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Reionization is thought to be driven by faint star-forming galaxies, but characterizing this population has long remained very challenging. Here, we utilize deep nine-band JADES (JWST Advanced Deep Extragalactic Survey)/NIRCam (Near-Infrared Camera) imaging to study the star-forming and ionizing properties of 756 |$z\sim 6-9$| galaxies, including hundreds of very ultraviolet (UV)-faint objects (⁠|$M_\mathrm{UV}\gt -18$|⁠). The faintest (⁠|$m\sim 30$|⁠) galaxies in our sample typically have stellar masses of |$M_\ast \sim (1-3)\times 10^7\ \mathrm{ M}_\odot$| and young light-weighted ages (⁠|$\sim$| 50 Myr), though some show strong Balmer breaks implying much older ages (⁠|$\sim$| 500 Myr). We find no evidence for extremely massive galaxies (⁠|$\gt 3\times 10^{10}\ \mathrm{ M}_\odot$|⁠) in our sample. We infer a strong (factor |$\gt $| 2) decline in the typical [O  iii ] |$+$| H  |$\beta$| equivalent widths (EWs) towards very faint |$z\sim 6-9$| galaxies, yet a weak UV luminosity dependence on the H  |$\alpha$| EWs at |$z\sim 6$|⁠. We demonstrate that these EW trends can be explained if fainter galaxies have systematically lower metallicities as well as more recently declining star formation histories relative to the most UV-luminous galaxies. Our data provide evidence that the brightest galaxies are frequently experiencing a recent strong upturn in star formation rate. We also discuss how the EW trends may be influenced by a strong correlation between |$M_\mathrm{UV}$| and Lyman continuum escape fraction. This alternative explanation has dramatically different implications for the contribution of galaxies along the luminosity function to cosmic reionization. Finally, we quantify the photometric overdensities around two |$z\,\gt\,7$| strong Ly  |$\alpha$| emitters. One Ly  |$\alpha$| emitter lies close to a strong photometric overdensity, while the other shows no significant nearby overdensity, perhaps implying that not all strong |$z\,\gt\, 7$| Ly  |$\alpha$| emitters reside in large ionized bubbles. [ABSTRACT FROM AUTHOR]

Nearly a decade ago, we began to see indications that reionization-era galaxies power hard radiation fields rarely seen at lower redshift. Most striking were detections of nebular C  iv emission in what appeared to be typical low-mass galaxies, requiring an ample supply of 48 eV photons to triply ionize carbon. We have obtained deep JWST/NIRSpec R = 1000 spectroscopy of the two z > 6 C  iv -emitting galaxies known prior to JWST. Here, we present a rest-UV to optical spectrum of one of these two systems, the multiply-imaged z = 6.1 lensed galaxy RXCJ2248-ID. NIRCam imaging reveals two compact (<22 pc) clumps separated by 220 pc, with one comprising a dense concentration of massive stars (>10 400 M⊙ yr−1 kpc−2) formed in a recent burst. We stack spectra of 3 images of the galaxy (J = 24.8–25.9), yielding a very deep spectrum providing a high-S/N template of strong emission line sources at z > 6. The spectrum reveals narrow high-ionization lines (He  ii , C  iv , N  iv ]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([O  iii ] EW = 2800 Å), albeit with weak emission from low-ionization transitions ([O  iii ]/[O  ii ] = 184). The electron density is found to be very high (6.4–31.0 × 104 cm−3) based on three UV transitions. The ionized gas is metal poor (⁠|$12+\log (\rm O/H)=7.43^{+0.17}_{-0.09}$|⁠), yet highly enriched in nitrogen (⁠|$\log (\rm N/O)=-0.39^{+0.11}_{-0.10}$|⁠). The spectrum appears broadly similar to that of GNz11 at z = 10.6, without showing the same AGN signatures. We suggest that the hard radiation field and rapid nitrogen enrichment may be a short-lived phase that many z > 6 galaxies go through as they undergo strong bursts of star formation. We comment on the potential link of such spectra to globular cluster formation. [ABSTRACT FROM AUTHOR]

