The James Webb Space Telescope has uncovered a hidden ingredient that could make some of the Universe’s earliest galaxies far more massive than previously believed. A study of nine massive, quiescent galaxies has found evidence that these systems contained an unusually large population of low-mass stars. The result suggests that the standard assumption used to estimate the masses of distant galaxies may be incomplete—and that some of the “impossibly early” galaxies observed by JWST could have been even heavier than their already surprising measurements indicate.
The findings, reported in Nature Astronomy, come from the JWST Initial Mass Function of Early Red NIRSpec Objects program, combined with exceptionally deep observations from the Very Large Telescope’s Large Early Galaxy Astrophysics Census survey. The galaxies examined lie at a redshift of approximately 0.7, meaning their light has travelled for billions of years before reaching Earth. Although these galaxies are not among the most distant objects detected by JWST, their stellar populations preserve important clues about how the first generations of massive galaxies assembled and evolved.
The central uncertainty concerns the initial mass function, or IMF. This is the distribution of stellar birth masses in a newly formed population: it describes how many stars are born extremely massive, intermediate in mass or relatively small. Astronomers cannot directly count every star in a remote galaxy, so they infer its total stellar mass from the light emitted by the entire population. Those calculations generally adopt an IMF similar to the one measured in the Milky Way, where low-mass stars are thought to dominate the number of stars and contribute a substantial share of the total mass.
Low-mass stars are especially important because they can survive for tens or even hundreds of billions of years. Massive stars burn through their fuel rapidly, exploding or collapsing after only a few million years, while stars below roughly the mass of the Sun remain visible for far longer. Yet the faintest low-mass stars in distant galaxies are almost impossible to observe individually. Their light is overwhelmed by brighter stars and by the combined emission of the galaxy, forcing researchers to estimate their abundance indirectly through subtle features in the integrated spectrum.
To make that inference, the researchers used full-spectrum modelling rather than relying on a small number of photometric measurements or isolated spectral lines. A galaxy’s spectrum contains a complex mixture of information about its stars, including their ages, chemical composition, motions and mass distribution. Certain absorption features are more sensitive to cool, low-mass stars than to luminous young stars. By comparing the observed spectra with detailed stellar-population models, astronomers can test whether a Milky Way-like IMF is sufficient or whether the galaxy requires an excess of low-mass stars.
The JWST Near-Infrared Spectrograph provided unusually clean and deep observations of the nine galaxies, while the Very Large Telescope data extended the spectral coverage toward bluer wavelengths. This broader range is essential because different parts of a spectrum respond to different physical properties. Red and near-infrared light can reveal the signatures of cool stars, whereas blue wavelengths help constrain age, metallicity and the contribution of warmer stars. Combining the two data sets reduces the chance that a misleading estimate of one property will be mistaken for evidence of an unusual IMF.
The analysis found that the most massive galaxies in the sample contained an excess of low-mass stars, a pattern commonly described as a bottom-heavy IMF. In this context, “bottom-heavy” does not mean that the galaxies were dominated by small stars in terms of luminosity. Low-mass stars are intrinsically faint, so they can contribute relatively little light while still accounting for a large amount of mass. Their presence would therefore allow a galaxy to hide substantial stellar material from conventional mass estimates based on a Milky Way-like stellar distribution.
The strongest signal came from the oldest galaxy in the sample. Its stellar population indicates a formation redshift greater than five, suggesting that much of its star formation occurred when the Universe was less than roughly 1.2 billion years old. This ancient system also showed the most bottom-heavy IMF, linking the abundance of low-mass stars to the conditions under which the earliest massive galaxies formed. The trend is notable because it points to a changing stellar birth-mass distribution rather than a simple error affecting all galaxies equally.
The researchers suggest that this ancient galaxy could be a later descendant of the unusually bright and massive galaxies JWST has found at extreme distances. Those objects, sometimes called “impossibly early” galaxies, appear to have assembled large stellar masses only a few hundred million years after the Big Bang, challenging many established models of galaxy formation. If their stellar populations were also bottom-heavy, the mass inferred from their light could be significantly underestimated. The study estimates that adopting the measured IMF effect could increase their stellar masses by approximately a factor of four, with an uncertainty of about one.
That possibility intensifies an existing debate over how rapidly galaxies formed in the early Universe. Standard models describe galaxies growing through the gradual accumulation of gas, star formation, mergers and the build-up of dark matter halos. Producing very massive, mature systems at such early times is already difficult in some simulations. Increasing their estimated masses would make the challenge more severe, requiring either more efficient conversion of gas into stars, unusually rapid assembly, different feedback processes or revisions to assumptions about the first stellar populations.
The result does not mean that every early galaxy has four times more mass than currently estimated, nor does it establish that a bottom-heavy IMF was universal. The study examined a small sample of nine quiescent galaxies, and interpreting integrated spectra requires models that account for age, chemical enrichment, dust, stellar remnants and the history of star formation. Degeneracies between these factors can imitate or obscure IMF-sensitive signatures. Even so, the use of ultra-deep spectra across a broad wavelength range provides a stronger test than earlier observations, and the relationship between stellar age, galaxy mass and IMF shape offers a physically suggestive pattern.
Future JWST observations will be needed to determine whether the finding applies to a wider range of galaxies, including actively star-forming systems and objects at much higher redshifts. Larger samples could reveal whether bottom-heavy IMFs were linked to dense environments, rapid bursts of star formation, high pressures in stellar nurseries or the unusually compact structure of early galaxies. If the trend survives those tests, astronomers may need to revise not only the masses assigned to the first galaxies but also the way models describe the birth of stars under conditions unlike anything common in the modern Milky Way.
Subject of Research: Low-mass stellar populations and the initial mass function in massive early galaxies
Article Title: Hidden mass in early galaxies revealed by bottom-heavy initial mass functions
Article References: Cheng, C.M., Slob, M., Kriek, M. et al. Hidden mass in early galaxies revealed by bottom-heavy initial mass functions. Nature Astronomy (2026). https://doi.org/10.1038/s41550-026-02932-4
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41550-026-02932-4
Keywords: James Webb Space Telescope, JWST, initial mass function, IMF, bottom-heavy IMF, low-mass stars, early galaxies, galaxy formation, stellar mass, quiescent galaxies, NIRSpec, Very Large Telescope, Nature Astronomy
