Outflows Driven by Quasars in High-Redshift Galaxies with Radiation Hydrodynamics

TL;DR

This study uses radiation-hydrodynamical simulations to explore how quasar radiation couples with galactic gas to produce large-scale outflows, emphasizing the role of infrared multi-scattering in driving energetic winds in high-redshift galaxies.

Contribution

It demonstrates how IR radiation and multi-scattering influence quasar-driven winds, providing insights into the mechanisms behind observed high-velocity outflows.

Findings

IR radiation is crucial for momentum transfer in dense clouds.

Outflows reach velocities of 10^2-10^3 km/s with mass rates around 10^3 M_sun/yr.

IR multi-scattering declines as clouds expand, affecting wind dynamics.

Abstract

The quasar mode of Active Galactic Nuclei (AGN) in the high-redshift Universe is routinely observed in gas-rich galaxies together with large-scale AGN-driven winds. It is crucial to understand how photons emitted by the central AGN source couple to the ambient interstellar-medium to trigger large-scale outflows. By means of radiation-hydrodynamical simulations of idealised galactic discs, we study the coupling of photons with the multiphase galactic gas, and how it varies with gas cloud sizes, and the radiation bands included in the simulations, which are ultraviolet (UV), optical, and infrared (IR). We show how a quasar with a luminosity of $10^{46}$ erg/s can drive large-scale winds with velocities of $10^2-10^3$ km/s and mass outflow rates around $10^3$ M$_\odot$/yr for times of order a few million years. Infrared radiation is necessary to efficiently transfer momentum to the gas via multi-scattering on dust in dense clouds. However, IR multi-scattering, despite being extremely important at early times, quickly declines as the central gas cloud expands and breaks up, allowing the radiation to escape through low gas density channels. The typical number of multi-scattering events for an IR photon is only about a quarter of the mean optical depth from the center of the cloud. Our models account for the observed outflow rates of $\sim$500-1000 M$_\odot$/yr and high velocities of $\sim 10^3$ km/s, favouring winds that are energy-driven via extremely fast nuclear outflows, interpreted here as being IR-radiatively-driven winds.