Hybrid-chimes: A model for radiative cooling and the abundances of ions and molecules in simulations of galaxy formation

TL;DR

This paper introduces a hybrid radiative cooling model for galaxy formation simulations that combines detailed chemical networks with efficient quasi-equilibrium calculations, improving the accuracy of ISM phase predictions.

Contribution

The model integrates non-equilibrium chemistry with quasi-equilibrium cooling rates, enhancing physical realism and computational efficiency in galaxy formation simulations.

Findings

Neutral gas pressure underestimates by up to 0.5 dex in simulations.

Atomic-to-molecular hydrogen transition shifts to lower densities without oxygen.

The model accurately reproduces thermal and chemical states of the multi-phase ISM.

Abstract

Radiative processes play a pivotal role in shaping the thermal and chemical states of gas across diverse astrophysical environments, from the interstellar medium (ISM) to the intergalactic medium. We present a hybrid cooling model for cosmological simulations that incorporates a comprehensive treatment of radiative processes, including parameterizations of the interstellar radiation field, cosmic ray rates, and dust physics. The model uses the chimes chemical network and combines on-the-fly non-equilibrium calculations with quasi-equilibrium cooling rates. The quasi-equilibrium rates account for the time-dependent free electron fractions of elements tracked in non-equilibrium, balancing computational efficiency with physical accuracy. We evaluate the performance under various conditions, including the thermal evolution of primordial gas at the cosmic mean density, the properties of the warm and cold neutral media in Milky Way-like galaxies, and the atomic-to-molecular hydrogen transition. We demonstrate that thermal equilibrium predictions for the neutral phases of the ISM underestimate the median gas pressures in simulations of isolated galaxies by up to 0.5 dex. Finally we find that the atomic-to-molecular hydrogen transition is shifted to lower densities by up to 1 dex if oxygen is not included in the chemical network. Our work provides a robust framework for studying the multi-phase ISM and its role in galaxy formation and evolution.