Liang et al., available on arXiv

Abstract: Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity (L[CII]) and star formation rate (SFR), suggesting that L[CII] may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower L[CII]/SFR than the local SFGs, including the infrared-luminous, starburst galaxies at low and high redshifts, as well as some moderately star-forming galaxies at the epoch of re-ionization (EoR). The origin of this ‘CII] deficit’ is unclear. In this work, we study the L[CII]-SFR relation of galaxies using a sample of z=0−8 galaxies with Mstar~10^7−5×10^11 Msun extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (FIRE) project. We find a simple analytic expression for L[CII]/SFR of galaxies in terms of the following parameters: mass fraction of [CII]-emitting gas (f[CII]), gas metallicity (Zgas), gas density (ngas) and gas depletion time (tdep=Mgas/SFR). We find two distinct physical regimes, where tdep (Zgas) is the main driver of the [CII] deficit in H2-rich (H2-poor) galaxies. The observed [CII] deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that [CII] deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant L[CII]-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming [CII] line intensity mapping experiments.