The spectral evolution of the first galaxies. I. James Webb Space
  Telescope detection limits and color criteria for population III galaxies
  The James Webb Space Telescope (JWST) is expected to revolutionize our
understanding of the high-redshift Universe, and may be able to test the
prediction that the first, chemically pristine (population III) stars formed
with very high characteristic masses. Since isolated population III stars are
likely to be beyond the reach of JWST, small population III galaxies may offer
the best prospects of directly probing the properties of metal-free stars.
Here, we present Yggdrasil, a new spectral synthesis code geared towards the
first galaxies. Using this model, we explore the JWST imaging detection limits
for population III galaxies and investigate to what extent such objects may be
identified based on their JWST colours. We predict that JWST should be able to
detect population III galaxies with stellar population masses as low as ~10^5
Msolar at z=10 in ultra deep exposures. Over limited redshift intervals, it may
also be possible to use colour criteria to select population III galaxy
candidates for follow-up spectroscopy. The colours of young population III
galaxies dominated by direct star light can be used to probe the stellar
initial mass function (IMF), but this requires almost complete leakage of
ionizing photons into the intergalactic medium. The colours of objects
dominated by nebular emission show no corresponding IMF sensitivity. We also
note that a clean selection of population III galaxies at z~7-8 can be achieved
by adding two JWST/MIRI filters to the JWST/NIRCam filter sets usually
discussed in the context of JWST ultra deep fields.
