A Characteristic Scale for Cold Gas
  We find that clouds of optically-thin, pressure-confined gas are prone to
fragmentation as they cool below $\sim10^6$ K. This fragmentation follows the
lengthscale $\sim{c}_{\text{s}}\,t_{\text{cool}}$, ultimately reaching very
small scales ($\sim{0.1} \text{pc}/n$) as they reach the temperature $\sim10^4$
K at which hydrogen recombines. While this lengthscale depends on the ambient
pressure confining the clouds, we find that the column density through an
individual fragment $N_{\text{cloudlet}}\sim10^{17} \text{cm}^{-3}$ is
essentially independent of environment; this column density represents a
characteristic scale for atomic gas at $10^4$ K. We therefore suggest that
"clouds" of cold, atomic gas may in fact have the structure of a mist or a fog,
composed of tiny fragments dispersed throughout the ambient medium. We show
that this scale emerges in hydrodynamic simulations, and that the corresponding
increase in the surface area may imply rapid entrainment of cold gas. We also
apply it to a number of observational puzzles, including the large covering
fraction of diffuse gas in galaxy halos, the broad line widths seen in quasar
and AGN spectra, and the entrainment of cold gas in galactic winds. While our
simulations make a number of assumptions and thus have associated
uncertainties, we show that this characteristic scale is consistent with a
number of observations, across a wide range of astrophysical environments. We
discuss future steps for testing, improving, and extending our model.
