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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)

   ABSTRACT 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. 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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3. An uncontaminated measurement of the escaping Lyman continuum at z ∼ 3

   https://doi.org/10.1093/mnras/stab1374  

   Pahl, Anthony J; Shapley, Alice; Steidel, Charles C; Chen, Yuguang; Reddy, Naveen A (June 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Observations of reionization-era analogues at z ∼ 3 are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 z ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum region. We present a method of removing foreground contamination from our sample using high-resolution, multiband Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually detected Lyman continuum sources and 107 individually undetected sources, including a sample-averaged absolute escape fraction of fesc, abs = 0.06 ± 0.01 and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of 〈f900/f1500〉out = 0.040 ± 0.006, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between 〈f900/f1500〉out and Lyα equivalent width (Wλ(Ly$$\rm \alpha$$)) and a negative correlation between 〈f900/f1500〉out and UV luminosity. As in previous work, we interpret the relationship between 〈f900/f1500〉out and Wλ(Ly$$\rm \alpha$$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a Wλ(Ly$$\rm \alpha$$)-weighted 〈f900/f1500〉out to estimate an ionizing emissivity from star-forming galaxies at z ∼ 3 as ϵLyC ≃ 5.5 × 1024 erg s−1 Hz−1 Mpc−3. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at z ∼ 3 is comparable to that of active galactic nuclei. 

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4. Ly{\alpha} profile shape as an escape-fraction diagnostic at high redshift

   Pahl, A; Shapley, A; Steidel, C; Reddy, N; Chen, Y; Rudie, G (January 2024, ApJ)

   While the shape of the Lyα profile is viewed as one of the best tracers of ionizing-photon escape fraction (fesc) within low redshift (z~0.3) surveys of the Lyman continuum, this connection remains untested at high redshift. Here, we combine deep, rest-UV Keck/LRIS spectra of 80 objects from the Keck Lyman Continuum Spectroscopic Survey with rest-optical Keck/MOSFIRE spectroscopy in order to examine potential correlations between Lyα profile shape and the escape of ionizing radiation within z~3 star-forming galaxies. We measure the velocity separation between double-peaked Lyα emission structure (vsep), between red-side Lyα emission peaks and systemic (vLyα,red), and between red-side emission peaks and low-ionization interstellar absorption lines (vLyα−LIS). We find that the IGM-corrected ratio of ionizing to non-ionizing flux density is significantly higher in KLCS objects with lower vLyα,red. We find no significant trend between measures of ionizing-photon escape and vLyα−LIS. We compare our results to measurements of z~0.3 "Green Peas" from the literature and find that KLCS objects have larger vsep at fixed vLyα,red, larger fesc at fixed vLyα,red, and higher vLyα,red overall than z~0.3 analogs. We conclude that the Lyα profile shapes of our high-redshift sources are fundamentally different, and that measurements of profile shape such as vLyα,red map on to fesc in different ways. We caution against building reionization-era fesc diagnostics based purely on Lyα profiles of low-redshift dwarf galaxies. Tracing vsep, vLyα,red, and fesc in a larger sample of z~3 galaxies will reveal how these variables may be connected for galaxies at the epoch of reionization. 

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5. The connection between the escape of ionizing radiation and galaxy properties at z ∼ 3 in the Keck Lyman continuum spectroscopic survey

   https://doi.org/10.1093/mnras/stad774  

   Pahl, Anthony J.; Shapley, Alice; Steidel, Charles C.; Reddy, Naveen A.; Chen, Yuguang; Rudie, Gwen C.; Strom, Allison L. (March 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The connection between the escape fraction of ionizing radiation (fesc) and the properties of galaxies, such as stellar mass ($$\rm M_{\rm *}$$), age, star-formation rate (SFR), and dust content, are key inputs for reionization models, but many of these relationships remain untested at high redshift. We present an analysis of a sample of 96 $$z$$ ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey (KLCS). These galaxies have both sensitive Keck/LRIS spectroscopic measurements of the Lyman continuum (LyC) region, and multiband photometry that places constraints on stellar population parameters. We construct composite spectra from subsamples binned as a function of galaxy property and quantify the ionizing-photon escape for each composite. We find a significant anti-correlation between fesc and $$\rm M_{\rm *}$$, consistent with predictions from cosmological zoom-in simulations. We also find significant anti-correlation between fesc and E(B−V), encoding the underlying physics of LyC escape in our sample. We also find no significant correlation between fesc and either stellar age or specific SFR (= SFR/$$\rm M_{\rm *}$$), challenging interpretations that synchronize recent star formation and favorable conditions for ionizing escape. The galaxy properties now shown to correlate with fesc in the KLCS are Lyα equivalent width, UV Luminosity, $$\rm M_{\rm *}$$, SFR, and E(B−V), but not age or sSFR. This comprehensive analysis of galaxy properties and LyC escape at high redshift will be used to guide future models and observations of the reionization epoch. 

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