Early Galaxies May Be Up to Four Times More Massive Than Thought, JWST Study Finds

Astronomers using NASA's James Webb Space Telescope (JWST) have found that some of the largest galaxies in the early universe held many more small, faint stars than previously realized, a discovery that could make the galaxies up to four times more massive than earlier estimates suggested. The findings, from an international team including researchers at Penn State University and led by Leiden Observatory, were published in the journal Nature Astronomy.

The team examined nine massive, mature galaxies that had stopped forming new stars billions of years ago, combining JWST observations with earlier data from the Very Large Telescope. By splitting each galaxy's light into a spectrum, they could estimate, for the first time at such distances, the ratio of small, faint stars to larger, brighter ones. Large, luminous stars dominate a galaxy's light, making the far dimmer small stars difficult to detect.

One galaxy stood out: it likely formed less than one and a half billion years after the Big Bang and could be up to four times more massive than earlier calculations indicated, said Leiden University doctoral candidate Martje Slob, a coauthor. Lead author Chloe Cheng, a recent doctoral graduate of Leiden University, said the models show that a much larger population of low-mass stars is hidden behind a galaxy's brightest stars.

The researchers found that the most massive galaxies in the early universe appear to hold a much greater share of low-mass stars than smaller galaxies such as the Milky Way, challenging the long-standing assumption that stars form in similar proportions throughout the universe. Study leader Mariska Kriek, professor of extragalactic astronomy at Leiden Observatory, said the result could also mean more planets formed in the early universe than previously assumed, since many planets orbit low-mass stars.

The researchers plan to use the same technique to study galaxies from even earlier periods in the universe's history.