Unveiling [C II] clumps in a lensed star-forming galaxy at z ~ 3.4

Context. Observations at UV and optical wavelengths have revealed that galaxies at z ∼ 1 − 4 host star-forming regions, dubbed “clumps”, which are believed to form due to the fragmentation of gravitationally unstable, gas-rich disks. However, the detection of the parent molecular clouds that give birth to such clumps is still possible only in a minority of galaxies, mostly at z ∼ 1. Aims. We investigated the [C II] and dust morphology of a z ∼ 3.4 lensed galaxy hosting four clumps detected in the UV continuum. We aimed to observe the [C II] emission of individual clumps that, unlike the UV, is not affected by dust extinction, to probe their nature and cold gas content. Methods. We conducted ALMA observations probing scales down to ∼300 pc and detected three [C II] clumps. One (dubbed “NE”) coincides with the brightest UV clump, while the other two (“SW” and “C”) are not detected in the UV continuum. We do not detect the dust continuum. Results. We converted the [C II] luminosity of individual clumps into molecular gas mass and found Mmol ∼ 108 M⊙. By complementing it with the star formation rate (SFR) estimate from the UV continuum, we estimated the gas depletion time (tdep) of clumps and investigated their location in the Schmidt–Kennicutt plane. While the NE clump has a very short tdep = 0.16 Gyr, which is comparable with high-redshift starbursts, the SW and C clumps instead have longer tdep > 0.65 Gyr and are likely probing the initial phases of star formation. The lack of dust continuum detection is consistent with the blue UV continuum slope estimated for this galaxy (β ∼ −2.5) and it indicates that dust inhomogeneities do not significantly affect the detection of UV clumps in this target. Conclusions. We pushed the observation of the cold gas content of individual clumps up to z ∼ 3.4 and showed that the [C II] line emission is a promising tracer of molecular clouds at high redshift, allowing the detection of clumps with a large range of depletion times.