The ionizing photon production efficiency of star-forming galaxies at z ∼ 4–10

Investigating the ionizing emission of star-forming galaxies and the escape fraction of ionizing photons is critical to understanding their contribution to reionization and their impact on the surrounding environment. The number of ionizing photons available to reionize the intergalactic medium (IGM) depends on not only the abundance of galaxies but also their efficiency in producing ionizing photons (ξion). This quantity is thus fundamental to quantify the role of faint versus bright sources in driving this process, as we must assess their relative contribution to the total ionizing emissivity. Aims. Our goal is to estimate the ξion using Balmer lines (Hα or Hβ) in a sample of 761 galaxies at 4 ≤ z ≤ 10 selected from different JWST spectroscopic surveys. We aim to determine the redshift evolution of ξion and the relation of ξion with the physical properties of the galaxies. Methods. We used the available HST and JWST photometry to perform a spectral energy distribution (SED) fitting in the sample to determine their physical properties and relate them with ξion. We used the BAGPIPES code for the SED fitting and assumed a delayed exponential model for the star formation history. We used the NIRSpec spectra from prism or grating configurations to estimate Balmer luminosities, and then constrained ξion values after dust correction. Results. We find a mean value of 1025.22 Hz erg−1 for ξion in the sample with an observed scatter of 0.42 dex. We find an increase in the median values of ξion with redshift from 1025.09 Hz erg−1 at z ∼ 4.18 to 1025.28 Hz erg−1 at z ∼ 7.14, confirming the redshift evolution of ξion found in other studies. Regarding the relation between ξion and physical properties, we find a decrease in ξion with increasing stellar mass, indicating that low-mass galaxies are efficient producers of ionizing photons. We also find an increase in ξion with increasing specific star formation rate (sSFR) and increasing UV absolute magnitude. This indicates that faint galaxies and galaxies with high sSFR are also efficient producers. We also investigated the relation of ξion with the rest-frame equivalent width (EW) of [OIII]λ5007 and find that galaxies with the higher EW([OIII]λ5007) are more efficient producers of ionizing photons, with the best fit leading to the relation log(ξion) = 0.43×log(EW[OIII])+23.99. Similarly, we find that galaxies with higher O32 = [OIII]λ5007/[OII]λλ3727,3729 and lower gas-phase metallicities (based on the R23 = ([OIII]λλ4959,5007+[OII]λλ3727,3729)/Hβ calibration) show higher ξion values.