The JWST EXCELS Survey: A spectroscopic investigation of the ionizing properties of star-forming galaxies at 1<z<8

Charting the Epoch of Reionization demands robust assessments of what drives the production of ionizing photons in high-redshift star-forming galaxies (SFGs), and requires better predictive capabilities from current observations. Using a sample of $N=159$ SFGs at $1<z<8$, observed with deep medium-resolution spectroscopy from the JWST/NIRSpec EXCELS survey, we perform a statistical analysis of their ionizing photon production efficiencies ($ξ_\rm{ion}$). We consider $ξ_\rm{ion}$, measured with Balmer line measurements, in relation to a number of key galaxy properties including; nebular emission line strengths ($W_λ(\rm{Hα})$ and $W_λ$( [OIII])), UV luminosity ($M_\rm{UV}$) and UV slope ($β_\rm{UV}$), as well as dust attenuation ($E(B-V)\rm{neb}$) and redshift. Implementing a Bayesian linear regression methodology, we fit $ξ\rm{ion}$ against the principal observables while fully marginalising over all measurement uncertainties, mitigating against the impact of outliers and determining the intrinsic scatter. Significant relations between $ξ_\rm{ion}$ and $ W_λ(\rm{Hα})$, $W_λ$([OIII]) and $β_\rm{UV}$ are recovered. Moreover, the weak trends with $M_\rm{UV}$ and redshift can be fully explained by the remaining property dependencies. Expanding our analysis to multivariate regression, we determine that $W_λ(\rm{Hα})$ or $W_λ$([OIII]), along with $β_\rm{UV}$ and $E(B-V)\rm{neb}$, are the most important observables for accurately predicting $ξ\rm{ion,0}$. The latter identifies the most common outliers as SFGs with relatively high $E(B-V)\rm{neb}\gtrsim0.5$, possibly indicative of obscured star-formation or strong differential attenuation. Combining these properties enable $ξ\rm{ion,0}$ to be inferred with an accuracy of $\sim0.15,$dex, with a population intrinsic scatter of $σ_\rm{int}\sim0.035,$dex.

💡 Research Summary

The study presented in this paper provides a groundbreaking spectroscopic investigation into the ionizing properties of star-forming galaxies (SFGs) during the critical Epoch of Reionization. Utilizing deep, medium-resolution spectroscopy (R $\approx$ 1000) from the JWST/NIRSpec EXCELS survey, the researchers analyzed a robust sample of 159 SFGs spanning a redshift range of $1 < z < 8$. The primary objective was to quantify the ionizing photon production efficiency ($\xi_{\text{ion}}$) and identify the physical drivers behind its variation, a task that has historically been hindered by the limitations of broadband photometric estimations.

The methodological core of this research lies in the direct measurement of Balmer lines (such as H$\alpha$ and H$\beta$), which allows for a much more precise calculation of $\xi_{\text{ion}}$ compared to previous photometric-based approaches. To ensure the highest level of data integrity, the team implemented empirical corrections for potential underestimations of uncertainty within the JWST pipeline and cross-validated slit losses against broadband photometry, achieving an absolute flux accuracy within the 5–8% range.

Employing a Bayesian linear regression framework, the study effectively marginalized over all measurement uncertainties, thereby mitigating the impact of outliers and precisely determining the intrinsic scatter of the population. The statistical results revealed significant correlations between $\xi_{\text{ion}}$ and key observables, specifically the nebular emission line strengths $W_\lambda(\text{H}\alpha)$ and $W_\lambda(\text{