The nature of high [OIII]88m/[CII]158m galaxies in the epoch of reionization: low carbon abundance and a top-heavy IMF?

ALMA observations of z>6 galaxies have revealed abnormally high [OIII]88μm/[CII]158μm ratios and [CII] deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionisation parameter, gas density, or PDR covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ SPHINX20, 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 ∼8× lower than Solar and the total metal production is ∼5.7× 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∼4.5. While we demonstrate that ionisation parameter, LyC escape fraction, 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. We provide estimates of these quantities for nine observed z>6 galaxies. Finally, we demonstrate that [OI]63μm can be used as a replacement for [CII] when [CII] is unobserved and argue that more observation time should be used to target [OI] at z>6