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

Abstract

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* ~ 1010M⊙ galaxies at z = 6. The first, 'Dahlia', adopts an equilibrium model for H2 formation, while the second, 'Althæa', features an improved nonequilibrium 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 100M⊙ yr-1 at z = 6. They both have SFR-stellar mass relation consistent with observations, and a specific SFR of ≈5Gyr-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 makingSNfeedback 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.