Newly revealed ancient galaxies generate excitement

When the first data from the James Webb Space Telescope (JWST) were released to the astrophysics community last summer, galaxy experts were amazed. Analyses of several deep-field images revealed ancient galaxies, dating back to less than a half-billion years after the Big Bang, that defied expectations. It had been thought that galaxies of that era would be few, small, disheveled, and colliding clumsily. Yet even narrow expanses of the ancient sky scanned by the JWST contain numerous galaxies, many of them brighter, larger, and more organized than previous observations had suggested they would be.

“The newly discovered galaxies are certainly challenging our expectations for what we thought early galaxies would look like,” says Christina Williams, an astronomer at NOIRLab and a member of the JWST‘s NIRCam instrument team. “Early galaxies might be growing faster than we previously thought they would.”

The initial findings do not break the standard model of cosmology. But if they are confirmed after more data and scrutiny, researchers may need to revise models of star formation, galaxy evolution, and other astrophysical mechanisms.

Here we detail three features of the newly discovered galaxies that contradict pre-JWST expectations: their ubiquity, their heftiness, and their mature structure.

1. A galactic bounty

After conducting more than three decades of observations with the Hubble Space Telescope, astrophysicists thought they had a good idea of some basic trends in the galactic populations at high redshifts. For example, previous measurements had suggested that the number of luminous galaxies would decline steadily as one looked farther back in the universe’s history. Then came the JWST data.

In one preliminary study , Steven Finkelstein and colleagues used IR and optical redshift estimates to identify 26 galaxies dating back to the first 250 million–600 million years after the Big Bang in a patch of sky smaller than a full moon. That pushes the theorized limit of how many galaxies should exist in such a small area, says Finkelstein, an astronomer at the University of Texas at Austin. A follow-up study , currently under peer review, confirmed the ages of seven of the galaxies by parsing the redshifts of individual lines in the galaxies’ spectra.

The bounty of early galaxies may be due to an unexpectedly high amount of baryonic matter available to help form stars, or it could be explained by the nuances of star formation. “When you form stars out of a cloud of gas, if the cloud is big enough you’ll form stars at all masses,” Finkelstein says. If researchers are underestimating the frequency at which the most massive stars emerge from such gas-rich clouds, he adds, then “it would make all galaxies brighter, so easier to see.” Another possibility is that the ancient galaxies have less dust than thought, which would make them more easily visible.

2. Bulking up rapidly

The ancient galaxies exposed by the JWST are also surprisingly massive. Incorporating observations by Hubble and other telescopes, theorists had established upper limits for the masses of galaxies of a given age. Yet in a study accepted for publication in Nature Astronomy, Mike Boylan-Kolchin of the University of Texas at Austin found two galaxies identified in the JWST data that exceed those limits, barring the unrealistic scenario that the galaxies managed to convert an unprecedentedly high percentage of their total baryonic matter into stars.

Theorists have some levers that they can pull to account for the hefty galaxies. One possibility (though disfavored in another preliminary study ) is that the star formation rate in the early universe was faster than expected.

If such explanations do not resolve the discrepancy, a more drastic hypothesis is that there was an unexpectedly high amount of dark matter in the early universe. According to the established cosmological model, ΛCDM, dark matter came together under the force of gravity to form seeds, or halos, which attracted sufficient densities of hydrogen gas to trigger star formation. Boylan-Kolchin explains that a change in the amount of dark matter could both alter the speed of halo assembly and cap the amount of drawn-in hydrogen. A larger halo would mean more material available to generate stars. However, “Within the standard cosmological model there is not much wiggle room,” he says. “You can’t really change the amount of dark matter, and therefore there is no way to get more atomic matter to build such massive galaxies.” If the galaxies he investigated are confirmed and more are found, he adds, researchers may need to start testing theories that allow a larger amount of dark matter.

3. Disks galore

Decades of Hubble observations had revealed another clear trend: The farther researchers looked into the past, the more irregularly shaped galaxies they saw, as opposed to the iconic disk-shaped ones. The observed irregular shapes were consistent with the merging of smaller galaxies, which is the preferred method of large-structure formation under the cold dark matter cosmological model (the CDM in ΛCDM). Cold, or low-speed, dark matter would favor the formation of small, amorphous cosmic structures that would need time to dynamically evolve into the mature galaxies we see today.

Yet when researchers used the JWST‘s superior vision to study 280 galaxies that existed between 5 billion and 600 million years after the Big Bang, they saw up to 10 times as many disk galaxies as expected. A follow-up study analyzing more than 4000 galaxies , currently under review, reports a similarly high fraction of disks.

Although the results are surprising, Leonardo Ferreira, a research fellow at the University of Victoria and first author of the two studies, says that they “only update our view of how early galaxy formation and evolution occur,” without contradicting ΛCDM. The morphologies of those distant galaxies are consistent so far with the results of recently run cosmological simulations, he adds.

Next steps

After the excitement following the fire-hose gush of JWST data released last summer, astronomers are preparing to wait a while for the next round of observations. Whereas the first data release was public and saw researchers vying for new discoveries, upcoming observations are allotted to specific teams.

For now, foremost in researchers’ minds is resisting the temptation to jump to unsubstantiated conclusions. The JWST era has only just begun. The greater accuracy of spectroscopic dating compared with the optical estimation of redshift may require adjustments to previous data. Longer-duration observations and wider fields will yield more precise measurements. “Some of our assumptions that help us interpret the early galaxies’ brightness could be inaccurate,” says NOIRLab’s Williams. Nonetheless, it’s exciting even if some of the properties derived from the initial observations are wrong, Boylan-Kolchin says, “because it’s teaching us something about the physics that is going on.”