Simulations of multi-phase gas in and around galaxies

TL;DR

The review surveys numerical simulations of multiphase gas in and around galaxies, spanning ISM, CGM, and ICM across idealized to cosmological scales. It emphasizes multi-scale, multi-physics challenges, diagnostic quantities, and the need to connect small-scale physics (mixing, cooling, conduction, magnetic fields, cosmic rays) with large-scale outflows and halo dynamics. Key themes include turbulent mixing layers, cloud–wind interactions, thermal instability, and halo-scale stratified and zoom-in simulations, highlighting criteria for gas survival/growth (e.g., $t_{\rm cool,mix}/t_{\rm cc}$ and $r_{\rm crit}$), the impact of enhanced halo resolution, and the role of cosmic rays and conduction in shaping multiphase structure. The paper argues for a unified, multi-scale framework that leverages idealized tests to inform subgrid models and large cosmological simulations, while stressing the importance of robust observationally anchored diagnostics and radiative transfer to constrain models. It also outlines persistent challenges, including convergence across scales, effective subgrid modeling for multiphase gas, and integrating diverse physics to accurately predict observable signatures of multiphase gas in galaxy environments.

Abstract

Multiphase gas -- ranging from cold molecular clouds ($\lesssim 100\,$K) to hot, diffuse plasma ($\gtrsim 10^6\,$K) is a defining feature of the interstellar, circumgalactic, intracluster, and intergalactic media. Accurately simulating its dynamics is critical to improving our understanding of galaxy formation and evolution, however, due to their multi-scale and multi-physics nature, multiphase systems are highly challenging to model. In this review, we provide a comprehensive overview of numerical simulations of multiphase gas in and around galaxies. We begin by outlining the environments where multiphase gas arises and the physical and computational challenges associated with its modeling. Key quantities that characterize multiphase gas dynamics are discussed, followed by an in-depth look at idealized setups such as turbulent mixing layers, cloud-wind interactions, thermal instability, and turbulent boxes. The review then transitions to less idealized and/or larger-scale simulations, covering radiative supernovae bubbles, tall box simulations, isolated galaxy models including dwarf and Milky Way-mass systems, and cosmological zoom-in simulations, with a particular focus on simulations that enhance resolution in the halo. Throughout, we emphasize the importance of connecting scales, extracting robust diagnostics, and comparing simulations to observations. We conclude by outlining persistent challenges and promising directions for future work in simulating the multiphase Universe.

Figures (12)

• Figure 1: Examples of observations and simulations of multiphase gas in and around galaxies. Sources/Credit: NASA/ESA/Hubble Heritage Project, Schneider2020(top left); Tonnesen2012, Sun2022(top right; NASA/ESA/Levay/STScI, Saxton/Lockman/NRAO/AUI/NSF/Mellinger, Heitsch2016(bottom left); fabian2008magnetic, vandeVoort2019.
• Figure 2: Left: Cooling (solid lines) and heating (dotted lines) rates for solar metallicity gas at two different number densities subject to photoionization from the metagalactic UV background. Rates are generated using the Cloudy code and the 'HM05' UV background table at redshift 0 Ferland2013. Right (figure from Marinacci2019): Phase diagram from an isolated galaxy simulation showing many of the density-temperature regimes discussed in this review, including hot, low-density gas (upper left), ionized gas at $T\sim 10^4$ K, high-density cold gas in the interstellar medium, and intermediate temperature unstable gas between these regimes, all of which can have comparable pressure and interact in many multiphase systems, i.e. the ISM. The sharp boundary at the left side of the gas distribution reflects where heating and cooling are balanced, i.e. where the two curves cross for a given density in the left panel.
• Figure 3: Illustrations of the various simulation setups discussed in this review. Art by Giana Deskovich.
• Figure 4: Left panel: Schematic of a turbulent mixing layer from Slavin1993 with the flow moving vertically upwards. Central panel: 3D rendering of the temperature isosurface of a turbulent mixing layer simulation Fielding2020. Right panels: Radiative, turbulent mixing layers shows two characteristic regimes both in morphology (top panels showing temperature slices) and radiated energy (bottom panel) separated by ${\rm Da}=1$Tan2020.
• Figure 5: Different physical mechanisms affecting 'cloud-wind' simulations (figure adapted from Li2019a). The panels show density slices of the 'destruction regime' (i.e. with no or weak cooling) with the hot wind coming from the top.