Of the almost 40 star forming galaxies at z \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle > $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 5 ( not counting QSOs ) observed in [ C II

] to date , nearly half are either very faint in [ C II

] , or not detected at all , and fall well below expectations based on locally derived relations between star formation rate and [ C II

] luminosity .
This has raised questions as to how reliable [ C II

] is as a tracer of star formation activity at these epochs and how factors such as metallicity might affect the [ C II

] emission .
Combining cosmological zoom simulations of galaxies with SÍGAME ( SImulator of GAlaxy Millimeter/submillimeter Emission ) we have modeled the multi-phased interstellar medium ( ISM ) and its emission in [ C II

] , as well as [ O I

] and [ O III

] , from 30 main sequence galaxies at z \simeq 6 with star formation rates \sim 3 - 23 M _ { \odot } yr ^ { -1 } , stellar masses \sim ( 0.7 - 8 ) \times 10 ^ { 9 } { \mbox { $M _ { \odot } $ } } , and metallicities \sim ( 0.1 - 0.4 ) \times \mbox { $Z _ { \odot } $ } .
The simulations are able to reproduce the aforementioned [ C II

] -faintness of some normal star forming galaxies sources at z \geq 5 .
In terms of [ O I

] and [ O III

] very few observations are available at z \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle > $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 5 – but our simulations match two of the three existing z \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle > $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 5 detections of [ O III

] , and are furthermore roughly consistent with the [ O I

] and [ O III

] luminosity relations with star formation rate observed for local starburst galaxies .
We find that the [ C II

] emission is dominated by the diffuse ionized gas phase and molecular clouds , which on average contribute \sim 66 \% and \sim 27 \% , respectively .
The molecular gas , which constitutes only \sim 10 \% of the total gas mass is thus a more efficient emitter of [ C II

] than the ionized gas , which makes up \sim 85 \% of the total gas mass .
A principal component analysis shows that the [ C II

] luminosity correlates with the star formation activity of a galaxy as well as its average metallicity .
The low metallicities of our simulations together with their low molecular gas mass fractions can account for their [ C II

] -faintness , and we suggest these factors may also be responsible for the [ C II

] -faint normal galaxies observed at these early epochs .