2022A&A...663A.172M

Context. A tight relation between [CII] 158 µm line luminosity and the star formation rate (SFR) has been observed for local galaxies. At high redshift (z > 5), galaxies instead deviate downwards from the local Σ_[CII] - Σ_SFR relation. This deviation might be caused by different interstellar medium (ISM) properties in galaxies at early epochs.
Aims. To test this hypothesis, we combined the [CII] and SFR data with CIII] 1909 Å line observations and our physical models. We additionally investigated how ISM properties, such as burstiness, κ_s, total gas density, n, and metallicity, Z, affect the deviation from the Σ_[CII] - Σ_SFR relation in these sources.
Methods. We present the VLT/X-shooter observations targeting the CIII] λ1909 line emission in three galaxies at 5.5 < z < 7.0. We include archival X-shooter data of two other sources at 5.5 < z < 7.0 and the VLT/MUSE archival data of six galaxies at z ∼ 2. We extend our sample of galaxies with eleven star-forming systems at 6 < z < 7.5, with either CIII] or [CII] detection reported in the literature.
Results. We detected CIII] λλ1907, 1909 line emission in HZ10 and we derived the intrinsic, integrated flux of the CIII] λ1909 line. We constrained the ISM properties for our sample of galaxies, κ_s, n, and Z, by applying our physically motivated model based on the MCMC algorithm. For the most part, high-z star-forming galaxies show subsolar metallicities. The majority of the sources have log(κ_s) ≳ 1, that is, they overshoot the Kennicutt-Schmidt (KS) relation by about one order of magnitude.
Conclusions. Our findings suggest that the whole KS relation might be shifted upwards at early times. Furthermore, all the high-z galaxies of our sample lie below the Σ_[CII] - Σ_SFR local relation. The total gas density, n, shows the strongest correlation with the deviation from the local Σ_[CII] - Σ_SFR relation, namely, low-density high-z systems have lower [CII] surface brightness, in agreement with theoretical models.