First Light
First light objects are expected to be at a redshift of z > 8, where the hot stellar continuum will lie in the NIRCam/NIRSpec spectral range. However, already by z ~ 5 features characteristic of red giants and supergiants, indicative of evolved stellar populations, shift out of the NIRCam and NIRSpec ranges. For z > 12, the Balmer limit is shifted out of the region accessible to these instruments. Thus, for z > 12, the photometric candidates will be detected in only a few NIRCam filter bands with no confirming detection of a second SED feature.
The NIRCam and MIRI filters convolved with the galaxy spectra are shown by the horizontal brown and orange bars, along with + 10% error bars. NIRCam would reliably identify both objects as very young galaxies and would allow estimates of their redshifts, but it would have great difficulty distinguishing their differing histories. As shown in the figure, high sensitivity imaging between 5 and 8 micron with the MIRI will detect the older stellar population and thus unambiguously identify objects with a longer star forming history than just a few million years.
Due to different spectral shapes, MIRI will provide a unique diagnostic for identifying first light sources. See graph below.
