How did an international team led by Leiden University use ultra deep James Webb Space Telescope NIRSpec spectra combined with ground based Very Large Telescope LEGA C data to study nine massive, quiescent galaxies at redshift about 0.7, what did they discover about the galaxies’

The team used spectroscopy sensitive to the otherwise faint light of low-mass dwarf stars to measure the stellar initial mass function (IMF), rather than assuming the Milky Way’s IMF. They found that all nine massive, quiescent galaxies at redshift ~0.7 have an excess of such stars—a bottom-heavy IMF—implying substantial “hidden” stellar mass.

Method

• They combined ultra-deep JWST/NIRSpec spectra from the JWST-IMFERNO programme with deep, bluer optical spectra from the VLT’s LEGA-C survey. This broad wavelength coverage separates IMF-sensitive dwarf-star absorption features in the rest-frame near-infrared from features that constrain ages, metallicity, and elemental abundances in the optical.
• The sample contains nine massive galaxies that had already stopped forming stars at z ~0.7, allowing their accumulated stellar populations to be studied with unusually high precision.

What they found

• The galaxies’ IMFs are more bottom-heavy than the Milky Way’s: relative to a Milky-Way-like distribution of birth masses, they contain many more faint, low-mass stars.
• Such dwarfs contribute little to a galaxy’s observed light but make up a large fraction of its stellar mass; conventional mass estimates that extrapolate a Milky Way IMF therefore undercount their total stellar mass.
• For the two oldest objects, identified as descendants of JWST’s unusually early mature galaxies, applying the measured IMF raises the inferred stellar masses by factors of three to four.

Why this worsens the early-galaxy puzzle

• One of those old populations likely formed less than 1.5 billion years after the Big Bang; if its descendant’s IMF was similarly bottom-heavy when its stars formed, the early galaxy had to assemble even more stellar mass, even faster, than prior estimates indicated.
• JWST has already found massive, evolved galaxies at very early cosmic times—including spectroscopically confirmed quiescent systems roughly 1.25 billion years after the Big Bang. Their unexpectedly rapid assembly strains galaxy-formation models; increasing their true masses strengthens rather than resolves that tension.
• The stated connection to galaxies observed within about 500 million years is an extrapolation to the earliest “impossibly early” systems, not a direct IMF measurement of z ≳ 10 galaxies. The present robust measurements are at z ~0.7.

Planet implication and next step

• If bottom-heavy star formation was common in the young Universe, it would mean more long-lived, low-mass stars—the principal potential hosts of planets—than assumed under a Milky-Way-like IMF. It could therefore raise the estimated number of planetary systems formed early on, although this study does not measure planet occurrence directly.
• The researchers’ next step is to apply the same ultra-deep, broad-rest-frame spectral approach to progressively higher-redshift galaxies, ultimately toward systems near the epoch of the first stars, where JWST can shift the relevant IMF-sensitive features into observable infrared wavelengths. This will test whether the excess of low-mass stars also characterized the earliest galaxies.