Outflows driven by quasars in high-redshift galaxies with radiation hydrodynamics

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 idealized 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, optical, and infrared (IR). We show how a quasar with a luminosity of 1046 erg s− 1 can drive large-scale winds with velocities of | 10²--10³ \, km \, s⁻¹ | and mass outflow rates around 103 M⊙ yr− 1 for times of the order of a few million years. IR radiation is necessary to efficiently transfer momentum to the gas via multiscattering on dust in dense clouds. However, IR multiscattering, 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 multiscattering events for an IR photon is only about a quarter of the mean optical depth from the centre of the cloud. Our models account for the observed outflow rates of ∼500–1000 M⊙ yr− 1 and high velocities of ∼ 103 km s− 1, favouring winds that are energy driven via extremely fast nuclear outflows, interpreted here as being IR radiatively driven winds.