Paper detail

RAMSES-RTZ: Non-Equilibrium Metal Chemistry and Cooling Coupled to On-The-Fly Radiation Hydrodynamics

Emission and absorption lines from elements heavier than helium (metals) represent one of our strongest probes of galaxy formation physics across nearly all redshifts accessible to observations. The vast majority of simulations that model these metal lines often assume either collisional or photoionisation equilibrium, or a combination of the two. For the few simulations that have relaxed these assumptions, a redshift-dependent meta-galactic UV background or fixed spectrum is often used in the non-equilibrium photoionisation calculation, which is unlikely to be accurate in the interstellar medium where the gas can self-shield as well as in the high-redshift circumgalactic medium where locally emitted radiation may dominate over the UV background. In this work, we relax this final assumption by coupling the ionisation states of individual metals to the radiation hydrodynamics solver present in RAMSES-RT. Our chemical network follows radiative recombination, dielectronic recombination, collisional ionisation, photoionisation, and charge transfer and we use the ionisation states to compute non-equilibrium optically-thin metal-line cooling. The fiducial model solves for the ionisation states of C, N, O, Mg, Si, S, Fe, and Ne in addition to H, He, and H$_2$, but can be easily extended for other ions. We provide interfaces to two different ODE solvers that are competitive in both speed and accuracy. The code has been benchmarked across a variety of gas conditions to reproduce results from CLOUDY when equilibrium is reached. We show an example isolated galaxy simulation with on-the-fly radiative transfer that demonstrates the utility of our code for translating between simulations and observations without the use of idealised photoionisation models.