High$-z$ [OI] emission lines: ColdSIM simulations and ALMA observations

Neutral-oxygen [OI] far-infrared emission lines at $63\mu$m and $145\mu$m are powerful probes of the physical conditions in the interstellar medium, although they have not been fully exploited in high-redshift studies. We investigate the connection between [OI] emission lines and key galaxy properties, such as star formation rate (SFR) and H$_2$ content. Our predictions are compared with existing observations and new data analysed in this work. We post-process the outputs of the ColdSIM cosmological simulations with the DESPOTIC model, taking into account [OI]$63\mu$m self-absorption by cold foreground material. A Random Forest algorithm is employed to accelerate computations and new observational ALMA data for galaxies at redshift $z\simeq 5-7$ are used to validate our model. Our predictions show significant [OI]$63\mu$m luminosities ($\approx 10^8\,\rm L_\odot$) for galaxies with SFRs of $\approx 10^2\,\rm M_\odot\,{\rm yr}^{-1}$. The $145\mu$m line luminosity is typically $15 \%$ the [OI]$63\mu$m one and is a factor $\approx 2-20$ below high-$z$ observations. Both [OI] lines correlate with SFR and molecular mass, but exhibit flattening in scaling relations with metallicity and stellar mass. Foreground self-absorption reduces the [OI] flux by a factor of $2-4$, consistent with empirical corrections in observational studies. We find typical line ratios of [OI]$63\mu$m / [CII]$158\mu$m $\approx 1$ and [OI]$145\mu$m / [CII]$158\mu$m $\approx 0.2 \, -$ consistent with $z\gtrsim 6$ observations, but only when [OI]$63\mu$m self-absorption is included. Both [OI]$63\mu$m and [OI]$145\mu$m lines serve as tracers of star formation and molecular gas at high redshift. Their joint detection can provide constraints on the properties of the early interstellar medium and self-absorption of the [OI]$63\mu$m line.