Webb Found 18 Galaxies Crammed Together 460 Million Years After the Big Bang. They Appear to Be Blowing the Earliest Bubble of Clear Space Ever Seen.

For its first few hundred million years, the universe was a fog. Hydrogen atoms that formed after the Big Bang absorbed ultraviolet light so efficiently that the cosmos was effectively opaque, until the first stars and galaxies switched on and began stripping electrons off those atoms, clearing pockets of transparent space that eventually merged into the see-through universe we live in. Astronomers call the process reionization, and where and how it began is one of the central open questions the James Webb Space Telescope was built to answer.

A team led by Zihao Wu of the Center for Astrophysics, Harvard & Smithsonian, now reports what may be the earliest such pocket ever identified. In a paper in The Astrophysical Journal drawing on the JWST Advanced Deep Extragalactic Survey, or JADES, the researchers describe a cluster of 18 galaxies at a redshift of about 10.5, which places them roughly 460 million years after the Big Bang, packed into a comoving group on the western side of the GOODS-South field. "We report a galaxy overdensity candidate at z ≈ 10.5," the authors write. The galaxy density there is four times the field average, and the group accounts for about a third of the comparably bright galaxies and nearly half the total star formation in that slice of sky at that epoch.

The galaxies are not unusually massive or unusually frantic. Their stellar masses and star formation rates run a little above the field average but stay within what models predict for that era. What sets them apart is company: more of them have close companions and internal substructure than galaxies elsewhere in the field, which the team reads as evidence that they are interacting with one another. And they appear to be doing something to the gas around them.

The clue is Lyman-alpha, the ultraviolet line hydrogen emits when an electron drops to its lowest energy level. Young stars pump it out in abundance, so it is a standard feature of star-forming galaxies. But Lyman-alpha photons scatter off neutral hydrogen with extreme efficiency, which is exactly why they are not supposed to be visible from deep inside the reionization era; the fog should swallow them. Detecting the line at redshift 10 is close to impossible unless the hydrogen along the line of sight has already been ionized.

Wu's team found Lyman-alpha transmission in the overdensity, and found that it varies across it in a telling way. "It is elevated near the center of the overdensity and decreases toward the outskirts," they write, a pattern "consistent with the formation of an ionizing bubble with radius ∼6 cMpc." In other words, the galaxies in the core appear to have cleared a cavity roughly six comoving megaparsecs in radius, about 20 million light-years, and light escapes most easily from the middle of it, where the bubble is deepest. "If confirmed, the spatial variation of Lyα transmission would mark the earliest ionized bubble produced by a galaxy overdensity known so far," the authors write. "It provides a rare laboratory to study the structure of ionizing bubbles."

The finding fits a picture in which reionization did not proceed evenly but was seeded by dense knots of galaxies that carved out bubbles first, with the rest of the universe clearing later as the bubbles grew and overlapped. The researchers are explicit that the conclusion is tentative, resting on photometric evidence rather than full spectra of every member. They plan to return to the field with JWST and the ALMA array in Chile to target hydrogen-alpha and doubly ionized oxygen lines, which will let them map the group in three dimensions and "provide a direct test of its role in cosmic reionization within the first 500 Myr of the universe." Webb was launched on Christmas Day 2021 after a quarter-century of delays and a near-cancellation by Congress in 2011. Findings like this one are the reason astronomers fought to keep it alive.