The impact of chemistry on the structure of high-z galaxies

TL;DR

This study compares two high-resolution galaxy simulations at redshift 6, highlighting how different H$_{2}$ chemistry models influence galaxy morphology, gas properties, and observable emissions, revealing significant impacts on star formation and line luminosities.

Contribution

It introduces a comparison between equilibrium and non-equilibrium H$_{2}$ chemistry models in high-z galaxy simulations, showing their effects on galaxy structure and observables.

Findings

Non-equilibrium chemistry shifts H$_{2}$ formation to higher densities.

Non-equilibrium model produces more clumpy, fragmented morphology.

Simulated [CII] and H$_{2}$ emissions differ significantly between models.

Abstract

To improve our understanding of high-z galaxies we study the impact of H$_{2}$ chemistry on their evolution, morphology and observed properties. We compare two zoom-in high-resolution (30 pc) simulations of prototypical $M_{\star}\sim 10^{10} {\rm M}_{\odot}$ galaxies at $z=6$. The first, "Dahlia", adopts an equilibrium model for H$_{2}$ formation, while the second, "Alth{\ae}a", features an improved non-equilibrium chemistry network. The star formation rate (SFR) of the two galaxies is similar (within 50\%), and increases with time reaching values close to 100 ${\rm M}_{\odot}/\rm yr$ at $z=6$. They both have SFR-stellar mass relation consistent with observations, and a specific SFR of $\simeq 5\, {\rm Gyr}^{-1}$. The main differences arise in the gas properties. The non-equilibrium chemistry determines the H$\rightarrow$ H$_{2}$~transition to occur at densities $> 300\,{cm}^{-3}$, i.e. about 10 times larger than predicted by the equilibrium model used for Dahlia. As a result, Alth{\ae}a features a more clumpy and fragmented morphology, in turn making SN feedback more effective. Also, because of the lower density and weaker feedback, Dahlia sits $3\sigma$ away from the Schmidt-Kennicutt relation; Alth{\ae}a, instead nicely agrees with observations. The different gas properties result in widely different observables. Alth{\ae}a outshines Dahlia by a factor of 7 (15) in [CII]~$157.74\,\mu{\rm m}$ (H$_{2}$~$17.03\,\mu{\rm m}$) line emission. Yet, Alth{\ae}a is under-luminous with respect to the locally observed [CII]-SFR relation. Whether this relation does not apply at high-z or the line luminosity is reduced by CMB and metallicity effects remains as an open question.