Too Massive, Too Early… and Still Not Massive Enough?

Key Takeaways

• JWST NIRSpec spectra reveal bottom‑heavy IMFs at z≈0.7
• IMF mismatch α_IMF >1 correlates with velocity dispersion and metallicity
• Bottom‑heavy IMFs increase stellar masses by 3‑4× for oldest galaxies
• Adjusted stellar masses align better with virial mass constraints
• Early massive galaxies may be more massive than previously thought

Pulse Analysis

The initial mass function (IMF) sets the relative numbers of high‑ and low‑mass stars and underlies all stellar‑mass estimates. While the Milky Way’s Kroupa/Chabrier IMF is often assumed universal, nearby massive ellipticals have shown signs of bottom‑heavy IMFs, meaning an excess of faint dwarf stars. JWST’s NIRSpec now provides the sensitivity to detect dwarf‑star absorption features in distant galaxies. This paper combines ultra‑deep NIRSpec data with LEGA‑C optical spectra to fit nine quiescent galaxies at z≈0.7. These observations also provide the first direct IMF constraints at look‑back times of roughly six billion years.

Full‑spectrum fitting allowed the authors to vary age, elemental abundances and the low‑mass IMF slope, producing an IMF‑mismatch parameter α_IMF. Most galaxies show α_IMF > 1, indicating mass‑to‑light ratios higher than a Milky Way IMF. The parameter rises with stellar velocity dispersion and metallicity, reproducing trends seen locally. For the two oldest systems, a bottom‑heavy IMF inflates stellar masses by three‑to‑four‑fold, yet these revised masses remain consistent with virial estimates, resolving previous dynamical tensions.

If bottom‑heavy IMFs are common among massive galaxies at intermediate redshift, the true stellar mass density of the early universe could be substantially higher than current surveys report. This exacerbates the “impossibly early” massive‑galaxy problem, demanding faster star‑formation or more efficient assembly in cosmological models. The study also demonstrates that JWST spectroscopy can directly probe IMF variations beyond the local universe, opening a new avenue for testing galaxy‑formation theories. Future larger samples will clarify whether the observed trends hold across cosmic time and how they influence the mass‑budget of the young cosmos.