Finding the first episode of star formation - the 'first light' in the Universe - is often described as the defining observation for the JWST. Initial surveys will be made by the near infrared imager, NIRCam, to find candidates through the very strong absorption by hydrogen gas, galaxies called 'Lyman dropouts' because they become undetectably black at wavelengths shorter than the Lyman limit (the wavelength needed to strip electrons in the lowest orbits from hydrogen atoms). (emergence of galaxies, simulation from Cosmology on a Computer, University of Illinois)

The role of the MIRI will be to demonstrate that these objects are really undergoing their first episode of star formation. A galaxy that had previously formed stars and was undergoing a second such event would look very similar to a true 'first light' object to NIRCam. The older stars would be glaringly obvious to the MIRI.

The importance of the MIRI observations is illustrated above. The figure shows the stellar continuum spectra of a galaxy truly undergoing its first episode of star formation, with no stars older than 5 million years, and of a galaxy that is undergoing a second 5-million-year-old episode, following one 80 million years previously, with the two of equal mass. The spectra have been normalized to the same flux in the NIRCam bands. The galaxies are assumed to be at a redshift of z = 15. We assume at these large redshifts that there is a high degree of absorption starting just to the blue of Lyman alpha. The differences between the spectra are subtle in the NIRCam bands, but the older stars produce about three times greater relative signal in the MIRI bands for the older galaxy. The figure also shows a composite quasar spectrum. Although its continuum slope is different from that for the galaxies, with the large emission line equivalent widths, its photometric spectral energy distribution will be similar to those of the star forming galaxies in the NIRCam bands. However, the quasar is very easily identified by MIRI, due to the rapid rise of its continuum toward longer wavelengths.

The image below is another version: it represents real data on a galaxy at a redshift of z = 6.6 - 6.8. The short wavelength measurements, obtained with HST and from the ground, are analogous to the NIRCam role above, while the long wavelength ones obtained with Spitzer are analogous to the MIRI points. The Spitzer data have been used by Eiichi Egami to show that the galaxy is past the peak of its star formation episode, with an age of 50 million years or so.