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

TL;DR

This paper investigates whether the enigmatic He II emitter Hebe is powered by Population III stars or an accreting supermassive black hole, using JWST data, simulations, and spectral modeling.

Contribution

It provides constraints on the possible power sources of Hebe, favoring a massive Pop III star cluster within standard first star formation models.

Findings

Hebe's properties are consistent with a Pop III star cluster of a few 10^5 solar masses.

Spectral modeling of an accreting SMBH can reproduce observed emission lines.

Pop III stars are the most plausible power source within current theoretical limits.

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.