2018MNRAS.473.5407M

We find that clouds of optically thin, pressure-confined gas are prone to fragmentation as they cool below ∼10⁶ K. This fragmentation follows the lengthscale ∼c_st_cool, ultimately reaching very small scales (∼0.1 pc/n), as they reach the temperature ∼10⁴ 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_cloudlet ∼ 10¹⁷ cm^–2 is essentially independent of environment; this column density represents a characteristic scale for atomic gas at 10⁴ 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 haloes, 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.