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Starts With A Bang

JWST confirms: The tiniest galaxies made the cosmos visible

JWST has puzzled astronomers by revealing large, bright, massive early galaxies. But the littlest ones pack the greatest cosmic punch.
Fractal pattern with a stark contrast of vibrant orange and deep blue hues, designed to make the universe visible.
Initially, at left, the Universe is filled with neutral, light-blocking matter back before any stars have formed. When stars begin to form, however, they create ionizing ultraviolet photons, which lead to pockets that behave as though they're transparent to visible light, as shown in red. Over time, as we move to the right, more and more of the Universe becomes reionized, until reionization completes around 550 million years after the Big Bang.
Thesan Collaboration / Big Think
Key Takeaways
  • Shortly after the hot Big Bang, once neutral atoms formed, the Universe was opaque to starlight, as those neutral atoms present throughout space blocked and absorbed any emitted starlight.
  • It took hundreds of millions of years for enough stars to form so that a sufficient number of ultraviolet photons would eventually reionize the Universe, making it transparent to starlight.
  • Who was the culprit behind this reionization process, though? While many thought it would be the largest, brightest, most luminous galaxies, they can't explain it. These tiny dwarf galaxies, however, can.

One remarkable patch of sky just solved a major cosmic puzzle.

full frame mosaic no annotation scale bar JWST
This image shows the full imaging field of the JWST UNCOVER Treasury Survey, which takes up about 0.007 square degrees in the sky. In this tiny patch of space, some ~50,000 objects are revealed, with the majority of them not associated with the imaged cluster, Abell 2744, at all, but rather as background galaxies that are affected by the gravity of the cluster itself. No signs of matter-antimatter annihilation are seen here, indicating that all stars and galaxies shown are made of matter, not antimatter. However, many gravitationally lensed background galaxies are among the most distant ever discovered.
Credit: R. Bezanson et al., ApJ submitted, JWST UNCOVER Treasury Survey, 2023

Shortly after the Big Bang — before any stars arose — the first stable, neutral atoms formed.

photon bath neutral CMB atoms
At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level.
Credit: E. Siegel/Beyond the Galaxy

These atoms, however, absorb visible light, rendering space opaque.

primordial black holes
The overdense regions that the Universe was born with grow and grow over time, but are limited in their growth by the initial small magnitudes of the overdensities, the cosmic scale on which the overdensities are found (and the time it takes the gravitational force to traverse them), and also by the presence of radiation that’s still energetic, which prevents structure from growing any faster. It takes tens-to-hundreds of millions of years to form the first stars; small-scale clumps of matter exist long before that, however. Until stars form, the atoms in these clumps remain neutral, requiring ionizing, ultraviolet photons to render them transparent to visible light.
Credit: Aaron Smith/TACC/UT-Austin

Energetic, ultraviolet photons are required to reionize atoms.

jwst first stars
An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As they shine and merge, radiation will be emitted, both electromagnetic and gravitational. But the conversion of matter into energy does something else: it causes an increase in radiation pressure, which fights against gravitation. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.
Credit: Pablo Carlos Budassi/Wikimedia Commons

Determining how early stars and galaxies reionized the Universe is a cosmic challenge.

galaxy evolution JWST UNCOVER
One of the science goals of the JWST UNCOVER survey is to track galaxy evolution across cosmic time. Here, a selection of nine galaxies pulled from the survey itself are highlighted in context with the cosmic time from which their light was emitted. JWST is shedding a whole new light on the story of galaxy evolution within our Universe, and is also helping us determine which types of galaxies, and when, helped our Universe transition from an opaque state to one transparent to light.
Credit: R. Bezanson et al., ApJ submitted, JWST UNCOVER Treasury Survey, 2023

It’s a challenge perfectly suited for JWST, however, with help from Einstein.

Abell 2744 pandora's cluster
The galaxies that compose Pandora’s Cluster, Abell 2744, are present within the three separate cluster components easily visually identifiable, while the remaining background sources are scattered all throughout the Universe, including many from the first ~1 billion years of cosmic history. This field of view is now known to contain many of the earliest galaxies ever found, as well as the youngest proto-cluster of galaxies ever discovered to date: just 650 million years after the Big Bang.
Credit: R. Bezanson et al., ApJ submitted, JWST UNCOVER Treasury Survey, 2023

This image shows Pandora’s cluster, Abell 2744: a massive collection of galaxy clusters.

strong lens map JWST UNCOVER
This gravitational lensing map shows the reconstructed magnification contours from JWST data owing to the lensing profile of the three bright components of Abell 2744, Pandora’s Cluster. All galaxy clusters have their own unique lensing magnification properties, providing maximum enhancement along specific contours.
(Credit: L.J. Furtak et al., MNRAS Submitted/arXiv:2212.04381, 2022)

This huge mass collection bends and distorts the surrounding spacetime.

lens reconstruction Abell 2744 JWST
The full set of light (left panels) received by JWST are a composite of the background sources of light with the foreground cluster sources. Attempts to map out the foreground cluster and subtract them leaves only the background signals, which can be further corrected for gravitational lensing effects. Making a lensing map is of the utmost importance if we wish to understand how the lensed objects themselves actually are, intrinsically.
Credit: L.J. Furtak et al., MNRAS Submitted/arXiv:2212.04381, 2022

Background objects, through gravitational lensing, appear magnified and stretched.

