Unveiling the main sequence of galaxies at z >= 5 with the JWST: predictions from simulations

We use two independent galaxy-formation simulations, flares, a cosmological hydrodynamical simulation, and shark, a semi-analytic model, to explore how well the JWST will be able to uncover the existence and parameters of the star-forming main sequence (SFS) at z = 5 -> 10, i.e. shape, scatter, normalization. Using two independent simulations allows us to isolate predictions (e.g. stellar mass, star formation rate, SFR, luminosity functions) that are robust to or highly dependent on the implementation of the physics of galaxy formation. Both simulations predict that JWST can observe >= 70-90 percent (for shark and flares, respectively) of galaxies up to z similar to 10 (down to stellar masses of approximate to 10(8.3) M-circle dot and SFRs of approximate to 10(0.5) M-circle dot yr(-1)) in modest integration times and given current proposed survey areas (e.g. the Web COSMOS 0.6deg(2)) to accurately constrain the parameters of the SFS. Although both simulations predict qualitatively similar distributions of stellar mass and SFR. There are important quantitative differences, such as the abundance of massive, star-forming galaxies with flares predicting a higher abundance than shark; the early onset of quenching as a result of black hole growth in flares (at z approximate to 8), not seen in shark until much lower redshifts; and the implementation of synthetic photometry with flares predicting more JWST-detected galaxies (similar to 90 per cent) than shark (similar to 70 per cent) at z = 10. JWST observations will distinguish between these models, leading to a significant improvement upon our understanding of the formation of the very first galaxies.