A strong He II $λ$1640 emitter with extremely blue UV spectral slope at $z=8.16$: presence of Pop III stars?

Abstract: Cosmic hydrogen reionization and cosmic production of first metals are major phase transitions of the universe occurring during the first billion years after the Big Bang, however these are still underexplored observationally. Using the JWST NIRSpec prism spectroscopy, we report the discovery of a sub-$L_\ast$ galaxy at $z_{\rm spec}=8.1623\pm0.0007$, dubbed RXJ2129-z8HeII, via the detection of a series of strong rest-frame UV/optical nebular emission lines and the clear Lyman break. RXJ2129-z8HeII shows a pronounced UV continuum with an extremely steep (i.e. blue) spectral slope of $\beta=-2.53_{-0.07}^{+0.06}$, the steepest amongst all spectroscopically confirmed galaxies at $z_{\rm spec}\gtrsim7$, in support of its very hard ionizing spectrum that could lead to a significant leakage of its ionizing flux. Therefore, RXJ2129-z8HeII is representative of the key galaxy population driving the cosmic reionization. More importantly, we detect a strong He II $\lambda$1640 emission line in its spectrum, one of the highest redshifts at which such a line is robustly detected. Its high rest-frame equivalent width (${\rm EW}=21\pm4$ Angstrom) and extreme flux ratios with respect to UV metal and Balmer lines raise the possibility that part of RXJ2129-z8HeII's stellar populations could be Pop III-like. Through careful photoionization modeling, we show that the physically calibrated phenomenological models of the ionizing spectra of Pop III stars with strong mass loss can successfully reproduce the emission line flux ratios observed in RXJ2129-z8HeII. Assuming the Eddington limit, the total mass of the Pop III stars within this system is estimated to be $7.8\pm1.4\times10^5 M_\odot$. To date, this galaxy presents the most compelling case in the early universe where trace Pop III stars might coexist with metal-enriched populations.

• The paper confirms a z=8.16 galaxy with an unprecedented blue UV continuum (β ≈ -2.53) and a strong He II 1640 emission line.
• It employs JWST NIRSpec prism spectroscopy to measure a high rest-frame equivalent width (21±4 Å) and distinctive flux ratios indicative of metal-poor stellar populations.
• Photoionization modeling estimates around 7.8±1.4×10^5 solar masses in Pop III stars, suggesting a mixed stellar population that may drive cosmic reionization.

Analysis of a He II $\lambda$1640 Emitter at $z=8.16$: Potential Insights into Population III Stars

The identification of Population III (Pop III) stars remains an elusive but highly significant goal in astronomy, offering critical insights into the early universe's stellar evolution. In a recent study utilizing JWST NIRSpec prism spectroscopy, a galaxy, dubbed RXJ2129-z8HeII, has been spectroscopically confirmed at a redshift of $z=8.1623\pm0.0007$. This galaxy, characterized as a sub-$L_*$ emitter, displays a distinct UV continuum with an exceptionally steep spectral slope ($\beta \approx -2.53$), which stands as the steepest among spectroscopically confirmed galaxies at $z_{\rm spec}\gtrsim7$. Such a slope is indicative of a hard ionizing spectrum, potentially facilitating significant leakage of ionizing flux, thereby implicating RXJ2129-z8HeII as a representative of the galaxy population likely driving cosmic reionization.

The strong detection of the He II $\lambda$1640 emission line in this galaxy's spectrum is particularly noteworthy, recorded as one of the highest redshifts for such a line detected to date. This is complemented by the line's high rest-frame equivalent width (EW = $21\pm4$ \AA) and notable flux ratios relative to both UV metal and Balmer lines. This combination raises the possibility that a portion of RXJ2129-z8HeII's stellar population could be Pop III-like.

Theoretical and Practical Implications

The presence of a pronounced He II emission line without accompanying strong metal lines is crucial. In typical stellar populations, such lines are usually observed in conjunction with emissions from other elements indicating substantial stellar metallicity; however, their absence here supports the hypothesis of metal-poor origins. Pop III stars, being metal-free, are expected to produce exceedingly hard ionizing spectra leading to significant He II emissions without corresponding metal lines.

Photoionization modeling within the study shows that simulated ionizing spectra of Pop III stars—particularly those incorporating strong mass loss—can replicate the observed line ratios in RXJ2129-z8HeII. This strengthens the argument for the coexistence of both metal-free and metal-enriched stellar populations within this galaxy. The modeling estimates a total stellar mass of Pop III stars in the system to be around $7.8\pm1.4\times10^5 M_\odot$, assuming conditions aligned with the Eddington limit.

Future Research Directions

Continued exploration of high-redshift galaxies like RXJ2129-z8HeII offers a meaningful pathway both to solidify the model of Pop III star formation within mixed stellar populations, and to refine the conditions necessary for their emergence in the early universe. As JWST continues operations, gathering more data from similar systems could hone our understanding of how these first stars contributed to the reionization and chemical enrichment phases.

Moreover, such research challenges existing stellar evolutionary models by demanding they accommodate scenarios involving significant mass loss and ionizing emissions from metal-free stars. This, in turn, could impact theories surrounding galaxy formation and the early evolution of cosmic structures.

In conclusion, RXJ2129-z8HeII offers a compelling glimpse into the characteristics and possible coexistence of the universe's first generations of stars with subsequent, metal-enriched populations. Future high-resolution spectroscopy and spatial analyses will be pivotal in unraveling the complexities of these early star-forming regions and refining models of both stellar and galactic evolution across cosmic time.