The most massive galaxies in the early universe appear to contain an unexpectedly large population of small, faint stars, suggesting that some of these ancient galaxies may have been several times more massive than earlier measurements indicated.
Astronomers reached this result by studying nine massive, mature galaxies that stopped forming stars billions of years ago. The international team, led by scientists at Leiden University, combined extremely deep observations from the James Webb Space Telescope with earlier ground-based observations from the Very Large Telescope.
The challenge was to measure stars that are difficult to see individually. When astronomers examine a distant galaxy, its light is dominated by massive, bright stars. Much smaller stars are far fainter, so their contribution can be difficult to separate from the overall light of the galaxy.
The researchers analyzed the galaxies’ spectra, which break their light into different wavelengths. Subtle differences in the resulting spectrum can reveal which kinds of stars are present.
Their analysis allowed the team to estimate the proportions of low-mass and high-mass stars in these distant galaxies.
“If a galaxy were a city, the brightest stars would be the skyscrapers that immediately catch your eye from afar,” lead author Chloe Cheng of Leiden University said. “Our models demonstrate that a far more numerous population of low-mass stars is concealed by those rare, bright stars, like houses hidden between skyscrapers. As a result, this galaxy turns out to be much more massive than previous estimates suggested.”
The early galaxies contained more small stars
Astronomers have historically estimated the unseen mass in stars by assuming that stars form in roughly similar proportions throughout the universe.
The new observations challenge that assumption for the most massive galaxies in the early universe. These galaxies contain a much larger fraction of low-mass stars than less massive galaxies such as the Milky Way.
That difference changes the estimated total mass of the galaxies because, although individual low-mass stars are faint, they can make up a much larger population.
For one galaxy in the sample, the effect is especially large. The galaxy likely formed less than 1.5 billion years after the Big Bang and may be up to four times more massive than earlier estimates suggested.
“Until recently, measurements like these were simply impossible,” co-author Martje Slob of Leiden University said. “We needed not only a telescope capable of magnifying very distant galaxies, but also spectra of exceptional quality and new analysis techniques to reliably detect the subtle signatures of faint, low-mass stars hidden within these cosmic titans.”
The findings add to the puzzle of massive early galaxies
Since the James Webb Space Telescope began observing the distant universe, astronomers have found massive, mature galaxies that already existed relatively soon after the Big Bang.
The new measurements indicate that these galaxies may have been even more massive than previously reported. Galaxy-formation models now need to account for how such large numbers of low-mass stars could have formed so early in cosmic history.
“This result shows that much more mass than previously thought is hidden in low-mass stars,” said Mariska Kriek of Leiden University, who led the research.
Kriek also noted that the result could have consequences for other areas of astronomy. Because many planets orbit low-mass stars, the finding could indicate that more planets formed in the early universe than previously assumed.
The researchers next plan to apply the same method to galaxies from even earlier in cosmic history, moving closer to the period when the first generations of stars and galaxies formed.
The study was published in Nature Astronomy.
