The [CII] emission as a molecular gas mass tracer in galaxies at low and high redshifts

We present ALMA Band 9 observations of the [CII]158 μm emission for a sample of 10 mainsequence galaxies at redshift z ~ 2, with typical stellar masses (logM*/M_⊙ ~ 10.0-10.9) and star formation rates (~35-115 M_⊙ yr^-1). Given the strong and well-understood evolution of the interstellar medium from the present to z = 2, we investigate the behaviour of the [CII] emission and empirically identify its primary driver. We detect [C II] from six galaxies (four secure and two tentative) and estimate ensemble averages including non-detections. The [C II]- to-infrared luminosity ratio (L_{[C II]}/L_IR) of our sample is similar to that of local main-sequence galaxies (~2 × 10^-3), and ~10 times higher than that of starbursts. The [CII] emission has an average spatial extent of 4-7 kpc, consistent with the optical size. Complementing our sample with literature data, we find that the [CII] luminosity correlates with galaxies' molecular gas mass, with a mean absolute deviation of 0.2 dex and without evident systematics: the [CII]- to-H₂ conversion factor (α_{[C II]} ~ 30 M_⊙/L_⊙) is largely independent of galaxies' depletion time, metallicity, and redshift. [C II] seems therefore a convenient tracer to estimate galaxies' molecular gas content regardless of their starburst or main-sequence nature, and extending to metal-poor galaxies at low and high redshifts. The dearth of [C II] emission reported for z > 6-7 galaxies might suggest either a high star formation efficiency or a small fraction of ultraviolet light from star formation reprocessed by dust.