Back in the Universe’s early days, starlight couldn’t travel very far at all.
After the Big Bang, the Universe formed neutral atoms, creating a problem.
Although they self-gravitate, forming stars and galaxies, atoms also exist between these luminous entities.
Most emitted starlight is energetic ultraviolet light: easily absorbed by these neutral atoms.
Only enough ultraviolet photons, cumulatively, can fully reionize these intergalactic atoms.
Until they do, the Universe lives in “the dark ages,” where emitted starlight gets absorbed before it’s observable.
Only the brightest galaxies, along the most serendipitously reionized line-of-sights, have previously been seen.
This includes the current cosmic record-holder: GN-z11.
But the brightest early galaxies, alone, can’t account for all of the photons that we need.
There must be additional early galaxies, yet unseen, contributing to the reionization process.
ALMA, the Atacama Large Millimetre/submillimetre Array, can detect longer-wavelength photons beyond Hubble’s limits.
Combining ALMA with infrared Spitzer data has revealed the first normal, pre-reionization galaxies.
Known as REBELS-29-2 and REBELS-12-2, they’re the first “less extreme” galaxies found before reionization completes.
Altogether, these previously unseen galaxies should contribute 10-25% of the needed early starlight.
James Webb’s novel capabilities, at last, will abundantly reveal and characterize these earliest galaxies.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.