of molecular outflows in local ultra-luminous infrared galaxies and in post-starburt winds, suggesting that active galactic nuclei (AGN) are capable of driving such outflows. Radiative shocks probably affect the multiphase structure of outflows in a range of other systems, including narrower and higher-ionization quasar absorption lines, and compact intergalactic absorbers ejected by star formation and AGN activity.