A physical model for [C II] line emission from galaxies

A tight relation between the [C II] 158 mu m line luminosity and star formation rate is measured in local galaxies. At high redshift (z > 5), though, a much larger scatter is observed, with a considerable (15-20 per cent) fraction of the outliers being [C II]-deficient. Moreover, the [C II] surface brightness (Sigma([C II])) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [C II]-deficiency, we have developed an analytical model that fits local [C II] data and has been validated against radiative transfer simulations performed with CLOUDY. The model predicts an overall increase of Sigma([C II]) with Sigma(SFR). However, for Sigma(SFR) greater than or similar to 1M(circle dot) yr(-1) kpc(-2), Sigma([C II]) saturates. We conclude that underluminous [C II] systems can result from a combination of three factors: (a) large upward deviations from theKennicutt-Schmidt relation (kappa(s) >> 1), parametrized by the 'burstiness' parameter kappa(s); (b) low metallicity; (e.g. CR7). Observations of [C II] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [OIII]88 mu m, C III]1909 degrees, CO lines). Simple formulae are given to interpret available data for low- and high- z galaxies.
QC 20191104