Driving gas shells with radiation pressure on dust in radiation-hydrodynamic simulations

Abstract

We present radiation-hydrodynamic simulations of radiatively-driven gas shells launched by bright active galactic nuclei (AGN) in isolated dark matter haloes. Our goals are (1) to investigate the ability of AGN radiation pressure on dust to launch galactic outflows and (2) to constrain the efficiency of infrared (IR) multi-scattering in boosting outflow acceleration. Our simulations are performed with the radiation-hydrodynamic code RAMSES-RT and include both single- and multi-scattered radiation pressure from an AGN, radiative cooling and self-gravity. Since outflowing shells always eventually become transparent to the incident radiation field, outflows that sweep up all intervening gas are likely to remain gravitationally bound to their halo even at high AGN luminosities. The expansion of outflowing shells is well described by simple analytic models as long as the shells are mildly optically thick to IR radiation. In this case, an enhancement in the acceleration of shells through IR multi-scattering occurs as predicted, i.e. a force dP/dt = tau_IR L/c is exerted on the gas. For high optical depths tau_IR > 50, however, momentum transfer between outflowing optically thick gas and IR radiation is rapidly suppressed, even if the radiation is efficiently confined. At high tau_IR, the characteristic flow time becomes shorter than the required trapping time of IR radiation such that the momentum flux dP/dt << tau_IR L/c. We argue that while unlikely to unbind massive galactic gaseous haloes, AGN radiation pressure on dust could play an important role in regulating star formation and black hole accretion in the nuclei of massive compact galaxies at high redshift.