The |$\rm {H}\alpha$| -to-UV luminosity ratio (⁠|$L(\text{H}\alpha)/L(\rm UV)$|⁠) is often used to probe bursty star formation histories (SFHs) of star-forming galaxies and it is important to validate it against other proxies for burstiness. To address this issue, we present a statistical analysis of the resolved distribution of star formation rate surface density (ΣSFR) as well as stellar age and their correlations with the globally measured |$L(\text{H}\alpha)/L(\rm UV)$| for a sample of 310 star-forming galaxies in two redshift bins of 1.37 < z < 1.70 and 2.09 < z < 2.61 observed by the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use the multiwaveband CANDELS/3D-HST imaging of MOSDEF galaxies to construct ΣSFR and stellar age maps. We analyse the composite rest-frame far-ultraviolet spectra of a subsample of MOSFIRE Deep Evolution Field (MOSDEF) targets obtained by the Keck Low Resolution Imager and Spectrometer (LRIS), which includes 124 star-forming galaxies (MOSDEF-LRIS) at redshifts 1.4 < z < 2.6, to examine the average stellar population properties, and the strength of age-sensitive far-ultraviolet spectral features in bins of |$L(\text{H}\alpha)/L(\rm UV)$|⁠. Our results show no significant evidence that individual galaxies with higher |$L(\text{H}\alpha)/L(\rm UV)$| are undergoing a burst of star formation based on the resolved distribution of ΣSFR of individual star-forming galaxies. We segregate the sample into subsets with low and high |$L(\text{H}\alpha)/L(\rm UV)$|⁠. The high- |$L(\text{H}\alpha)/L(\rm UV)$| subset exhibits, on average, an age of |$\log [\rm {Age/yr}]$|  = 8.0, compared to |$\log [\rm {Age/yr}]$| = 8.4 for the low- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies, though the difference in age is significant at only the 2σ level. Furthermore, we find no variation in the strengths of Si  iv λλ1393, 1402 and C  iv λλ1548, 1550 P-Cygni features from massive stars between the two subsamples, suggesting that the high- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies are not preferentially undergoing a burst compared to galaxies with lower |$L(\text{H}\alpha)/L(\rm UV)$|⁠. On the other hand, we find that the high- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies exhibit, on average, more intense He  ii λ1640 emission, which may possibly suggest the presence of a higher abundance of high-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

Spitzer /Infrared Array Camera (IRAC) imaging has revealed that the brightest z ∼ 7−8 galaxies often exhibit young ages and strong nebular line emission, hinting at high ionizing efficiency among early galaxies. However, IRAC's limited sensitivity has long hindered efforts to study the fainter, more numerous population often thought largely responsible for reionization. Here, we use Cosmic Evolution Early Release Science (CEERS) JWST /NIRCam data to characterize 116 ultraviolet (UV)-faint (median M UV = −19.5) z ∼ 6.5−8 galaxies. The spectral energy distributions are typically dominated by young (∼10–50 Myr), low-mass (M * ∼ 108 M⊙) stellar populations, and we find no need for extremely high stellar masses (∼1011 M⊙). Considering previous studies of UV-bright (M UV ∼ −22) z ∼ 7−8 galaxies, we find evidence for a strong (5–10 times) increase in specific star formation rate (sSFR) toward lower luminosities (median sSFR = 103 Gyr−1 in CEERS). The larger sSFRs imply a more dominant contribution from OB stars in the relatively numerous UV-faint population, perhaps suggesting that these galaxies are very efficient ionizing agents (median ξion = 1025.7 erg−1 Hz). In spite of the much larger sSFRs, we find little increase in [O  iii ] + H  β equivalent widths towards fainter M UV (median ≈780  |$\mathrm{\mathring{A}}$|⁠). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities (≲3 per cent Z ⊙) where [O  iii ] emission is suppressed. Alternatively, high ionizing photon escape fractions or bursty star formation histories can also weaken the nebular lines in a subset of our sample. While the majority of galaxies in our sample are very blue (median β  = −2.0), we identify a significant tail of very dusty galaxies (β ∼ −1) at ≈0.5 |$L_\mathrm{UV}^\ast$| which may contribute significantly to the z ∼ 7−8 star formation rate density. [ABSTRACT FROM AUTHOR]

