Hidden Low-Mass Stars May Make Early Galaxies Far Heavier Than Thought

Some of the earliest massive galaxies in the universe may be hiding far more mass than astronomers had estimated, according to a study published in Nature Astronomy. An international research team, including physicists from Penn State, found that faint, low-mass stars concealed inside these galaxies could add up to four times more stellar mass than previous calculations suggested.

The team used NASA's James Webb Space Telescope (JWST) together with earlier observations from the ground-based Very Large Telescope to study nine massive, mature galaxies that had already stopped forming new stars billions of years ago. By splitting each galaxy's light into a spectrum, they could for the first time reliably estimate how many small, faint stars were hidden behind the glare of brighter, more massive stars.

"If a galaxy were a city, the brightest stars would be the skyscrapers that immediately catch your eye from afar," said lead author Chloe Cheng, a recent doctoral graduate of Leiden University. "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." One galaxy in the sample likely formed less than one and a half billion years after the Big Bang and may contain up to four times the mass previously estimated, according to co-author Martje Slob, a doctoral candidate at Leiden University.

The findings challenge the long-standing assumption that stars form in similar proportions throughout the universe. Joel Leja of Penn State, a co-author of the study, said the extra mass deepens a puzzle that has grown since JWST began operating: astronomers keep finding galaxies that were already surprisingly massive and mature not long after the Big Bang. Mariska Kriek of Leiden Observatory, who led the research, said that because many planets orbit low-mass stars, the result could also mean more planets formed in the early universe than previously assumed.

The researchers plan to extend the same method to galaxies from even earlier periods of cosmic history.