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] 158µm emission in high-redshift intervening damped Lyman-\(\alpha\) (DLA) 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-redshift 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. 

https://doi.org/10.1093/mnrasl/slab119