Was the IMF top-heavy in the first galaxies?
In plain words
Metal-poor gas in the early universe cools less well, which might have favored more massive stars. This would change how bright and how heavy early galaxies appear.
Precise statement
Determine the IMF slope above $10\ M_{\mathrm{sun}}$ in galaxies at $z>8$ with gas metallicity $Z<0.1\ Z_{\mathrm{sun}}$, using UV luminosity per unit stellar mass, He II and nitrogen emission lines, and chemical abundance ratios. The answer is the slope or an upper and lower bound on it compared with the local Salpeter value $\Gamma=1.35$ in $dN/d\operatorname{log} m\sim m^{-\Gamma}$.
What would settle it
Deep rest-UV and optical spectroscopy of $z > 8$ galaxies giving line ratios and abundance patterns that discriminate IMF slopes from burstiness and AGN contributions.
Status in the literature
Unverified note
2025-2026 studies invoke top-heavy IMFs for the UV luminosity and nitrogen enrichment at $z > 10$, while 2026 modeling finds that a top-heavy IMF alone cannot reproduce the $z \sim 14$ observations without bursty star formation.