The Webb telescope reveals an unexpected abundance of bright galaxies in the early universe | Science

The James Webb Space Telescope has been observing the sky for just a few weeks and has already made a startling discovery: dozens, hundreds, perhaps even 1,000 times more bright galaxies in the early universe than astronomers expected.

“No one expected anything like this,” says Michael Boylan-Kolchin of the University of Texas at Austin. “Galaxies explode from the tree,” says Rachel Somerville of the Flatiron Institute.

Galaxy formation models may now need revision, as current ones argue that gas clouds must be much slower to coalesce into stars and galaxies than suggested by the galaxy-rich Webb images of the early universe, less than 500 million years after the big bang. “This is way outside the range of model predictions,” says Garth Illingworth of the University of California (UC), Santa Cruz.

Webb, a NASA-led orbiting observatory with contributions from the European and Canadian space agencies, began observing in late June from its vantage point 1.5 million kilometers from Earth. Much of its time so far has been devoted to projects designed to demonstrate its capabilities, such as the Cosmic Evolution Early Release Science (CEERS) survey. Webb was designed to delve deeper into space history than its predecessor, the Hubble Space Telescope. Its 6.5-meter mirror – with six times the area of ​​Hubble’s – can capture more light from distant sources and, unlike Hubble, operates at infrared wavelengths, making Webb more sensitive to those distant sources , whose light is stretched to longer, redder wavelengths by cosmic expansion.

Within days of Webb starting observations, he spotted a candidate galaxy that appeared to glow brightly when the universe was only 230 million years old, 1.7% of its current age, which would make it the most distant ever seen . Studies since then have shown the object to be just one of a staggering abundance of early galaxies, each small by today’s standards but brighter than astronomers had expected.

Some researchers warn that the abundance, based on images of a small part of the sky, may be an illusion. Boylan-Kolchin wonders if Webb just got “extra lucky” and happened upon a huge cluster of galaxies denser than the rest of the early universe. This question will be resolved when CEERS expands its scope later this year and results from other large-scale studies emerge.

It is also possible for astronomers to misidentify galaxies from a slightly more recent time as very early. Spectra are the gold standard for measuring the age of a galaxy because they allow the precise measurement of the reddening of its light. But collecting spectra from many galaxies takes time. Instead, Webb’s studies have so far estimated the age of galaxies by the color they appear in the images, a relatively crude method. Webb’s near-infrared camera filters their light into several broad-wavelength bins, giving astronomers a rough measurement of color; redder equals more distant. But dust around a galaxy can fool observers because it can absorb starlight and re-emit it at longer wavelengths, making the galaxy appear redder.

Early Webb science teams have already identified several such masked galaxies, as reported in several recent preprints. But if the abundance of early galaxies is real, astronomers may have to fundamentally rethink galaxy formation or the prevailing cosmology.

Looking at nearby galaxies, the researchers concluded that the heat in the gas clouds slows the rate at which gravity would otherwise condense matter into stars—making star formation about 100 times longer than if gravity were to rule alone. As the first stars in the protogalaxy begin to shine, they inject more heat into the gas, pumping the brakes on further star formation. And the first stars were short-lived giants; when they explode as supernovae, they heat the gas clouds even more or eject them completely from the forming galaxy.

Hubble studies show that the rate of star formation was relatively constant for about 600 million years after the big bang, says Charlotte Mason of the Niels Bohr Institute. But Webb’s results suggest that at earlier times its pace was much faster — so fast, Somerville suggests, as if the gas clouds were collapsing freely, without any stopping from heat or supernovae.

In fact, UCLA’s Tommaso Treu, who leads another Webb study called GLASS, says his team sees these early galaxies “forming stars like crazy.” They look, he adds, “like giant balls of star formation and nothing else.”

Theorists do not know whether the higher density of matter and higher temperatures of the early universe may have accelerated star formation. Another theory is that the first stars were able to form more quickly because they formed only from the primordial matter left over from the big bang — hydrogen and helium — without the heavier elements forged by later generations of stars.

Or something may be wrong with the current understanding of how the universe evolves. The prevailing theory in cosmology, known as lambda-CDM (referring to cold dark matter), describes how, soon after the big bang, the invisible dark matter that makes up most of the matter in the universe clumped together under its own gravity into “ halos.” These halos then attract normal matter and create the conditions for it to condense into galaxies. Lambda-CDM predicts the number and size of haloes that must have existed in the early universe, and hence the number of galaxies. “There’s not a lot of wiggle room,” says Boylan-Kolchin.

Somerville says it may be possible to tune the lambda-CDM to create something closer to what Webb sees. Or, she says, cosmologists may be forced to reevaluate the first moments of the big bang itself: the era of inflation, a period of rapid growth when quantum fluctuations grow into regions of higher or lower matter density—the seeds of more the late halos. “If inflation is wrong, it can be very fundamental,” she says. “But I wouldn’t bet it is.”

Having uncovered the early galaxy problem, Webb could provide the data needed to answer it. So far, Webb sees only young, hot, bright stars in the newly discovered early galaxies. Follow-up observations of these galaxies at longer wavelengths with the Webb Mid-Infrared Instrument or ground-based radio telescopes sensitive to submillimeter waves could reveal the gas clouds that are actively forming stars. These observations may help astronomers confirm that early galaxies were unusually prodigious star factories—and hold clues to how they did it.

“In six months, we’ll have a much better picture of all of this,” says Boylan-Kolchin. “It’s a very exciting time.”

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