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

Abstract

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 (PopulationIII) stars are formed with very high characteristic masses. Since isolated PopulationIII stars are likely to be beyond the reach of JWST, small PopulationIII 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 toward the first galaxies. Using this model, we explore the JWST imaging detection limits for PopulationIII galaxies and investigate to what extent such objects may be identified based on their JWST colors. We predict that JWST should be able to detect PopulationIII galaxies with stellar population masses as low as 10 5 M ∞ at z 10 in ultra deep exposures. Over limited redshift intervals, it may also be possible to use color criteria to select PopulationIII galaxy candidates for follow-up spectroscopy. The colors of young PopulationIII galaxies dominated by direct starlight 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 colors of objects dominated by nebular emission show no corresponding IMF sensitivity. We also note that a clean selection of PopulationIII 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. © 2011. The American Astronomical Society. All rights reserved.