On the model of the circumgalactic mist: the implications of cloud sizes in galactic winds and halos

TL;DR

This paper introduces a multiphase model of the circumgalactic medium, suggesting cold gas exists as small cloudlets embedded in hot halos, which explains observed properties and highlights the need for higher resolution in simulations.

Contribution

The study proposes a novel semi-analytic and MHD simulation-based model of the circumgalactic mist, emphasizing the importance of cloudlet sizes for accurate cold gas representation.

Findings

Cold gas conforms to a characteristic column density of ~10^{17} cm^{-2}.

The model predicts many small cold clumps with high covering fractions.

Spatial resolution mismatch affects cold gas modeling in galaxy simulations.

Abstract

Ubiquitous detections of cold/warm gas around galaxies indicate that the circumgalactic medium (CGM) is multiphase and dynamic. Recent state-of-the-art cosmological galaxy simulations have generally underproduced the column density of cold halo gas. We argue that this may be due to a mismatch of spatial resolution in the circumgalactic space and the relevant physical scales at which the cold gas operates. Using semi-analytic calculations and a set of magnetohydrodynamic (MHD) simulations, we present a multiphase model of the gaseous halos around galaxies, the circumgalactic mist (CGm). The CGm model is based on the idea that the observed cold halo gas may be a composite of cold, dense and small cloudlets embedded in a hot diffuse halo, resembling terrestrial clouds and mist. We show that the resulting cold gas from thermal instabilities conforms to a characteristic column density of $N_{\rm H}\approx 10^{17}\rm{cm^{-2}}$ as predicted by the $c_s t_{\rm cool}$ ansatz. The model implies a large number of cold clumps in the inner galactic halo with a small volume filling factor but large covering fraction. The model also naturally gives rise to spatial extents and differential covering fractions of cold, warm and hot gas. To self-consistently model the co-evolution of the CGM and star formation within galaxies, future simulations must address the mismatch of the spatial resolution and characteristic scale of cold gas.