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

TL;DR

This study investigates how deviations from the Kennicutt-Schmidt relation, metallicity, and density influence the [OIII]/[CII] surface brightness ratios in high-redshift galaxies, revealing that burstiness and density are key factors.

Contribution

It introduces a detailed analysis of the factors affecting [OIII]/[CII] ratios in early galaxies, highlighting the dominant role of burstiness and providing insights into their starburst nature.

Findings

High [OIII]/[CII] ratios indicate starburst galaxies with short gas depletion times.

The ratio strongly depends on the burstiness parameter $s$, with secondary dependence on density.

Galaxies show significant enrichment and high density, supporting rapid ISM enrichment in starbursts.

Abstract

We study the impact of deviations from the Kennicutt-Schmidt relation (quantified by the `burstiness' parameter $\kappa_s$), gas metallicity ($Z$), and density ($n$) on the observed [OIII]88$\mu$m/[CII]158$\mu$m surface brightness ratios ($\Sigma_{[OIII]}/\Sigma_{[CII]}$) in nine galaxies at $z\approx6-9$. We first discuss possible biases in the measured $\Sigma_{[OIII]}/\Sigma_{[CII]}$ ratios by comparing the data with zoom-in cosmological simulations, and then use a Markov Chain Monte Carlo algorithm to derive the best fit values of ($\kappa_s, Z, n$). We find that (i) the strongest dependence of $\Sigma_{[OIII]}/\Sigma_{[CII]}$ is on $\kappa_s$; (ii) high ratios identify starburst galaxies with short gas depletion times ($t_{dep}=6-49\,\rm Myr$); (iii) a secondary dependence on density is found, with $\Sigma_{[OIII]}/\Sigma_{[CII]}$ anticorrelating with $n$ as a result of the lower [OIII] critical density, (iv) the ratio only weakly depends on $Z$. The nine galaxies are significantly enriched (Z=0.2-0.5 $Z_\odot$), and dense ($n=10^{1-3} {\rm cm}^{-3}$). This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.