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1. An empirical reionization history model inferred from the low-redshift Lyman continuum survey and the star-forming galaxies at z > 8

   https://doi.org/10.1093/mnras/stad3483  

   Lin, Yu-Heng; Scarlata, Claudia; Williams, Hayley; Chen, Wenlei; Kelly, Patrick; Langeroodi, Danial; Hjorth, Jens; Chisholm, John; Koekemoer, Anton M; Zitrin, Adi; et al (November 2023, Monthly Notices of the Royal Astronomical Society)

   We present a new analysis of the rest-frame ultraviolet (UV) and optical spectra of a sample of three z > 8 galaxies discovered behind the gravitational lensing cluster RX J2129.4+0009. We combine these observations with z > 7.5 galaxies from the literature, for which similar measurements are available. As already pointed out in other studies, the high [O iii]λ5007/[O ii]λ3727 ratios (O32) and steep UV continuum slopes (β) are consistent with the values observed for low-redshift Lyman continuum emitters, suggesting that such galaxies contribute to the ionizing budget of the intergalactic medium. We construct a logistic regression model to estimate the probability of a galaxy being a Lyman continuum emitter based on the measured MUV, β, and O32. Using this probability and the UV luminosity function, we construct an empirical model that estimates the contribution of high-redshift galaxies to reionization. The preferred scenario in our analysis shows that at z ∼ 8, the average escape fraction of the galaxy population [i.e. including both LyC emitters (LCEs) and non-emitters] varies with MUV, with intermediate UV luminosity (−19 < MUV < −16) galaxies having larger escape fraction. Galaxies with faint UV luminosity (−16 < MUV < −13.5) contribute most of the ionizing photons. The relative contribution of faint versus bright galaxies depends on redshift, with the intermediate UV galaxies becoming more important over time. UV bright galaxies, although more likely to be LCEs at a given log(O32) and β, contribute the least of the total ionizing photon budget. 

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2. On the contribution of dwarf galaxies to reionization of the Universe

   https://doi.org/10.33232/001c.121193  

   Wu, Zewei; Kravtsov, Andrey (January 2024, The Open Journal of Astrophysics)

   We present estimates of the ultraviolet (UV) and Lyman continuum flux density contributed by galaxies of luminosities from to at redshifts 5≤z≤10 using a galaxy formation model that reproduces properties of local dwarf galaxies down to the luminosities of the ultra-faint satellites. We characterize the UV luminosity function (LF) of galaxies and their abundance as a function of the ionizing photon emission rate predicted by our model and present accurate fitting functions describing them. Although the slope of the LF becomes gradually shallower with decreasing luminosity due to feedback-driven outflows, the UV LF predicted by the model remains quite steep at the luminosities . After reionization, the UV LF flattens at due to UV heating of intergalactic gas. However, before reionization, the slope of the LF remains steep and approximately constant from to . We show that for a constant ionizing photon escape fraction the contribution of faint galaxies with to the UV flux and ionizing photon budget is ≈40−60% at z>7 and decreases to ≈20% at z=6. Before reionization, even ultra-faint galaxies of contribute ≈10−25% of ionizing photons. If the escape fraction increases strongly for fainter galaxies, the contribution of galaxies before reionization increases to ≈60−75%. Our results imply that dwarf galaxies fainter than , beyond the James Webb Space Telescope limit, contribute significantly to the UV flux density and ionizing photon budget before reionization alleviating requirements on the escape fraction of Lyman continuum photons. 

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3. Delayed Photons from Binary Evolution Help Reionize the Universe

   https://doi.org/10.3847/1538-4357/abaefa  

   Secunda, Amy; Cen, Renyue; Kimm, Taysun; Götberg, Ylva; de Mink, Selma E. (September 2020, The Astrophysical Journal)

   Abstract High-resolution numerical simulations including feedback and aimed at calculating the escape fraction (f_esc) of hydrogen-ionizing photons often assume stellar radiation based on single-stellar population synthesis models. However, strong evidence suggests the binary fraction of massive stars is ≳70%. Moreover, simulations so far have yielded values off_escfalling only on the lower end of the ∼10%–20% range, the amount presumed necessary to reionize the universe. Analyzing a high-resolution (4 pc) cosmological radiation-hydrodynamic simulation, we study howf_escchanges when we include two different products of binary stellar evolution—stars stripped of their hydrogen envelopes and massive blue stragglers. Both produce significant amounts of ionizing photons 10–200 Myr after each starburst. We find the relative importance of these photons to be amplified with respect to escaped ionizing photons, because peaks in star formation rates (SFRs) andf_escare often out of phase by this 10–200 Myr. Additionally, low-mass, bursty galaxies emit Lyman continuum radiation primarily from binary products when SFRs are low. Observations of these galaxies by the James Webb Space Telescope could provide crucial information on the evolution of binary stars as a function of redshift. Overall, including stripped stars and massive blue stragglers increases our photon-weighted mean escape fraction () by ∼13% and ∼10%, respectively, resulting in. Our results emphasize that using updated stellar population synthesis models with binary stellar evolution provides a more sound physical basis for stellar reionization. 