We use the large spectroscopic data set of the MOSFIRE Deep Evolution Field survey to investigate some of the key factors responsible for the elevated ionization parameters (U) inferred for high-redshift galaxies, focusing in particular on the role of star-formation-rate surface density (ΣSFR). Using a sample of 317 galaxies with spectroscopic redshifts z spec ≃ 1.9–3.7, we construct composite rest-frame optical spectra in bins of ΣSFR and infer electron densities, n e, using the ratio of the [O ii ] λ λ 3727, 3730 doublet. Our analysis suggests a significant (≃3 σ) correlation between n e and ΣSFR. We further find significant correlations between U and ΣSFR for composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences of U. The increase in n e—and possibly also the volume filling factor of dense clumps in H ii regions—with ΣSFR appear to be important factors in explaining the relationship between U and ΣSFR. Further, the increase in n e and SFR with redshift at a fixed stellar mass can account for most of the redshift evolution of U. These results suggest that the gas density, which sets n e and the overall level of star formation activity, may play a more important role than metallicity evolution in explaining the elevated ionization parameters of high-redshift galaxies. [ABSTRACT FROM AUTHOR]

We use medium-resolution JWST/NIRSpec observations from the Cosmic Evolution Early Release Science Survey to place the first constraints on dust attenuation and star formation based on Paschen lines for a sizable sample of 63 galaxies at redshifts z = 1.0–3.1. Our analysis indicates strong correlations between the Balmer decrement, H α /H β, and line ratios that include Paschen lines (i.e., Pa α /H β, Pa β /H β, and the Paschen decrement, Pa α /Pa β), suggesting that the former is sensitive to the overall dust obscuration toward H ii regions in high-redshift galaxies. The line ratios are used to derive nebular reddening, E (B − V)neb, and star formation rates (SFRs). There is marginal evidence that the SFRs deduced from Paschen lines may exceed by ≈25% those derived from Balmer lines alone, suggesting the presence of star formation that is optically thick in Balmer lines, though deeper observations are needed to confirm this result. Using the Paschen-line constraints on the bolometric SFRs, we reevaluate the relationship between dust obscuration and UV spectral slope, and find a reddening of the UV continuum that, on average, follows the SMC extinction curve. This analysis highlights the need for deeper spectroscopy of more representative samples to evaluate nebular dust attenuation and bolometric SFRs in high-redshift galaxies, and their relationship to the reddening of the UV continuum. [ABSTRACT FROM AUTHOR]

With JWST , new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of z ≳ 7 galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual z ≳ 8 objects could be detected with Spitzer , making it difficult to robustly constrain activity at z ≳ 10. Here, we leverage the greatly improved rest-optical sensitivity of JWST at z ≳ 8 to constrain the ages of seven UV-bright (⁠|$M_{\rm uv}\lesssim -19.5$|⁠) galaxies selected to lie at z ∼ 8.5–11, then investigate implications for z ≳ 15 star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright z ∼ 8.5–11 galaxies. We find a median age of ∼20 Myr, younger than that inferred at z ∼ 7 with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only ∼3 per cent of bright z ∼ 8.5–11 galaxies would be similarly luminous at z ≳ 15, implying that the number density of bright galaxies declines by at least an order of magnitude between z ∼ 8.5–11 and |$z \sim 15$|⁠. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright z ∼ 15 galaxies are observed during a burst of star formation. [ABSTRACT FROM AUTHOR]

