JWST Finds Hidden Low-Mass Stars That Make Early Galaxies Up to Four Times More Massive

The galaxies have bottom-heavy initial mass functions

The initial mass function, or IMF, describes how many stars form at different masses. A bottom-heavy IMF contains a larger proportion of low-mass stars than the Milky-Way-like distribution commonly used in galaxy-mass estimates.

The team found evidence for this bottom-heavy pattern in all nine massive, quiescent galaxies. These low-mass stars are dim, so they can remain effectively hidden in ordinary observations. However, they are long-lived and collectively add significant mass. If researchers estimate a distant galaxy’s mass using a Milky-Way-like IMF when its true IMF is bottom-heavy, they will underestimate the total mass in stars.

The oldest two galaxies become three to four times more massive

The effect is especially important for the two oldest objects in the sample. They are considered direct descendants of the unusually early mature galaxies revealed by JWST. Applying the measured bottom-heavy IMF increases their inferred stellar masses by factors of three to four.

That is not a small correction. It changes the amount of matter that galaxy-formation models must assemble in a short period of cosmic history. One of the old stellar populations likely formed less than 1.5 billion years after the Big Bang, so its progenitor would have needed to build an even larger stellar mass, even faster, than earlier estimates suggested.

Why this sharpens the early-galaxy problem

JWST has already identified massive, evolved galaxies at surprisingly early times. For example, the quiescent galaxy GS-9209 was spectroscopically confirmed at redshift 4.658, when the Universe was about 1.25 billion years old. Its inferred stellar mass was approximately 3.8 × 10^10 solar masses, based on the assumptions used in that analysis.

The new IMF result does not directly measure galaxies at the first few hundred million years after the Big Bang. Instead, it provides evidence at redshift about 0.7 that some descendants of very early galaxies contain more low-mass stars than expected. Extending that conclusion to the earliest JWST galaxies is therefore an inference, not a direct measurement of their IMFs.

If the same bottom-heavy star-formation pattern applied when those ancient galaxies formed, their true stellar masses could be substantially larger than initial estimates. That would strengthen the challenge to galaxy-formation models: the models would need to explain not only how mature galaxies appeared so early, but how they assembled even more stellar mass than previously thought.

Could more low-mass stars mean more planets?

Low-mass stars live for long periods and are potential hosts for planetary systems. If bottom-heavy star formation was common in the young Universe, more of these stars may have formed than standard Milky-Way-like assumptions imply. In turn, estimates of how many planetary systems could have existed early in cosmic history might increase.

That implication remains speculative. The study measured stellar populations, not planets or planet occurrence rates. Its result changes the possible number of low-mass stellar hosts; it does not establish how often those stars formed planets or whether such planets were habitable.

What researchers will test next

The next step is to apply the same strategy to galaxies at progressively higher redshifts. By combining very deep spectra across a wide rest-frame wavelength range, researchers can test whether the excess of low-mass stars persists closer to the era of the first stars. At sufficiently high redshift, IMF-sensitive features are shifted into infrared wavelengths that JWST can observe.

Those measurements will help determine whether the bottom-heavy IMF is a characteristic of the massive quiescent galaxies studied so far, a broader feature of early star formation, or a pattern that changes with cosmic time and environment.