What is Powering the Enigmatic He II Emitter Hebe: The First Stars or Black Holes?

Junehyoung Jeon, Tae Bong Jeong, Saiyang Zhang, Volker Bromm

First listed 2026-04-21 | Last updated 2026-04-21

Abstract

Recent high-resolution spectroscopy with the James Webb Space Telescope (JWST) has confirmed the presence of a strong He II, $\lambda1640$ emitting clump in the vicinity of GN-z11, with only upper limits on its metallicity. To explain the peculiar properties of this source, now termed Hebe, a cluster of metal-free, Population III (Pop III) stars has been invoked. A less likely source for the hard UV ionizing radiation could be an accreting supermassive black hole (SMBH) embedded inside Hebe. We here provide further constraints on what could power the observed emission lines in Hebe. Comparing with cosmological simulations of Pop III star cluster formation, we assess the maximum Pop III stellar mass that could plausibly form at the location of Hebe, finding stellar masses of a few $10^5\,M_{\odot}$, consistent with those inferred from the observations. Modeling the continuum spectral energy distribution arising from an accreting SMBH, we derive He II and H I ionizing rates and the resulting recombination line luminosities, roughly in line with the observations. We thus confirm the interpretation of Hebe as a remarkable, primordial object, with the most plausible power source provided by a massive cluster of Pop III stars, at the limit of what is allowed within the standard model of first star formation.

Short digest

This paper asks whether the He II-bright source Hebe is more naturally powered by first stars or by a black-hole engine. The main result is that the authors favor an extreme Pop III stellar population, while still laying out the heavy-seed black-hole configuration that could mimic some of the same observables. The paper matters because Hebe is one of the cleanest laboratories for separating primordial-star and accreting-black-hole interpretations in the JWST era.

Key figures to inspect

• Figure 4 is the must-see comparison plot: it puts the accreting-black-hole SEDs and line predictions next to the Pop III benchmark and the observed He II / H constraints, which is where the two scenarios are most directly confronted.
• Figure 1 shows the required LW-flux environment and how rare such halos are, so it is the place to look if you want to judge whether the Pop III pathway is cosmologically plausible.
• Figure 2 translates that LW requirement into a Pop III burst mass estimate for Hebe, which is the key bridge between the environmental argument and the observed system.
• Figure 3 is the geometry figure for the black-hole scenario, illustrating the configuration in which a heavy seed black hole lives near, but not inside, the main stellar component.