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