Early Galaxies Were Messy, New Study Finds

As astronomers gaze farther into the universe, they also peer back in time. That lets them see how some of the earliest galaxies formed. While most star-forming galaxies today have settled into orderly disks of gas and dust, a new study has found that galaxies in the early universe were far more turbulent.

A team led by Angelica Lola Danhaive (University of Cambridge, UK) used the James Webb Space Telescope to examine hot gas in 213 star-forming galaxies in a universe only 850 million to 1.6 billion years old. They found that most of these galaxies had messy, puffed-up disks, unlike the thin, orderly disks of today. What’s more, the higher the starbirth rate, the more turbulent the galaxy.

“Determining when the very first disks formed in the early universe is a key question,” says Andrea Ferrara (Scuola Normale Superiore, Italy), who was not involved in the paper. Previous studies have largely concentrated on galaxy evolution during “cosmic noon,” a period between 10 and 11 billion years ago when the most stars were forming in the universe. Astronomers never had the chance to look at such a large population of low-mass galaxies before that stellar baby boom.

The Webb observations also illuminate the specific formation processes causing these turbulent effects during this early period in the universe. The results, published in the Monthly Notices of the Royal Astronomical Society, trace galaxy formation from its chaotic beginnings in the early universe to today.

“The most exciting outcome, I think, is that we are now seeing the transition phase — the dawn of ordered disks emerging from turbulent, clumpy systems,” says Sandro Tacchella (University of Cambridge, UK), a coauthor on the paper. “It’s a key missing link between the earliest galaxies [...] and the more settled, rotation-supported disks we observe a few billion years later at cosmic noon.”

The Dawn of Disks

The team used JWST’s Near-Infrared Camera (NIRCam) to measure warm ionized gas via hydrogen alpha emission — a specific wavelength of light that hydrogen gas emits when it’s been heated, such as by newborn stars. By mapping this spectral emission line across each galaxy, the team could observe the structure and motion of its ionized gas.

The team determined each galaxy’s rotational support — how well the galaxy can sustain a disk — and the velocity dispersion, which quantifies turbulent motions in the gas that puff up the galaxy.

The team found that turbulence dominates over rotation in more than 50% of galaxies; only a small number of early star-forming galaxies have the rotational support to form disks. The results suggest that most galaxies only settle into neatly rotating disks during and after cosmic noon. In other words, the Webb observations are probing the very beginning of galactic disk formation in the early universe.

Even galaxies of the same mass or level of star formation could have different amounts of turbulence, hinting at complex processes. “This suggests that the path toward ordered disk formation was neither uniform nor monotonic,” Tachella says. “Some galaxies were already beginning to settle, while others remained violently unstable.”

“Our results were partly anticipated but still striking,” he adds. Some previous studies had identified single, massive galaxies in this early epoch with stable, rotating disks, challenging galaxy formation scenarios that had predicted turbulent, unsettled systems in the early universe.

“With our study statistically characterizing more ‘typical’ lower-mass galaxies,” Danhaive says, “we found that actually most galaxies are in a turbulent state of assembly, which is more in line with what we expected.”

Mahsa Kohandel (Scuola Normale Superiore, Italy), who was not involved in the study, agrees that the work aligns with predictions. “What impressed me most was that the authors could recover such kinematic information from [the near-infrared spectra], a technically demanding task given the complexity of [the instrument],” she adds.

However, Kohandel cautions that measuring only warm, ionized gas can make galaxies appear more chaotic than they actually are. Systematic effects can become more pronounced at greater distances, underscoring the importance of observing multiple sources of emission over different wavelengths.

From Chaotic Systems to Graceful Spirals

The team’s findings shed light on early galaxy evolution. Since galaxies forming more stars tend to be more disorderly, it’s possible the effects of star formation — such as stellar winds, supernovae, and the pressure of the newborn stars’ intense light — could be heating and redistributing gas within the galaxy and disrupting the formation of a stable disk.

Messy periods of instability might disrupt a galaxy, but it could also signal processes that support its growth. Turbulence might come from galaxy mergers or large inflows of gas, which ultimately push matter inward and trigger star formation.

To understand those effects, the team plans to combine the Webb observations with those from the Atacama Large Millimeter/Submillimeter Array (ALMA), which measures longer-wavelength light coming from cold gas and dust, to study the assembly of the first galaxies.

“JWST has opened a completely new window for this type of study,” Tacchella says, “and future programs combining data from the Webb Near-Infrared Camera, Webb Near-Infrared Spectrograph, and the Atacama Large Millimter/submillimeter Array will allow us to follow how these chaotic systems gradually matured into the graceful spiral galaxies we see today.”