The <i>JWST</i> EXCELS survey: A spectroscopic investigation of the ionizing properties of star-forming galaxies at 1&amp;lt;z&amp;lt;8

ABSTRACT 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 $1\lesssim z \lesssim 8$, observed with ultra-deep medium-resolution spectroscopy from the JWST/NIRSpec EXCELS survey, we perform a statistical analysis of their ionizing photon production efficiencies ($\xi _\mathrm{ion}$). We consider $\xi _\mathrm{ion}$, accurately measured with spectroscopic Balmer line measurements, in relation to a number of key galaxy observables including; nebular emission line equivalent widths ($W_\lambda (\mathrm{H\alpha })$ and $W_\lambda$([O iii])), UV luminosity ($M_\mathrm{UV}$) and spectral slope ($\beta _\mathrm{UV}$), as well as dust attenuation ($E(B-V)_\mathrm{neb}$) and redshift. Implementing a Bayesian linear regression methodology, we fit $\xi _\mathrm{ion}$ against the principal observables while fully marginalizing over all measurement uncertainties, mitigating against the impact of outliers and determining the intrinsic scatter. Significant relations between $\xi _\mathrm{ion}$ and $W_\lambda (\mathrm{H\alpha })$, $W_\lambda$([O iii]) and $\beta _\mathrm{UV}$ are recovered. Moreover, the weak trends with $M_\mathrm{UV}$ and redshift can be fully explained by the remaining property dependencies. Expanding our analysis to multivariate regression, we determine that $W_\lambda (\mathrm{H\alpha })$ or $W_\lambda$([O iii]), along with $\beta _\mathrm{UV}$ and $E(B-V)_\mathrm{neb}$, are the most important observables for accurately predicting $\xi _\mathrm{ion}$. The latter identifies the most common outliers as SFGs with relatively high $E(B-V)_\mathrm{neb}\gtrsim 0.5$, possibly indicative of intense obscured star-formation or strong differential attenuation. Combining these properties enable $\xi _\mathrm{ion,0}$ to be inferred with an accuracy of $\simeq 0.15\,$ dex, with a population intrinsic scatter of $\sigma _\mathrm{int}\simeq 0.035\,$ dex.