2021MNRAS.505.5543V

We study the impact of deviations from the Kennicutt-Schmidt relation (quantified by the 'burstiness' parameter κ_s), gas metallicity (Z), and density (n) on the observed [O III]88 µm/[C II]158 µm surface brightness ratios (Σ_[O III]_/Σ[C II]_) in nine galaxies at z ≃ 6-9. We first discuss possible biases in the measured Σ_[O III]_/Σ[C II]_ ratios by comparing the data with zoom-in cosmological simulations and then use a Markov Chain Monte Carlo algorithm to derive the best-fitting values of (κ_s, Z, n). We find that (i) the strongest dependence of Σ_[O III]_/Σ[C II]_ is on κ_s; (ii) high ratios identify starburst galaxies with short gas depletion times (t_ dep_=6-49 Myr); (iii) a secondary dependence on density is found, with Σ_[O III]_/Σ[C II]_ anticorrelating with n as a result of the lower [O III] critical density; and (iv) the ratio weakly depends only on Z. The nine galaxies are significantly enriched (Z = 0.2-0.5 Z_☉) and dense n ≃ 10^1–3 cm^–3. This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.