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4. Reionization after JWST : a photon budget crisis?

   https://doi.org/10.1093/mnrasl/slae086  

   Muñoz, Julian B; Mirocha, Jordan; Chisholm, John; Furlanetto, Steven R; Mason, Charlotte (September 2024, Monthly Notices of the Royal Astronomical Society: Letters)

   ABSTRACT New JWST observations are revealing the first galaxies to be prolific producers of ionizing photons, which we argue gives rise to a tension between different probes of reionization. Over the last two decades, a consensus has emerged where star-forming galaxies are able to generate enough photons to drive reionization, given reasonable values for their number densities, ionizing efficiencies $$\xi _{\rm ion}$$ (per unit ultraviolet luminosity), and escape fractions $$f_{\rm esc}$$. However, some new JWST observations infer high values of $$\xi _{\rm ion}$$ during reionization and an enhanced abundance of earlier ($$z\gtrsim 9$$) galaxies, dramatically increasing the number of ionizing photons produced at high z. Simultaneously, recent low-z studies predict significant escape fractions for faint reionization-era galaxies. Put together, we show that the galaxies we have directly observed ($$M_{\rm UV} < -15$$) not only can drive reionization, but would end it too early. That is, our current galaxy observations, taken at face value, imply an excess of ionizing photons and thus a process of reionization in tension with the cosmic microwave background and Lyman-$$\alpha$$ forest. Considering galaxies down to $$M_{\rm UV}\approx -11$$, below current observational limits, only worsens this tension. We discuss possible avenues to resolve this photon budget crisis, including systematics in either theory or observations. 

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5. A GLIMPSE into the very faint end of the H β +[O III] λλ 4960,5008 luminosity function at z ∼ 7 – 9 behind Abell S1063

   https://doi.org/10.1051/0004-6361/202556868  

   Korber, Damien; Chemerynska, Iryna; Furtak, Lukas J; Atek, Hakim; Endsley, Ryan; Schaerer, Daniel; Chisholm, John; Kokorev, Vasily; Saldana-Lopez, Alberto; Adamo, Angela; et al (April 2026, Astronomy & Astrophysics)

   We used the ultra-deep GLIMPSE JWST/NIRCam survey to constrain the faint end of the [O III]+Hβluminosity function (LF) down to 10³⁹erg s⁻¹atz ∼ 7 − 9 behind the lensed Hubble Frontier Field galaxy cluster Abell S1063. We applied a spectral energy distribution fitting on a Lyman-break galaxy selected sample of 164 lensed galaxies and measured their combined Hβ+[O III]λλ4960, 5008 flux to build the emission line LF. We found a [O III]+HβLF with a faint-end slope (α= −1.78^+0.06_−0.06forz = 7 − 8 andα= −1.55^+0.11_−0.11forz = 8 − 9), which is flatter than the UV LF at similar redshifts (α ≤ −2) and suggests a lower number density of weak [O III]+Hβemitting galaxies at fixedM_UV. We analysed several possible explanations: (i) a decrease in the [O III]+Hβ-to-UV ratio due to bursty star formation histories (SFHs), (ii) the effect of metallicity on the [O III]-to-Hβratio, or (iii) signs of a faint-end turnover in the UV LF. Under the assumption of an evolving [O III]-to-Hβratio, we separated the contribution of [O III]λ5008 and Hβand obtained a flatter [O III]λ5008 LF (α= −1.66^+0.05_−0.05forz = 7 − 8 andα= −1.45^+0.09_−0.10forz = 8 − 9) but steeper HβLF ($$ \alpha=-1.95_{-0.08}^{+0.08} $$ forz = 7 − 8 andα= −1.68^+0.13_−0.14forz = 8 − 9). The combination of a decreasing metallicity and bursty SFH can reconcile the observed differences between the UV and [O III]+HβLF. By converting this LF into the ionising photon-production rateṄ_ion, we show that galaxies withL_Hα ≥ 10³⁹erg s⁻¹, that is, with a star formation rate (SFR) (Hα)  ≥  5 × 10⁻³M_⊙yr⁻¹) cause 31%−90% and 46%−156% of the ionising photon budget (at 7 < z < 8 and 8 < z < 9), when we assume a constant escape fraction of Lyman-continuum photon (f_esc = 0.14). The shape of the LF further shows the negligible contribution of faint galaxies to theṄ_ion. Additionally, we derived the cosmic star formation rate density (SFRD), finding results consistent with previous estimates. However, the sensitivity of GLIMPSE to lower SFRs reinforces the conclusion that very faint galaxies contribute very little toṄ_ionand the SFRD. Our results suggests that GLIMPSE has detected the bulk of the total [O III]+Hβemission from star-forming galaxies, and that galaxies below our detection limits are likely minor contributors to cosmic re-ionisation. 

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