Cold Gas Subgrid Model (CGSM): A Two-Fluid Framework for Modeling Unresolved Cold Gas in Galaxy Simulations

TL;DR

The paper introduces the Cold Gas Subgrid Model (CGSM), a two-fluid framework that models unresolved cold gas in galaxy simulations, enabling more accurate representation of cold CGM dynamics without requiring extremely high resolution.

Contribution

The CGSM is a novel two-fluid subgrid model that captures unresolved cold gas behavior in galaxy simulations, bridging the gap between resolved scales and microphysics.

Findings

Successfully models cold gas mass and distribution in idealized tests.

Captures cloud destruction and growth timescales accurately.

Overcomes resolution limitations of traditional hydrodynamics simulations.

Abstract

The cold ($\sim 10^{4}\,{\rm K}$) component of the circumgalactic medium (CGM) accounts for a significant fraction of all galactic baryons. However, using current galaxy-scale simulations to determine the origin and evolution of cold CGM gas poses a significant challenge, since it is computationally infeasible to directly simulate a galactic halo alongside the sub-pc scales that are crucial for understanding the interactions between cold CGM gas and the surrounding ''hot'' medium. In this work, we introduce a new approach: the Cold Gas Subgrid Model (CGSM), which models unresolved cold gas as a second fluid in addition to the standard ''normal'' gas fluid. The CGSM tracks the total mass density and bulk momentum of unresolved cold gas, deriving the properties of its unresolved cloudlets from the resolved gas phase. The interactions between the subgrid cold fluid and the resolved fluid are modeled by prescriptions from high-resolution simulations of ''cloud crushing'' and thermal instability. Through a series of idealized tests, we demonstrate the CGSM's ability to overcome the resolution limitations of traditional hydrodynamics simulations, successfully capturing the correct cold gas mass, its spatial distribution, and the timescales for cloud destruction and growth. We discuss the implications of using this model in cosmological simulations to more accurately represent the microphysics that govern the galactic baryon cycle.