The Nature of High ${[\mathrm{OIII}]}_{88\mu \mathrm{m}}$/${[\mathrm{CII}]}_{158\mu \mathrm{m}}$ Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

Abstract

ALMA observations of $z>6$ galaxies have revealed abnormally high [OIII]${}_{88\mu \mathrm{m}}$/[CII]${}_{158\mu \mathrm{m}}$ ratios and [CII]${}_{158\mu \mathrm{m}}$ deficits compared to local galaxies. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ SPHINX${}^{20}$, a state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization. We find that the observed $z>6$ [OIII]-SFR and [CII]-SFR relations can only be reproduced when the C/O abundance ratio is $\sim 8\times$ lower than Solar and the total metal production is $\sim 4\times$ higher than that of a Kroupa IMF. This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core-collapse supernovae with a more top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [CII]-SFR and [CII] luminosity functions are predicted to converge to observed values at $z\sim 4.5$. While we demonstrate that ionisation parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [OIII]/[CII], observed values at $z>6$ can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [CII] and [OIII] can be used to predict the values of ionisation parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed $z>6$ galaxies. Finally, we demonstrate that [OI]${}_{63\mu \mathrm{m}}$ can be used as a replacement for [CII] in high-redshift galaxies where [CII] is unobserved and argue that more observation time should be used to target [OI] at $z>6$. (Abridged)

20 pages, 18 figures, Accepted by MNRAS

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