Exploring the Physical Properties of the Cool Circumgalactic Medium with
  a Semi-Analytic Model
  We develop a semi-analytic model to explore the physical properties of cool
pressure-confined circumgalactic clouds with mass ranging from $10$ to $10^{8}
\, \rm M_{\odot}$ in a hot diffuse halo. We consider physical effects that
control the motion and mass loss of the clouds, and estimate the lifetime and
the observed properties of individual cool gas clouds inferred from the CLOUDY
simulation. Our results show that the cool pressure-confined gas clouds have
physical properties consistent with absorption line systems with neutral
hydrogen column densities $N_{\rm HI}\geq10^{18.5} \rm cm^{-2}$ such as strong
metal absorbers, sub-DLAs, and DLAs. The cool circumgalactic clouds are
transient due to evaporation and recycling and therefore a constant
replenishment is needed to maintain the cool CGM. We further model the ensemble
properties of the cool CGM with clouds originated from outflows, inflows,
or/and in-situ formation with a range of initial cloud mass function and
velocity distribution. We find that only with a certain combination of
parameters, an outflow model can broadly reproduce three cool gas properties
around star-forming galaxies simultaneously: the spatial distribution,
down-the-barrel outflow absorption, and gas velocity dispersion. Both a
constant insitu model and gas inflow model can reproduce the observed covering
fractions of high $\rm N_{HI}$ gas around passive galaxies but they fail to
reproduce sufficient number of low $\rm N_{HI}$ systems. The limitations and
the failures of the current models are discussed. Our results illustrate that
semi-analytic modeling is a promising tool to understand the physics of the
cool CGM which is usually unresolved by state-of-the-art cosmological
hydrodynamic simulations.
