The findings, published in Nature Astronomy by Chloe M. Cheng, Martje Slob, and Daniel R. Weisz, suggest that early massive galaxies may be substantially heavier than previously calculated. The result deepens a long-running puzzle in cosmology: how the universe assembled such massive objects so soon after the Big Bang.
A Hidden Population of Small Stars
Galaxies are weighed indirectly. Astronomers measure the light a galaxy emits and then use stellar population models to convert that light into mass. The conversion depends on assumptions about how stars are distributed by size — a relationship called the initial mass function, or IMF.
Most nearby galaxies appear to follow a fairly standard IMF, with a predictable ratio of massive, luminous stars to small, dim ones. But the new JWST data show that distant massive galaxies deviate from this norm. They host a disproportionate number of low-mass stars — a “bottom-heavy” IMF — which previous measurements failed to account for.
Because small stars contribute little light but substantial mass, overlooking them causes astronomers to underestimate total galactic mass. Cheng and colleagues found that correcting for the hidden small-star population can increase inferred masses by as much as a factor of four.
The Impossibly Early Galaxy Problem
The discovery lands squarely inside a debate that has intensified since the first JWST observations began arriving in 2022. In 2016, Charles Steinhardt and colleagues identified what they called the “impossibly early galaxy problem” — the existence of massive galaxies at redshifts that seemed too early for standard cosmological models to produce.
JWST has only sharpened the tension. The EXCELS survey, reported by A. C. Carnall and collaborators in 2024, found numerous ultra-massive quiescent galaxies at redshifts between 3 and 5. These galaxies had already stopped forming stars when the universe was less than two billion years old. Standard models struggle to explain how such objects assembled and quenched so quickly.
If those galaxies are actually up to four times more massive than previously thought, the problem becomes more acute, not less. Each upward revision of early galactic mass pushes the timeline for structure formation closer to the theoretical limit set by the age of the universe itself.
Ground and Space Working Together
The new analysis draws on JWST observations of distant galaxies but supplements them with ground-based spectroscopy from the Large Early Galaxy Astrophysics Census, or LEGA-C. The third data release of LEGA-C, published by Arjen van der Wel and colleagues in 2021, provided roughly 3,000 high-quality spectra of galaxies at redshifts greater than 0.6.
Combining space-based infrared imaging with ground-based spectra allowed the team to test how well different IMF assumptions fit the data. The bottom-heavy models, originally developed by Charlie Conroy and Pieter van Dokkum in 2012 and updated in 2018, proved the strongest match for the distant massive galaxy sample.
An Alternative Reconciliation
Not every researcher agrees that the answer lies in revising the IMF upward. In 2024, van Dokkum and Conroy proposed an alternative framework — a “concordance IMF” — that would adjust the proportion of small and large stars in a way that lowers the inferred masses of distant galaxies rather than raising them.
The two approaches point in opposite directions. One says early galaxies are heavier than measured. The other says they are lighter than the new analysis suggests. Resolving the disagreement will require larger samples, tighter spectroscopic constraints, and independent cross-checks using gravitational lensing or dynamical mass estimates.
What Happens Next
The mass revision, if confirmed across larger samples, has cascading consequences. Cosmological simulations that model the growth of structure in the early universe may need to be recalibrated. The efficiency with which gas cools, condenses, and forms stars in massive halos could require rethinking. So could the timeline for the emergence of the first quiescent galaxies.
Watch for follow-up work from the EXCELS and LEGA-C teams, as well as independent tests using JWST’s near-infrared spectrograph on individual galaxies at redshifts above 5. The concordance IMF proposal from van Dokkum and Conroy will also face direct observational tests. The central question — whether the early universe built galaxies faster and heavier than theorists expected — now hinges on which stellar population model survives the next round of data.
— Priya Nair, science desk, AXO News