On August 22, Nature Astronomy published two papers in the same issue: the primary study by Cheng et al. (first author from Leiden, DOI 10.1038/s41550-026-02932-4) plus a companion commentary (DOI 10.1038/s41550-026-02947-x). The one-line conclusion: nine early, massive galaxies that had already stopped forming stars are actually 3 to 4 times more massive than previous estimates once a bottom-heavy initial mass function (IMF) correction is applied—one galaxy formed less than 1.5 billion years after the Big Bang was revised to four times its prior mass.
What JWST Saw
The team combined high-quality JWST NIRSpec spectra with ground-based VLT LEGA-C survey data to, for the first time, reliably estimate the ratio of low-luminosity to high-luminosity stars in nine mature, long-quiescent massive galaxies. Cheng (a Leiden PhD graduate) offered a "skyscraper-residents" analogy: bright galaxies in the night sky are like skyscrapers in a city—visually prominent—but between the towers hide vast numbers of low-rise residents that were previously undetectable in spectra. Penn State co-author Joel Leja put the result more bluntly: 3–4 times.
Why a Bottom-Heavy IMF
The classical IMF assumes stars form in the same proportions throughout cosmic history; observations of the local universe over the past 20 years do support this (e.g., in Milky Way-like galaxies). Cheng's team draws a boundary around that assumption: in early massive galaxies, the fraction of low-mass stars is significantly higher than average. This is not fine-tuning of a model parameter, but a structural change in the ratios.
The "Impossibly Early" Problem Deepens
Since JWST launched, astronomers have repeatedly found anomalously large, anomalously mature galaxies at surprisingly early epochs—a persistent tension with existing galaxy formation models, formally named the "impossibly early" problem by Steinhardt et al. in 2016. Cheng's team's result means early galaxy masses must be revised upward by up to 4x, tightening that tension further. Existing models of "too-rapid star formation / too-efficient gas accumulation" now need an additional mechanism explaining why early IMFs favored many small stars. In Leja's words: if these galaxies really host four times more stars, "more physical explanation is needed for why such small stars appeared so early."
Undercurrents Accompanying This Work
- The temptation to extrapolate earlier. The paper states the next step is applying the same technique to even earlier galaxies. If galaxies within 1.5 billion years show a 3–4x correction, will galaxies a few hundred million years old show 6–8x? Classical cosmology and structure formation will be pushed further.
- Planet formation. Leja and Kriek both emphasize: many planets orbit low-mass stars, so if low-mass stars in the early universe were systematically undercounted, early planet counts were too. This pushes the scale of "cradles for life" earlier and wider—though planet formation and habitability remain separate judgments, the numerical revision is explicit.
- A renewed Penn State vs. Yale/Leiden contest. Van Dokkum and Conroy proposed a "concordance IMF" in 2024 to keep early galaxy masses from ballooning into conflict with contemporary physics; Cheng's team's work explicitly rejects that middle path, pulling the IMF toward the "bottom-heavy" pole. If future observations accumulate in the same direction, galaxy formation models may be forced to make "when and by what mechanism did bottom-heavy IMFs emerge" the standard question of the next decade.
Back to JWST Itself
JWST gained no new mirrors and no new detectors. It was the combination of a spectrograph (NIRSpec) with ground-based high-precision radial velocity data (VLT) that upgraded "how many small stars are in early galaxies" from a guess to an empirical result. This suggests the biggest puzzle of the JWST era may not be "what are the unseen objects," but "what we thought was invisible was there all along." Whether the next JWST time allocation shifts more aggressively toward the early-IMF question will be an early signal from the 2027 observing-cycle competition.