The search for Population III: Confirmation of a HeII emitter with no metal lines at z=10.6

Abstract

We report the confirmation of a HeII$λ$1640 emitter located at 3 pkpc from the galaxy GN-z11, at z=10.6. The detection, based on JWST NIRSpec-IFU high-resolution spectroscopy, confirms a previous claim based on medium-resolution spectroscopy. The HeII$λ$1640 identification is further supported by the independent detection of H$γ$ obtained by Übler et al. (2026) at the same location. The HeII emission is spectrally resolved in two components separated by 120 km/s. The Equivalent Width of the HeII emission is extremely high ($>$20 A). No metal lines are detected. Population III stars appear to be the most plausible explanation for the observed HeII emission. We argue that Population III stars are the most plausible explanation for the observed He II emission, with no satisfactory alternative from other classes of sources or mechanisms.

Figures (9)

• Figure 1: Top: Hebe's spectrum around the wavelength of HeII$\lambda$1640. Bottom: Hebe's spectrum around H$\gamma$, from Ubler2026. The long-dashed (orange) and dot-dashed (green) lines show the simultaneous fit of HeII and H$\gamma$ with two components (C1 and C2), respectively, while the short-dashed red line shows the total; only C2 is detected in H$\gamma$. The flux scale is not corrected for aperture losses nor for lensing magnification.
• Figure 2: Continuum-subtracted map of the HeII emission at the redshift of Hebe in the halo of GN-z11 (see text). Contours indicate the 3$\sigma$ (thin), 4$\sigma$ and 5$\sigma$ (thick) levels. The white circle indicates the location of GN-z11.
• Figure 3: Same map of the HeII emission as in Fig.\ref{['fig:heii_map']} (contours), overlayed on the RGB image of the field from the NIRCam filters F115W, F150W and F200W, and with a $1.5\arcsec\times1.5\arcsec$ Field of View, adjusted to show the location of both GN-z11 and the foreground galaxy at z=2.03.
• Figure 4: Overlay of the continuum-subtracted medium resolution R1000 map of HeII reported in Maiolino:2024_Halo (magenta contours) on the continuum-subtracted R2700 map of HeII obtained in this paper (background image). The white circle indicates location of the continuum of GN-z11 and its size shows the relative uncertainty of positioning the two continua for registering the two maps. Note that at R1000 emission is also seen close to GN-z11, as at low resolution the broader integrating wavelength window includes some HeII emission from GN-z11. Also note that in the R1000 map the top-left corner is affected by an artefact originating from the fact that, in that observation, Westward of GN-z11, there were fewer frames and edge effects, due to a guiding problem Maiolino:2024_Halo.
• Figure 5: EW(HeII1640) versus HeII/H$\gamma$ diagnostic diagram. Various symbols show models from Nakajima2022 for different classes of objects, specifically: squares - PopIII (blue: density $n=10^3~\rm cm^{-3}$; purple: densities $n=10^5~\rm cm^{-3}$ and $10^6~\rm cm^{-3}$); red stars - PopII with decreasing metallicity, from darker to lighter, as indicated in the legend; orange pentagons - AGN. The sizes of the PopIII symbols reflect different IMFs. The small points are pristine PopIII models from Rusta2026, from pure PopIII (dark symbols) to cases with PopII contribution but still PopIII-dominated (PopIII $>$50% in mass, light symbols). The large golden symbols are the values inferred for Hebe, both the total and individual components, as indicated.