2017MNRAS.471.4128P

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, 'Althaea', 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, Althaea 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; Althaea, instead nicely agrees with observations. The different gas properties result in widely different observables. Althaea outshines Dahlia by a factor of 7 (15) in [C II]157.74 µm (H₂17.03 µm) line emission. Yet, Althaea 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.