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Harvard ADS: The evolution of [OIII]+\rm{H}\beta equivalent width from \mathbf{z\simeq3-8}: implications for the production and escape of ionizing photons during reionization

Paper abstract: Accurately quantifying the ionizing photon production efficiency (\xi_\rm{ion}) of z>6 star-forming galaxies (SFGs) is necessary to understand their contribution to reionization. We investigate the ionizing properties of N=289 SFGs selected at z=6.9-7.6 from the JWST Cycle-1 imaging programmes; PRIMER and JADES. We use BAGPIPES to consistently infer the equivalent widths of their [OIII]+\rm{H\beta} emission lines (W_\lambda) and their physical properties. To supplement our high-redshift galaxies, we measure W_\lambda photometrically for a sample of N=253 z=3.2-3.6 SFGs selected from the VANDELS spectroscopic survey. Comparing these samples, we find a strong apparent redshift evolution in their median W_\lambda, increasing from W_\lambda =310\pm25\,\r{A} in VANDELS to W_\lambda =630\pm30\,\r{A} in our JWST-based sample. In the JWST sample at z>7, we find that W_\lambda correlates with both stellar mass and UV luminosity, with high-mass, M_{ UV}-faint galaxies producing systematically weaker emission lines. Moreover, we discover a departure from the standard log-normal shape of the W_\lambda distribution, characterised by a more pronounced tail at lower W_\lambda, consistent with increasingly bursty star formation. Using W_\lambda as a proxy for \xi_\rm{ion}, and UV spectral slope as a proxy for LyC escape fraction (f_\rm{esc}), we find a minority of galaxies with high \xi_\rm{ion} and f_\rm{esc} (e.g., \rm{log(\xi_{ion}/erg^{-1}Hz})~25.6 and f_\rm{esc}~0.15). However, we find that the LyC photon budget at z>7 is dominated by galaxies with more moderate output, close to the sample median of \rm{log(\xi_{ion}/erg^{-1}Hz})~25.3 and f_\rm{esc}~0.05. This is consistent with estimates for the number of LyC photons required to power reionization at z>7, with no evidence for over or under-production.