CII*/CII ratio in high-redshift DLAs: ISM phase separation drives the
  observed bimodality of [CII] cooling rates
  We discuss observations of CII*/CII ratios and cooling rates due to
[CII]$\,$158$\mu$m emission in high-redshift intervening damped Lyman-$\alpha$
systems towards quasars. We show that the observed bimodality in the CII
cooling rates actually reflects a bimodality in the CII*/CII-metallicity plane
that can be naturally explained by phase segregation of the neutral medium,
without invoking differences in star-formation scenarios. Assuming realistic
distributions of the physical parameters to calculate the phase diagrams, we
also reproduce qualitatively the metallicity dependence of this bimodality. We
emphasize that high-z DLAs mostly probe low-metallicity gas ($Z\lesssim 0.1
Z_{\odot}$), where heating is dominated by cosmic rays (and/or turbulence), and
not by photoelectric heating. Therefore even if the gas of DLA is predominantly
cold (where the cooling is dominated by [CII]), the excitation of CII can be
used to derive the cosmic ray ionization rate (and/or turbulent heating), but
not the UV field, as was previously considered. Alternatively, if the gas in
DLA is predominantly warm, CII*/CII can be used to constrain its number
density. Finally, we also discuss the importance of the ionized medium, which,
if also present along the line of sight, can significantly increase the average
CII*/CII ratio.
