2025ApJ...986...18Z

The topology of reionization and the environments where galaxies efficiently produce ionizing photons are key open questions. For the first time, we investigate the trend between ionizing photon production efficiency, ξ_ion, and galaxy overdensity, $\mathrm{log}(1+\delta )$. We analyze the ionizing properties of 79 galaxies between 1.0 < z < 5.2 using JWST NIRSpec medium-resolution spectra from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES) program. Among these, 67 galaxies have Hα coverage, spanning 1.0 < z < 3.1. The galaxy overdensity, $\mathrm{log}(1+\delta )$, is measured using the JADES photometric catalog, which covers the SMILES footprint. For the subset with Hα coverage, we find that $\mathrm{log}{\xi }_{{\rm{ion}}}$ is positively correlated with $\mathrm{log}(1+\delta )$, with a slope of $0.9{4}_{-0.46}^{+0.46}$. Additionally, the mean ξ_ion for galaxies in overdense regions ($\mathrm{log}(1+\delta )\gt 0.1$) is 2.43 times that of galaxies in lower density regions ($\mathrm{log}(1+\delta )\lt 0.1$). This strong trend is found to be independent of redshift evolution. Furthermore, our results confirm the robust correlations between ξ_ion and the rest-frame equivalent widths of the [O III] or Hα emission lines. Our results suggest that galaxies in high-density regions are efficient producers of ionizing photons.