The Pristine HeII Emitter near GN-z11: Constraining the Mass Distribution of the First Stars

TL;DR

This study uses new observations of a HeII emitter near GN-z11 to constrain the initial mass function and properties of the first metal-free stars, suggesting a top-heavy IMF.

Contribution

It provides the first data-driven constraints on the PopIII star IMF using high-redshift observations, linking stellar mass, age, and IMF characteristics.

Findings

HeII/Hγ ratio favors top-heavy IMFs for young PopIII stars.

Estimated PopIII stellar mass range is 2×10^4 to 6×10^5 solar masses.

Observations exclude the flattest IMFs, constraining the characteristic mass and slope.

Abstract

The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their mass distribution (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the HeII emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (PopIII) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with $>50\%$ of their stellar mass in PopIII stars. We find that C1 is consistent with a purely PopIII system and adopt a simple parametric approach to infer the implications for the PopIII IMF and stellar mass. The observed $\rm HeII/H_\gamma$ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages ($\leq 1$ Myr). Combined with the HeII luminosity, this implies a total PopIII stellar mass of $2 \cdot 10^4 < M_\star/M_\odot < 6 \cdot 10^5$. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations ($M_\star < 10^5\,M_\odot$) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable PopIII IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-$z$ PopIII systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties.