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

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 $\rm HeII/H_γ$ 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.

Figures (5)

• Figure 1: Density distributions of our NEFERTITI models at $z=10.6$ for PopIII galaxies at different evolutionary stages (red: pristine; orange: self-polluted; green: PopIII-rich hybrids) and for metal-poor PopII galaxies ( purple). The solid contours include $68\%$ of the galaxy population, for $\rm logU = [-2, -1, -0.5, 0]$ altogether. The errorbars are the $3\sigma$ observational upper limits presented in Maiolino et al., (submitted) and Übler et al., (submitted).
• Figure 2: EW(HeII) vs $\rm HeII/H_\gamma$ for the sub-sample of NEFERTITI galaxies at $z=10.6$ that match the upper limits on metal emission lines (\ref{['fig:fig1']}), shown for three PopIII IMFs as indicated in the top-right scheme (Salpeter slope $x=2.35$, peak mass $M_{ch} = [1, 10, 70] M_\odot$). For each IMF, we show the PDFs of different PopIII galaxy types: pure (red), self-polluted (orange) and PopIII-rich hybrids (green). Arrows indicate how dust and/or higher gas densities would modify the observed properties of these PopIII galaxies. The errorbars are the $3\sigma$ data from Maiolino et al., (submitted) and Übler et al., (submitted).
• Figure 3: Left: predicted $\rm HeII/H_\gamma$ emission from PopIII stars for ages 1 Myr ( top) and 2 Myr ( bottom), for different $M_{ch}$ and IMF slope ( gray regions). The red areas indicate the $3\sigma$, $2\sigma$ and $1\sigma$ lower limits measured by Maiolino et al., (submitted) in the C1 component of Hebe. Right: mass of PopIII stars required to reproduce the observed $\rm L_{HeII}$ value of C1 for ages 1 Myr ( top) and 2 Myr ( top), using different $M_{ch}$ and IMF slopes. Red areas mark IMFs excluded by the $\rm HeII/H_\gamma$ constraints (left panels). In both left and right panels the gray scale shows different PopIII IMF slopes (see legend in the top-right panel) with solid lines highlighting the flat, Salpeter, and steep ($x=4.35$) slopes. For these three examples, the dotted lines represent the scatter due to $\rm logU=[0,-2]$, and the error of the measured $L_{HeII}$ value.
• Figure 4: Constraints on PopIII IMF shapes combining our results with near-field cosmology studies Koustouridou2024. Red shaded regions show IMF peaks ($M_{ch}$) and slope ($x$) excluded by this work based on the 1, 2, and 3$\sigma$ upper limits of $\rm He\, II/H_\gamma$ in the C1 component of Hebe (left panels in Fig. \ref{['fig:fig3']}). Dotted regions are valid if we are not observing PopIII stars at the peak of their HeII emission, i.e., at ages close to 2 Myr.
• Figure 5: Luminosity of HeII$\lambda1640$ versus stellar mass for a single burst of PopIII stars with an age of 1 Myr, forming according to PopIII IMF with different slopes: Flat (dark gray) or Salpeter ($x=2.35$, light gray) with $M_{ch}=10 M_\odot$ (see eq. \ref{['e:Larson']}). The stars represent a fully sampled IMF, while the shaded region shows the range obtained by randomly sampling the IMF. The orange region showcases the range of masses and luminosities of pure PopIII NEFERTITI galaxies (IMF slope Salpeter, $M_{ch}=10 M_\odot$). The red line is the measured HeII of C1 in Hebe reported by Maiolino et al., (submitted).