High [{OIII}]/[{CII}] surface brightness ratios trace early starburst galaxies

We study the impact of deviations from the Kennicutt–Schmidt relation (quantified by the ‘burstiness’ parameter κs), 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 (κs, Z, n). We find that (i) the strongest dependence of Σ[O III]/Σ[C II] is on κs; (ii) high ratios identify starburst galaxies with short gas depletion times (tdep = 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 ≈ 101−3 cm−3. This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.