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Accepted · first revision · record

Title: Was the IMF top-heavy in the first galaxies?
Plain: 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: 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 $dN/d\operatorname{log} m\sim m^{-\Gamma}$.
Settled by: 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.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status 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.
Posed since: (empty)