This deep-field region of the GOODS-South field contains 18 galaxies forming stars so quickly that the number of stars inside will double in just 10 million years: just 0.1% the lifetime of the Universe. The deepest views of the Universe, as revealed by space telescopes, take us back into the early history of the Universe, where star formation was much greater, and to times where most of the Universe’s stars hadn’t even formed. Many of the most distant galaxies are found in close proximity to other foreground galaxies, whose mass distorts and magnifies the light from background objects.
Credit: NASA, ESA, A. van der Wel (Max Planck Institute for Astronomy), H. Ferguson and A. Koekemoer (Space Telescope Science Institute), and the CANDELS team

This reveals galaxies too small, faint, and distant to otherwise be glimpsed.

This image shows how much of the sky (x-axis) versus how deep, in terms of magnitude (y-axis) a variety of Hubble and JWST surveys have reached. The dotted lines include enhancements due to gravitational lensing. Note that the UNCOVER survey, with lensing enhancements included, reaches the most deeply of any (cycle 1) JWST program. The deeply imaged galaxy cluster Abell 2744, also known as Pandora’s cluster, is a key part of the UNCOVER survey area.
Credit: R. Bezanson et al., ApJ submitted, JWST UNCOVER Treasury Survey, 2023

Ultra-distant, massive galaxies create too little ultraviolet light to explain cosmic reionization.

JADES deep image
This image reveals about 20% of the full JADES survey area: around 25 square arc-minutes in the sky. It would take nearly 6 million boxes of this size to fill the entire sky. Already, a whopping 45,000 galaxies are known within this tiny region of space, showcasing just how vast and wondrous our Universe is. The most bright, distant, massive galaxies found in surveys such as these, however, cannot account for the majority of ultraviolet, ionizing photons needed to make the Universe transparent to light.
Credit: NASA, ESA, CSA, B. Robertson (UC Santa Cruz), B. Johnson (CfA), S. Tacchella (Cambridge), M. Rieke (University of Arizona), D. Eisenstein (CfA); Processing: A. Pagan (STScI)

They account for between 5% and 20% of the needed ionizing photons.

JWST reionization infographic
More than 13 billion years ago, during the Era of Reionization, the Universe was a very different place. The gas between galaxies was largely opaque to energetic light, making it difficult to observe young galaxies. The James Webb Space Telescope (JWST) is peering deep into space to gather more information about objects that existed during the Era of Reionization to help us understand this major transition in the history of the Universe.
Credit: NASA, ESA, J. Kang (STScI)

Despite the hopes of some, neither can quasar emissions.

quasar-galaxy hybrid
This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, showcases how bright and powerful quasars can be. However, despite their early presence and remarkable luminosities, they cannot account for all or even most of the reionizing photons needed to render the Universe transparent to optical light.
Credit: NASA, ESA, G. Illingworth (UCSC), P. Oesch (UCSC, Yale), R. Bouwens (LEI), I. Labbe (LEI), Cosmic Dawn Center/Niels Bohr Institute/University of Copenhagen, Denmark

However, gravitational lensing newly revealed eight tiny, faint, distant dwarf galaxies.

Nasa image makes universe visible, showing a cluster of galaxies.
These eight very faint, low-mass galaxies would be invisible to even JWST at these great distances under normal circumstances. Only from gravitational lensing’s severe brightness enhancement, an effect of Einstein’s general relativity, can these galaxies be revealed at all.
Credit: H. Atek et al., Nature, 2024

With only ~1 million stars apiece, they are ~100 times as common as their bigger, brighter counterparts.

A graph making the results of experiment ID 1655 visible.
Even at early times and great distances, the properties of the faintest, lowest-mass galaxies identified appear very similar to the same mass galaxies (with ~10^6 solar masses worth of stars inside) seen today.
Credit: H. Atek et al., Nature, 2024

They were caught producing four times as many ionizing photons as previously assumed.

A graph displaying the impact of chemotherapeutic drugs on a study.
Whereas young stars produce ionizing photons that freely travel throughout the Universe, early galaxies, back when the intergalactic medium was filled with neutral atoms, have their light absorbed, and only a tiny fraction escapes. However, with such great numbers of dwarf galaxies, even a small fraction of ionizing photons can be primarily responsible for reionizing the Universe.
Credit: H. Atek et al., Nature, 2024

It’s these cosmic dwarfs — not quasars or bright galaxies — that primarily reionize the Universe.

the last of the universe's neutral atoms becoming reionized during the end of the cosmic dark ages
For the first 550 million years of the Universe, neutral, light-blocking atoms persist in the space between galaxies, continuing what’s known as the cosmic dark ages. Once the last of that neutral matter becomes reionized, starlight can propagate freely through the Universe, marking the end of the reionization epoch.
Credit: M. Alvarez, R. Kaehler, and T. Abel

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.


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