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimetre wavelengths to resolve. Here, we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [C  ii ] and dust emission in UV-selected galaxies at z ∼ 7, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation f obs = SFRIR/SFRUV+IR depends on stellar mass, similar to what is observed at lower redshift, and ranges from f obs ≈ 0.3 − 0.6 for galaxies with log10(M ⋆/M⊙) = 9.4–10.4. We further adopt the z ∼ 7 stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between 3 ≲ z ≲ 7, with dust-obscured star formation still contributing |${\sim}30{{\ \rm per\ cent}}$| at z ∼ 7. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization. [ABSTRACT FROM AUTHOR]

We present ultradeep Keck/MOSFIRE rest-optical spectra of two star-forming galaxies at z = 2.18 in the COSMOS field with bright emission lines, representing more than 20 hr of total integration. The fidelity of these spectra enabled the detection of more than 20 unique emission lines for each galaxy, including the first detection of the auroral [O ii ] λλ 7322, 7332 lines at high redshift. We use these measurements to calculate the electron temperature in the low-ionization O+ zone of the ionized interstellar medium and derive abundance ratios of O/H, N/H, and N/O using the direct method. The N/O and α /Fe abundance patterns of these galaxies are consistent with rapid formation timescales and ongoing strong starbursts, in accord with their high specific star formation rates. These results demonstrate the feasibility of using auroral [O ii ] measurements for accurate metallicity studies at high redshift in a higher-metallicity and lower-excitation regime previously unexplored with the direct method in distant galaxies. These results also highlight the difficulty in obtaining the measurements required for direct-method metallicities from the ground. We emphasize the advantages that the JWST/NIRSpec instrument will bring to high-redshift metallicity studies, where the combination of increased sensitivity and uninterrupted wavelength coverage will yield more than an order of magnitude increase in efficiency for multiplexed auroral-line surveys relative to current ground-based facilities. Consequently, the advent of JWST promises to be the beginning of a new era of precision chemical abundance studies of the early universe at a level of detail rivaling that of local galaxy studies. [ABSTRACT FROM AUTHOR]

We present specific star formation rates (sSFRs) for 40 ultraviolet (UV)-bright galaxies at z ∼ 7–8 observed as part of the Reionization Era Bright Emission Line Survey (REBELS) Atacama Large Millimeter/submillimeter Array (ALMA) large programme. The sSFRs are derived using improved star formation rate (SFR) calibrations and spectral energy distribution (SED)-based stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [C  ii ]-based spectroscopic redshifts. The median sSFR of the sample is |$18_{-5}^{+7}$|  Gyr−1, significantly larger than literature measurements lacking constraints in the FIR, reflecting the larger obscured SFRs derived from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly dependent on the assumed star formation history in reionization-era galaxies. When large sSFR galaxies (a population that is common at z > 6) are modelled with non-parametric star formation histories, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [C  ii ] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses capable of accommodating an old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties among derived parameters, we find that sSFRs increase rapidly toward higher redshifts for massive galaxies (9.6 < log (M */M⊙) < 9.8), evolving as (1 + z)1.7 ± 0.3, broadly consistent with expectations from the evolving baryon accretion rates. [ABSTRACT FROM AUTHOR]

