A physical model for [CII] line emission from galaxies

Abstract

A tight relation between the [CII]158$\hat{I}¼$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\%) fraction of the outliers being [CII]-deficient. Moreover, the [CII] surface brightness ($\hat{I}{\pounds }_{\mathrm{CII}}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [CII]-deficiency we have developed an analytical model that fits local [CII] data, and has been validated against radiative transfer simulations performed with CLOUDY. The model predicts an overall increase of $\hat{I}{\pounds }_{\mathrm{CII}}$ with the surface star formation rate ($\hat{I}{\pounds }_{\ast }$). However, for $\hat{I}{\pounds }_{\ast }>1M_{\odot }{\mathrm{yr}}^{-1}{\mathrm{kpc}}^{-2}$, $\hat{I}{\pounds }_{\mathrm{CII}}$ saturates. We conclude that underluminous [CII] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt-Schmidt relation ($\hat{I}{º}_s\gg 1$), parameterized by the "burstiness" parameter $\hat{I}{º}_s$; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [CII] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [OIII]88$\hat{I}¼$m, CIII]1909A, CO lines). Simple formulae are given to interpret available data for low and high-$z$ galaxies.

Accepted for publication in MNRAS. 12 pages, 6 Figures. Comments welcome