How the Webb and Gaia missions bring a new perspective on galaxy formation

Webb’s view of the early universe

Webb’s observations come with their own challenges, though. Determining the age of a galaxy is not a simple matter, even using a powerful tool, for reasons that have as much to do with definitions as with available data.

“How do you define the age of a galaxy? Is it the first time it formed or the most recent set of star formation? We’re talking about an ensemble of objects, all of which are going to have different ages,” said Micaela Bagley of the University of Texas at Austin, a researcher on Webb’s CEERS (Cosmic Evolution Early Release Science Survey) project. CEERS has identified some of the oldest galaxies ever seen, at redshifts of up to 12, which corresponds to as early as just 500 million years after the Big Bang.

These galaxies observed by Webb are too far away to resolve individual stars, so its instruments collect data on the galaxies as a whole. Using cameras like NIRCam, Webb observes many of these distant galaxies as a cluster of pixels, but using its spectroscopy instruments like NIRSpec, it can obtain more detailed data, like signatures of recent or older star formation.

The very distant galaxies observed so far, which are part of the early Universe, are typically showing very active, very recent star formation. Webb can’t track the age of individual stars or groups of stars, but it can look at a galaxy’s history of star formation as a whole—whether a galaxy has had multiple periods of star formation or whether its star formation spiked and then declined, for example.

With these early galaxies, “We’re looking at their populations of stars to try to figure out the history of their star formation,” Bagley explained. “Not necessarily how old it is but how it has gone about its life.”

Early evidence suggests that galaxies have typically experienced star formation in waves, peaking and troughing over time. But all that active star formation at such an early stage in the Universe poses some problems.

A brighter early universe

Early results from Webb have been exciting for many reasons, but astronomers are particularly intrigued to learn that the early Universe is not quite as we imagined it to be. From almost the first week that Webb began its science operations, it has been turning up brighter and more massive galaxies much earlier than anyone expected, and subsequent observations have backed these findings up.

When it comes to galaxies in the early Universe, “Across the board, everything is more,” Bagley said. Webb has found early galaxies that are brighter and more numerous than predicted, and these galaxies have developed in unexpected ways: “There are more black holes than we thought, and there are more dusty galaxies than we thought. These galaxies are able to form dust, which should take a long time, but they’re doing it earlier and more efficiently than we thought.”

The early Universe “is just a much busier place than we thought,” Bagley summed up.

As part of this busy development, galaxies have been found with structures similar to the Milky Way much earlier than had been expected as well. The very earliest galaxies observed by Webb, at around 12 or 13 billion years old, tend to be diffuse and chaotic, with not much structure. But structured galaxies with features like disks, spiral arms, or bars have been found as far back as around 11 billion years (z≈3), far earlier than predicted.

“We thought that it would take a very long time to form a galaxy that was gravitationally stable enough to have a disk and spiral arms like the Milky Way does. But we’re starting to see some of those pretty early in the Universe. Not as early as the first galaxies, but way earlier than we expected,” Bagley said.

Even galaxies from just a few hundred million years after the Big Bang (redshifts of 7 and above), which had previously been only observed by Hubble as blobs of light, seem to be being resolved by Webb as having some kind of structure.

“Galaxies seem to be forming very quickly, then reaching that equilibrium that allows them to rotate and flatten into a disk. And once you’re in that shape, you can start to have structures form, like spiral arms,” Bagley explained.