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Illustration showing the reionization of the Universe.

An illustration of the reionization of the Universe, which transitioned from a cauldron (red, right) of subatomic particles to a sea of neutral hydrogen gas dotted with early stars (middle) to its current transparent state (left).

Credit: Mark Garlick/SPL

Astronomers have used the

James Webb Space Telescope
(JWST) to show that faint miniature galaxies cleared the early Universe of its

obfuscating fog of atomic hydrogen
— allowing starlight to shine through the cosmos for the first time.

The research, published today in



, provides evidence that

dwarf galaxies
roughly 100 times smaller than the

Milky Way
triggered the process known as reionization, which changed the course of cosmic history. “The Universe became transparent,” says Hakim Atek, an astrophysicist at the Paris Institute of Astrophysics and lead author of the study. “It’s because of reionization that we are able to see distant galaxies.”

Emerging from a cosmic dark age

For around 380,000 years after the Big Bang, the Universe was a hot, dense furnace of subatomic particles. As the cosmos cooled, the free electrons and protons combined to form a gas of neutral hydrogen atoms.

What followed was a dreary period of darkness. This lasted until the gas collapsed in places to fuse and form the first stars, which produced ultraviolet (UV) light. However, the remaining gas permeating the Universe either absorbed or scattered this light. As a result, the Universe resembled a foggy forest speckled with dim, flickering fireflies, and light sources were visible only for short distances.

To render space transparent, something needed to bombard this gas with powerful ‘ionizing’ radiation, which could transform the neutral hydrogen atoms into charged particles, or ions, of hydrogen. The three candidates were energetic light jets called quasars, which are powered by supermassive black holes; massive galaxies roughly the same size as the Milky Way; and, finally, the minnows — dwarf galaxies.

Massive galaxies would have absorbed much of their own UV light, says Claudia Scarlata, an astrophysicist at the University of Minnesota in Minneapolis. And there might have been too few quasars to orchestrate the whole process. Dwarf galaxies, however, were small enough to allow easy escape of the UV light that they generated.

Observations of

younger dwarf galaxies,
closer to Earth, suggest that they can emit ionizing radiation. All the same, “there’s nothing like actually having the data from the early galaxies to confirm that”, says James Rhoads, an astrophysicist at NASA Goddard Space Flight Center in Greenbelt, Maryland. But dwarf galaxies from the epoch of reionization are too tiny and too dim to detect — even for the JWST.

Tale of two telescopes

To overcome this, the authors took advantage of a ‘natural telescope’: a cluster of galaxies located about 1.2 million parsecs from Earth. This cluster is so enormous that it warps light passing through it, thereby magnifying any light source located behind the lens, as observed from Earth.

The authors harnessed this lens to observe eight dwarf galaxies from the era of reionization, when the Universe was less than one billion years old. The galaxies are the faintest objects ever observed from that time.

Using data gathered by the JWST, the astronomers analysed the wavelengths of UV light from these galaxies. This allowed the team to estimate that even these faint, small galaxies could have expunged hydrogen gas around them easily. The researchers also estimate that dwarf galaxies were abundant enough up to one billion years after the Big Bang to have ionized the entire Universe, even if 5% of their ionizing radiation escaped into intergalactic space.

Small galaxies were the first to form in the Universe, which “probably makes it easier to start the [reionization] process early” in the history of the cosmos, Rhoads says. As each galaxy emitted radiation, it effectively blew a bubble of transparency that expanded into neutral gas. Eventually, all the bubbles from all the galaxies overlapped to complete the transformation.

Dwarf galaxies would have blown bubbles smaller than those produced by quasars and massive galaxies, and such small bubbles might have ensured that reionization proceeded homogeneously across the Universe. This, in turn, had implications for the architecture of the present-day Universe, Atek says.

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