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The first galaxies (seen by JWST)
2022· Discovery / experiment
ProvenTested many times; part of working physics.
Galaxies already shining a few hundred million years after the start, now seen directly by the James Webb Space Telescope.
See it on the Science map →How it works
Expansion stretched the earliest galaxies' starlight into infrared, which the James Webb Space Telescope, launched December 25, 2021, was built to catch. It has confirmed galaxies at about 290 million years, such as JADES-GS-z14-0 (2024), beating Hubble's record of about 400 million (GN-z11, 2016). Many are brighter, more compact and more chemically developed than expected, and some host surprisingly large black holes. Compact red objects nicknamed 'little red dots' remain a puzzle.
Why it matters
They are where first stars, black holes and chemistry came together, and they test how quickly standard cosmology lets structure grow.
How we know
Distances are confirmed by spectra that measure how far known light lines are stretched: by about 15 times for JADES-GS-z14-0. In 2025 the ALMA radio array found oxygen in it, so earlier stars had already lived and died.
Still a mystery
Why so many bright galaxies so early? Stars may have formed more efficiently, or black holes added light; most astronomers think standard cosmology survives. What the 'little red dots' are is debated.
Connections (4)
▸The first stars gather into The first galaxies (seen by JWST)Well supported
The relationship
First stars formed alone or in small groups inside minor dark matter clumps. Their explosions blew gas out and enriched it. Over tens of millions of years those clumps merged into bigger ones, of tens to hundreds of millions of Suns, whose gravity could hold hot gas and keep forming stars continuously. Those merged systems, with mixed generations of stars, are the first galaxies.
How the link was found
The 'small things merge into big things' picture comes from Simon White and Martin Rees in 1978. Volker Bromm and Naoki Yoshida reviewed how the first galaxies assembled in 2011.
Is it proven?
JWST galaxies at about 300 million years already contain oxygen, which requires earlier stars. Simulations and JWST's images of clumpy early galaxies show merging.
Open questions and mysteries
Did pure first-generation stars survive inside galaxies bright enough for JWST to see? How did the earliest galaxies grow so bright so fast?
▸The first galaxies (seen by JWST) finish Reionization: starlight clears the fogWell supported
The relationship
Galaxies, packed with hot young stars, produced most of the ultraviolet light that ionized gas between them. Their ionized bubbles grew and merged, like holes in Swiss cheese until little cheese is left. By about a billion years, almost all intergalactic hydrogen was ionized. Faint small galaxies, being so numerous, probably supplied much of the light; growing black holes may have added some.
How the link was found
The galaxy-driven picture was developed in the 1990s and 2000s by Piero Madau, Abraham Loeb, Rennan Barkana and others. Since 2022 JWST has measured how much ionizing light early galaxies produce.
Is it proven?
Quasar spectra show the last neutral patches vanishing at about 1.1 billion years, later than once thought (2022 analyses). JWST counts of early galaxies give enough ultraviolet to do the job.
Open questions and mysteries
What share of ionizing light escapes its home galaxy? The galaxy's own gas absorbs much of it, and at these distances the escaping share cannot be measured directly, because intergalactic hydrogen absorbs it too. JWST's light-budget puzzle hinges on it.
▸The first galaxies (seen by JWST) grow into Galaxies and the cosmic webProven
The relationship
Early galaxies sat at the densest knots of the dark-matter web. Over billions of years they kept pulling in gas along filaments and merging with neighbors, growing into today's large galaxies and clusters. The Milky Way assembled this way: it has absorbed many smaller galaxies, including a sizable one roughly 10 billion years ago.
How the link was found
Growth by mergers comes from White and Rees (1978); feeding by cold gas streams along filaments from Avishai Dekel and colleagues in the 2000s. The Milky Way's big merger was found in 2018 Gaia data by Amina Helmi's and Vasily Belokurov's teams.
Why it's proven
Deep images show galaxies growing in size and mass over cosmic time. Gaia's map of star motions reveals the debris of past mergers in our own galaxy.
Open questions and mysteries
JWST finds galaxies that had already stopped forming stars within the first billion years or so. How they shut down so early is debated.
▸The first galaxies (seen by JWST) came earlier and brighter than forecast, testing ΛCDM: the standard model of cosmologyOpen question
The relationship
ΛCDM sets how many dark-matter clumps exist at each era; galaxy-formation models then estimate how much starlight each clump makes. JWST found more bright galaxies within 300–500 million years than those models expected. The clumps themselves seem numerous enough, so most researchers think the surprise lies in star formation, which may have been faster, burstier or less dusty, rather than in ΛCDM itself.
How the link was found
JWST teams (CEERS, JADES, GLASS and others) from mid-2022. In 2023 Michael Boylan-Kolchin showed the most extreme early claims would need nearly all the ordinary matter in their dark-matter clumps turned into stars, straining ΛCDM.
Is it proven?
Early claims of 'universe-breaking' massive galaxies (2023) shrank once spectra and light from growing black holes were accounted for. Spectra confirm the bright early galaxies; how hard they strain ΛCDM is still being assessed.
Open questions and mysteries
If star formation cannot explain it, cosmology might: early dark energy or stronger small-scale ripples would grow clumps sooner. And are the 'little red dots', compact red sources 0.5–1.5 billion years in, overgrown black holes?