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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 modelled 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 time-scales 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. 

Abstract We presentCloudFlex, an open-source tool for predicting absorption-line signatures of cool gas in galaxy halos with small-scale structure. Motivated by analyses of ∼10⁴K material in hydrodynamical simulations of turbulent, multiphase media, we model cool gas structures as complexes of cloudlets sampled from a power-law distribution of mass $\propto m_{\mathrm{cl}}^{-\alpha }$with velocities drawn from a turbulent velocity field. The user may specifyα, the lower limit of the cloudlet mass distribution ( $m_{\mathrm{cl},\min }$), and several other parameters that set the mass, size, and velocity distribution of the complex. This permits investigation of the relation between these parameters and absorption-line observables. As a proof-of-concept, we calculate the Mgiiλ2796 absorption induced by the cloudlets in background quasi-stellar object (QSO) spectra. We demonstrate that, at fixed metallicity, the covering fraction of sight lines with equivalent widthsW₂₇₉₆< 0.3 Å increases significantly with decreasing $m_{\mathrm{cl},\min }$, cloudlet number density (n_cl), and complex size. We then use this framework to predict the halo-scaleW₂₇₉₆distribution around ∼L^*galaxies. We show that the observed incidences ofW₂₇₉₆> 0.3 Å sight lines with impact parameters 10 kpc <R_⊥< 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with $m_{\mathrm{cl},\min }\ge 100M_{\odot }$andn_cl≥ 0.03 cm⁻³, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (m_cl≲ 100M_⊙) with a volume filling factor ≲1%. When used to model absorption-line data sets built from multi-sight line and/or spatially extended background probes,CloudFlexenables detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM.