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

Abstract

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 simulations, we present a multiphase model of the gaseous haloes around galaxies, the circumgalactic mist (CGmist). The CGmist 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 cstcool ansatz. The model implies a large number of cold clumps in the inner galactic halo with a small volume filling factor but a 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.