The Impact of Enhanced Halo Resolution on the Simulated Circumgalactic Medium

Abstract

Traditional cosmological hydrodynamics simulations fail to spatially resolve\nthe circumgalatic medium (CGM), the reservoir of tenuous gas surrounding a\ngalaxy and extending to its virial radius. We introduce the technique of\nEnhanced Halo Resolution (EHR), enabling more realistic physical modeling of\nthe simulated CGM by consistently forcing gas refinement to smaller scales\nthroughout the virial halo of a simulated galaxy. We investigate the effects of\nEHR in the Tempest simulations, a suite of Enzo-based cosmological zoom\nsimulations following the evolution of an L* galaxy, resolving spatial scales\nof 500 comoving pc out to 100 comoving kpc in galactocentric radius. Among its\nmany effects, EHR (1) changes the thermal balance of the CGM, increasing its\ncool gas content and decreasing its warm/hot gas content; (2) preserves cool\ngas structures for longer periods; and (3) enables these cool clouds to exist\nat progressively smaller size scales. Observationally, this results in a boost\nin "low ions" like H I and a drop in "high ions" like O VI throughout the CGM.\nThese effects of EHR do not converge in the Tempest simulations, but\nextrapolating these trends suggests that the CGM in reality is a mist\nconsisting of ubiquitous, small, long-lived, cool clouds suspended in a hot\nmedium at the virial temperature of the halo. Additionally, we explore the\nphysical mechanisms to explain why EHR produces the above effects, proposing\nthat it works both by (1) better sampling the distribution of CGM phases\nenabling runaway cooling in the denser, cooler tail of the phase distribution;\nand (2) preventing cool gas clouds from artificially mixing with the ambient\nhot halo and evaporating. Evidence is found for both EHR mechanisms occurring\nin the Tempest simulations.\n