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The James Webb Space Telescope (JWST) is expected to revolutionize ourunderstanding of the high-redshift Universe, and may be able to test theprediction that the first, chemically pristine (population III) stars formedwith very high characteristic masses. Since isolated population III stars arelikely to be beyond the reach of JWST, small population III galaxies may offerthe best prospects of directly probing the properties of metal-free stars.Here, we present Yggdrasil, a new spectral synthesis code geared towards thefirst galaxies. Using this model, we explore the JWST imaging detection limitsfor population III galaxies and investigate to what extent such objects may beidentified based on their JWST colours. We predict that JWST should be able todetect population III galaxies with stellar population masses as low as ~10^5Msolar at z=10 in ultra deep exposures. Over limited redshift intervals, it mayalso be possible to use colour criteria to select population III galaxycandidates for follow-up spectroscopy. The colours of young population IIIgalaxies dominated by direct star light can be used to probe the stellarinitial mass function (IMF), but this requires almost complete leakage ofionizing photons into the intergalactic medium. The colours of objectsdominated by nebular emission show no corresponding IMF sensitivity. We alsonote that a clean selection of population III galaxies at z~7-8 can be achievedby adding two JWST/MIRI filters to the JWST/NIRCam filter sets usuallydiscussed in the context of JWST ultra deep fields.

The spectral evolution of the first galaxies. I. James Webb Space Telescope detection limits and color criteria for population III galaxies