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.
Categories: astro-ph.GA astro-ph.CO