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