JWST’s ‘impossible’ early galaxies may be even more massive than we thought

The James Webb Space Telescope has revealed that distant galaxies may contain far more stellar mass than previously thought, according to a study led by Mariska Kriek of Leiden Observatory.

Chloe Cheng of Leiden University and colleagues used JWST to obtain deep spectra of nine massive galaxies observed as they appeared seven billion years ago. They combined these data with observations from the European Southern Observatory's Very Large Telescope in Chile. By measuring subtle absorption features in the galaxies' light, the team estimated populations of faint, low-mass stars that are too dim to see individually at such distances.

The researchers found that several galaxies contained substantially more low-mass stars than expected based on the stellar birth distribution observed in the Milky Way, a pattern astronomers call the initial mass function, or IMF. These galaxies displayed what researchers describe as a "bottom-heavy" IMF, with disproportionately large numbers of small stars.

"If a galaxy were a city, the brightest stars would be the skyscrapers that immediately catch your eye from afar," Cheng 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."

Two of the oldest galaxies in the study appear to be descendants of remarkably massive galaxies JWST detected within the first billion years after the Big Bang. If those early galaxies also contain unexpectedly large populations of faint stars, their estimated stellar masses could be three to four times greater than current calculations, intensifying the puzzle of how such enormous galaxies formed so quickly in the young Universe.

Kriek noted that the result "could even indicate that more planets formed in the early universe than we had previously assumed," since low-mass stars commonly host planets. The connection to JWST's earliest galaxies remains an inference, however, as Cheng's team studied possible descendants rather than the early galaxies themselves.