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

To improve our understanding of high-z galaxies, we study the impact of H2 chemistry on their evolution, morphology and observed properties. We compare two zoom-in high-resolution (30 pc) simulations of prototypical M⋆ ∼ 1010 M⊙ galaxies at z = 6. The first, ‘Dahlia’, adopts an equilibrium model for H2 formation, while the second, ‘Althæa’, features an improved non-equilibrium chemistry network. The star formation rate (SFR) of the two galaxies is similar (within 50 per cent), and increases with time reaching values close to 100 M⊙ yr−1 at z = 6. They both have SFR–stellar mass relation consistent with observations, and a specific SFR of ≃5 Gyr−1. The main differences arise in the gas properties. The non-equilibrium chemistry determines the H → H2 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, Althæa 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 3σ away from the Schmidt–Kennicutt relation; Althæa, instead nicely agrees with observations. The different gas properties result in widely different observables. Althæa outshines Dahlia by a factor of 7 (15) in [C II]157.74 μm (H217.03 μm) line emission. Yet, Althæa is underluminous with respect to the locally observed [C II]–SFR relation. Whether this relation does not apply at high-z or the line luminosity is reduced by cosmic microwave background and metallicity effects remain as an open question.