Galaxies that Shine: radiation-hydrodynamical simulations of disk galaxies

TL;DR

This study uses radiation-hydrodynamical simulations at 18 pc resolution to evaluate the impact of radiation feedback on galaxy evolution, finding it less influential than previously assumed, mainly affecting dense cloud formation.

Contribution

First galaxy-scale RHD simulations including photo-ionisation heating and radiation pressure, providing new insights into radiation feedback effects on galaxy disks.

Findings

Photo-heating thickens disks and suppresses star formation similarly to supernova feedback.

Radiation pressure has minimal impact on galaxy evolution in these simulations.

Increasing IR optical depths reduces star formation by smoothing gas, not by driving outflows.

Abstract

Radiation feedback is typically implemented using subgrid recipes in hydrodynamical simulations of galaxies. Very little work has so far been performed using radiation-hydrodynamics (RHD), and there is no consensus on the importance of radiation feedback in galaxy evolution. We present RHD simulations of isolated galaxy disks of different masses with a resolution of 18 pc. Besides accounting for supernova feedback, our simulations are the first galaxy-scale simulations to include RHD treatments of photo-ionisation heating and radiation pressure, from both direct optical/UV radiation and multi-scattered, re-processed infrared (IR) radiation. Photo-heating smooths and thickens the disks and suppresses star formation about as much as the inclusion of ("thermal dump") supernova feedback does. These effects decrease with galaxy mass and are mainly due to the prevention of the formation of dense clouds, as opposed to their destruction. Radiation pressure, whether from direct or IR radiation, has little effect, but for the IR radiation we show that its impact is limited by our inability to resolve the high optical depths for which multi-scattering becomes important. While artificially boosting the IR optical depths does reduce the star formation, it does so by smoothing the gas rather than by generating stronger outflows. We conclude that although higher-resolution simulations, and potentially also different supernova implementations, are needed for confirmation, our findings suggest that radiation feedback is more gentle and less effective than is often assumed in subgrid prescriptions.