JWST Reveals Distant “Galaxy” Is Six-way Galactic Crash

Peering deep into the past, the James Webb Space Telescope’s keen detectors are revealing unprecedented details of some of the oldest structures in the universe. Formerly fuzzy images become sharp at the telescope's highest resolution. One such re-examination of an energetic primordial galaxy has left many astronomers starstruck: Webb reveals the single galactic unit is actually six galaxies, crashing together to create a deluge of fresh stars.

In 2013 astronomers using data from the Herschel Space Observatory discovered the earliest star-forming galaxy yet seen. Bursting at the seams with new stars when the universe was only about 850 million years old, the existence of the object, christened HFLS3, defied accepted scenarios for how quickly galaxies could grow. The colossal stellar factory ignited new stars at a rate about 2,000 times greater than our Milky Way, despite having roughly the same mass.

In the universe’s infancy, astronomers had believed, galaxies should not nearly be this big with such a high birth rate. Several teams attempted to image the galaxy again using the Hubble Space Telescope and several ground-based telescopes. Yet the photos only hinted at the rough signatures of other nearby sources—with the potential influence of gravitational lensing, in which massive objects closer to us warp and magnify the light rays from distant objects behind them.

Now with new data, scientists have proposed that HFLS3 is not a single giant starburst galaxy after all. “This galaxy was actually an interacting system of galaxies in the early universe,” says team lead Gareth Jones (University of Oxford), “which are still very bright and starbursting, but as a system rather than a single source.”

The new observation was part of a JWST program called Galaxy Assembly with NIRSpec Integral Field Spectroscopy. This collaboration is targeting 40 of the most distant and massive galaxies to resolve even the smallest pockets of space around them in great detail. The spectrograph’s Integral Field Unit cast its eye on HFLS3’s radiant neighborhood in September 2022.

The team led by Jones produced a set of images that displayed brightness across different wavelengths and fields of view. They then analyzed the motions and heating of the gases in the galaxy, and modeled how much gravitational lensing could have affected the perceived light. In doing so, they reconstructed a more accurate image of HFLS3.

Jones had expected to see the typical rotational movement of gas seen in similar early galaxies. “But instead of a single rotating [galaxy], we just had lots of little galaxies,” he says.

Based on the analysis, the bulk of HFLS3 is composed of three pairs of small, closely interacting galaxies. Just one of those pairs is magnified by two separate foreground galaxies. Given the incredibly dense field, scientists suggest that the sextuplet system is colliding, triggering a surge of new stars.

Follow-up observations nearly a decade ago had shown prominently bright emissions around HFLS3. This made Dominik Riechers (University of Cologne, Germany), who led the 2013 discovery at a lower resolution with Herschel, consider the possibility of neighboring mergers. However, he recognized that the telescopes available at the time did not yet have the capability to expose the true nature of those interactions.

“We speculated that these could be galaxies associated with starburst, but JWST shows this beyond any doubt by measuring precise distances and sizes of these ‘companion’ systems,” says Riechers, who was not involved in the new study.

Riechers also commends Jones and his team for verifying that we see only a weak gravitational lensing of the cluster. This proves HFLS3 is indeed massive and chock full of bright new stars, not just a smaller magnified system, he explains.

Scientists can now view HFLS3 as a chaotic jumble of galaxies entangled in webs of star formation. This reassures Kenneth Duncan (University of Edinburgh, UK), who wasn’t involved in the study, that the results actually align with current galaxy formation theories. Had HFLS3 been one gargantuan galaxy, Duncan notes, astronomers would have had to modify that framework. “With these observations confirming that it is in fact a group of galaxies undergoing a series of mergers, it supports our picture of mergers playing a critical role in the build-up of galaxies,” writes Duncan. But still, the images are striking, he remarks: “What is surprising is really just how messy and complex the system turns out to be.”

Chiara Villanueva is a graduate student in the Science Communication M.S. Program at the University of California, Santa Cruz. They have a background in astrophysics and are interested in covering stories about the universe.