JWST discovers how we’re able to see the Universe at all

The Dawn of Transparency: Unveiling the Universe’s First Light

The James Webb Space Telescope (JWST) isn’t just about pretty pictures. Its core mission centers on answering profound scientific questions. One of the most significant is understanding how the early Universe transitioned from a fog of neutral atoms to the transparent cosmos we observe today.

The Reionization Era: A Cosmic Renovation

Before the first stars and galaxies, the Universe was opaque. Neutral hydrogen atoms absorbed starlight, preventing it from traveling freely. This “reionization” process, driven by the energy from the first stars, transformed the cosmos. But, where did this energy come from?

Tiny Galaxies: The Universe’s Early Architects

Recent studies, like the one led by Isak Wold of the UNCOVER collaboration, have revealed a surprising answer: small, relatively common galaxies are the primary culprits. These galaxies, though faint, were incredibly numerous and energetic, emitting enough ultraviolet light to ionize the neutral hydrogen.

This low-resolution image shows the full field of the COSMOS-Web survey conducted with JWST. Spanning 0.54 square degrees in the sky, or nearly three full Moons’ worth of area, this represents the largest, deepest wide-field view of the Universe ever acquired. Space, as we can clearly see here, is full of luminous sources of light in practically all directions.

Understanding the Early Universe: The Role of Neutral Atoms

Initially, the Universe was filled with neutral atoms, absorbing starlight. The transition to a transparent Universe required a significant amount of high-energy photons, primarily ultraviolet (UV) light. But, where did this UV radiation originate?

• Stars don’t form everywhere but in regions where enough matter accumulates.
• JWST can see further than Hubble into early galaxies.
• Early stars form from neutral atoms.

This animation shows the Bok globule Barnard 68 in a variety of visible and infrared wavelengths. As the longer wavelengths reveal, this is not a hole in the Universe but simply a dusty cloud of gas, where the longer (redder) wavelengths of light penetrate and pass through the dust. As dust clouds form and dissipate, the dust density can be revealed by examining the light blocked and transmitted by fixed, background objects.

JWST’s Advanced Capabilities: A New Era of Discovery

JWST is a significant leap over Hubble. Its larger size, superior light-gathering power, and advanced instrumentation enable it to observe more distant, fainter objects. This includes the ability to detect specific emission lines, such as those from doubly ionized oxygen, critical for identifying star-forming galaxies in the early Universe.

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.

Finding the Sources: The Search for UV Photons

The key to understanding reionization is identifying the sources of the UV photons. Starburst galaxies, with their rapid star formation, are prime candidates. JWST, with its ability to detect the signature of doubly ionized oxygen, allows us to pinpoint these galaxies in the early Universe.

Around a variety of stellar corpses and dying stars, doubly-ionized oxygen atoms produce a characteristic green glow, as electrons cascade down the various energy levels when heated to extreme temperatures often exceeding ~50,000 K. Here, the planetary nebula IC 1295 shines brilliantly. These conditions are present in intense star-forming regions (including in the early Universe) and around stellar corpses, where the green phenomenon also helps color the so-called “green pea” galaxies, as well as Earth’s aurorae.

Gravitational Lensing: A Cosmic Magnifying Glass

To observe these faint, distant galaxies, researchers use a technique called gravitational lensing. Massive galaxy clusters, like Abell 2744, bend and magnify the light from objects behind them. This “cosmic magnifying glass” allows JWST to probe deeper into the early Universe.

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.

Groundbreaking Results: Reionization’s Culprits Identified

By studying the field behind Abell 2744, the UNCOVER collaboration found a surprising number of these galaxies, finding a remarkable 83 galaxies dating back to the first 550 million years of cosmic history. Further spectroscopic analysis of the galaxies confirmed the existence of doubly ionized oxygen, high escape fractions, and determined that these galaxies emit enough ultraviolet light to account for the reionization of the Universe.

In this field of view, which includes massive galaxy cluster Abell 2744, a total of 83 young galaxies were found from within the first ~800 million years of cosmic history. By combining the gravitational lensing enhancements of the foreground clusters with deep JWST imagery, scientists identified 83 candidate young galaxies that were doubly ionized oxygen emitters. The 20 located inside the white diamonds were chosen for follow-up with NIRSpec, with all 20 being confirmed as young, star-forming galaxies.

These low-mass, star-forming galaxies appear to provide the majority of the ultraviolet light needed to reionize the Universe.

Although galaxies like 41028, shown here, are very hard to image directly due to their intrinsic faintness and great distances, gravitational lensing can enhance the brightness of galaxies like this to make them visible to JWST’s eyes. With an estimated stellar mass of just ~2 million Suns, comparable to large star-forming regions (like 30 Doradus) found in the Local Group, these low-mass but quite numerous galaxies, starbursting early on in cosmic history, provide the majority of ultraviolet light needed to reionize the Universe.

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