Some of the earliest massive galaxies in the universe may be hiding enormous numbers of small, faint stars, suggesting they contain much more mass than astronomers previously believed.
Using the James Webb Space Telescope (JWST), an international team led by researchers at Leiden University studied nine massive galaxies from the early universe.
They discovered that these galaxies contain a surprisingly large proportion of small stars that are difficult to see because they are overwhelmed by much brighter stars.
The findings, published in Nature Astronomy, could change scientists’ understanding of how quickly galaxies grew after the Big Bang.
Astronomers have already received several surprises from JWST. The powerful space telescope has revealed large, mature-looking galaxies that existed when the universe was still relatively young. Scientists have struggled to explain how galaxies could have grown so large so quickly.
The new research suggests the mystery may be even greater because some of these galaxies could actually be far more massive than earlier estimates indicated.
The researchers studied galaxies that had already stopped producing significant numbers of new stars billions of years ago. They combined extremely detailed observations from JWST with earlier observations from the Very Large Telescope in Chile.
To understand what kinds of stars were inside the galaxies, the scientists separated their light into different wavelengths. This produces a spectrum that contains subtle clues about the stars producing the light.
The difficulty is that bright, massive stars dominate what astronomers can see, while much smaller stars are extremely faint.
Lead author Chloe Cheng compares the situation to looking at a city from far away. Tall skyscrapers immediately attract attention, while thousands of smaller houses between them may be difficult to see.
The same thing can happen with galaxies. A relatively small number of bright stars can hide a much larger population of faint stars. Individually, these small stars contribute little light, but together they can contain an enormous amount of mass.
Astronomers have traditionally estimated this hidden population by assuming that stars form in roughly similar proportions throughout the universe.
The new findings challenge that assumption.
The most massive early galaxies studied by the team appear to contain a much larger proportion of low-mass stars than less massive galaxies such as our Milky Way.
One galaxy produced an especially striking result. It likely formed less than 1.5 billion years after the Big Bang, yet the researchers estimate that it could contain up to four times more stellar mass than previously thought.
That presents a major challenge for theories explaining how galaxies formed and evolved. Scientists must now determine how the young universe could have produced such enormous populations of small stars in such a short period.
The discovery could have another intriguing implication.
Low-mass stars commonly have planets orbiting them. If the early universe contained many more of these stars than scientists realized, planets may also have begun forming in greater numbers earlier in cosmic history than previously suspected.
The researchers now plan to use their technique to examine even older and more distant galaxies.
By looking deeper into space—and therefore further back in time—they hope to determine whether vast populations of hidden stars were already forming during the earliest chapters of the universe.
Source: Netherlands Research School for Astronomy.
