The earliest galaxies — and how the gravity of foreground clusters magnifies these faint, far-away sources into view.
01 — Cosmic Dawn / high redshift
As the universe expands, a galaxy's light is stretched to longer wavelengths. Neutral hydrogen in the early universe absorbs everything blueward of Lyman-α, so a distant galaxy simply drops out of the bluer filters — the higher the redshift, the redder the band where it vanishes. This Lyman-break technique is how galaxies from the first few hundred million years of our universe are selected with JWST.
Flux blueward of (1+z)·1216 Å is absorbed by the intergalactic medium. Bands: JWST NIRCam wide filters. Ages from a flat ΛCDM cosmology.
02 — Lensed Candidates / the explorer
This is how Population III candidates are hunted at z = 5.6–6.6 behind lensing clusters: from a faint dropout, to fitting its z = 6 spectral fingerprint (Pop III vs. galaxy templates), to undoing the cluster’s lensing to recover the true source, to its place in the source-plane density field. The candidate shown is fabricated for illustration.
—
Population III galaxies are expected to form from nearly pristine gas. The open question is whether unusually metal-poor, high-redshift candidates in JWST lensing fields show the photometric and environmental signatures expected from the first generations of stars.