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