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, and requires better predictive capabilities from current observations. Using a sample of N = 159 star-forming galaxies (SFGs) at 1 ≲ z ≲ 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 (ξion). We consider ξion, accurately measured with spectroscopic Balmer line measurements, in relation to a number of key galaxy observables including; nebular emission line equivalent widths (Wλ(Hα) and Wλ([O iii])), UV luminosity (MUV) and spectral slope (βUV), as well as dust attenuation (E(B − V)neb) and redshift. Implementing a Bayesian linear regression methodology, we fit ξion 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 ξion and Wλ(Hα), Wλ([O iii]) and βUV are recovered. Moreover, the weak trends with MUV and redshift can be fully explained by the remaining property dependencies. Expanding our analysis to multivariate regression, we determine that Wλ(Hα) or Wλ([O iii]), along with βUV and E(B − V)neb, are the most important observables for accurately predicting ξion. The latter identifies the most common outliers as SFGs with relatively high E(B − V)neb ≳ 0.5, possibly indicative of intense obscured star-formation or strong differential attenuation. Combining these properties enable ξion, 0 to be inferred with an accuracy of ≃ 0.15 dex, with a population intrinsic scatter of σint ≃ 0.035 dex.