The combination of the MOSDEF and KBSS-MOSFIRE surveys represents the largest joint investment of Keck/MOSFIRE time to date, with ∼3000 galaxies at 1.4 ≲ z ≲ 3.8, roughly half of which are at z ∼ 2. MOSDEF is photometric- and spectroscopic-redshift selected with a rest-optical magnitude limit, while KBSS-MOSFIRE is primarily selected based on rest-UV colours and a rest-UV magnitude limit. Analysing both surveys in a uniform manner with consistent spectral-energy-distribution (SED) models, we find that the MOSDEF z ∼ 2 targeted sample has higher median M * and redder rest U − V colour than the KBSS-MOSFIRE z ∼ 2 targeted sample, and smaller median SED-based SFR and sSFR (SFR(SED) and sSFR(SED)). Specifically, MOSDEF targeted a larger population of red galaxies with U − V and V − J ≥1.25, while KBSS-MOSFIRE contains more young galaxies with intense star formation. Despite these differences in the z ∼ 2 targeted samples, the subsets of the surveys with multiple emission lines detected and analysed in previous work are much more similar. All median host-galaxy properties with the exception of stellar population age – i.e. M *, SFR(SED), sSFR(SED), A V, and UVJ colours – agree within the uncertainties. Additionally, when uniform emission-line fitting and stellar Balmer absorption correction techniques are applied, there is no significant offset between both samples in the [O  iii ]λ5008/H β versus [N  ii ]λ6585/H α diagnostic diagram, in contrast to previously reported discrepancies. We can now combine the MOSDEF and KBSS-MOSFIRE surveys to form the largest z ∼ 2 sample with moderate-resolution rest-optical spectra and construct the fundamental scaling relations of star-forming galaxies during this important epoch. [ABSTRACT FROM AUTHOR]

We analyse the rest-optical emission-line ratios of z ∼ 1.5 galaxies drawn from the Multi-Object Spectrometer for Infra-Red Exploration Deep Evolution Field (MOSDEF) survey. Using composite spectra, we investigate the mass–metallicity relation (MZR) at z  ∼ 1.5 and measure its evolution to z = 0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z  ∼ 1.5 to z = 0 depends on stellar mass, evolving by |$\Delta \rm log(\rm O/H) \sim 0.25$| dex at M *< |$10^{9.75}\, \mathrm{M}_{\odot }$| down to |$\Delta \rm log(\rm O/H) \sim 0.05$| at M * ≳ |$10^{10.5}\, \mathrm{M}_{\odot }$|⁠. In contrast, the O3N2-based MZR shows a constant offset of |$\Delta \rm log(\rm O/H) \sim 0.30$| across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z  ∼ 1.5 sample. We investigated the secondary dependence of the MZR on star formation rate (SFR) by measuring correlated scatter about the mean M *-specific SFR and M *− |$\log (\rm O3N2)$| relations. We find an anticorrelation between |$\log (\rm O/H)$| and sSFR offsets, indicating the presence of a M *−SFR−Z relation, though with limited significance. Additionally, we find that our z  ∼ 1.5 stacks lie along the z = 0 metallicity sequence at fixed μ = log (M */ M ⊙) − 0.6 × |$\log (\rm SFR / M_{\odot } \, yr^{-1})$| suggesting that the z  ∼ 1.5 stacks can be described by the z = 0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [N  ii ]/H α scales with galaxy properties is crucial to accurately describe the effects of blended [N  ii ] and H α on redshift and H α fiux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope. [ABSTRACT FROM AUTHOR]

We analyse the rest-optical emission-line spectra of z ∼ 2.3 star-forming galaxies in the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. In investigating the origin of the well-known offset between the sequences of high-redshift and local galaxies in the [O  iii ]λ5008/H β versus [N  ii ]λ6585/H α ('[N  ii ] BPT') diagram, we define two populations of z ∼ 2.3 MOSDEF galaxies. These include the high population that is offset towards higher [O  iii ]λ5008/H β and/or [N  ii ]λ6585/H α with respect to the local SDSS sequence and the low population that overlaps the SDSS sequence. These two groups are also segregated within the [O   iii ]λ5008/H β versus [S  ii ]λλ6718,6733/H α and the [O  iii ]λλ4960,5008/[O  ii  ]λλ3727,3730 (O32) versus ([O   iii ]λλ4960,5008+[O  ii ]λλ3727,3730)/H β (R23) diagrams, which suggests qualitatively that star-forming regions in the more offset galaxies are characterized by harder ionizing spectra at fixed nebular oxygen abundance. We also investigate many galaxy properties of the split sample and find that the high sample is on average smaller in size and less massive, but has higher specific star formation rate (SFR) and SFR surface density values and is slightly younger compared to the low population. From Cloudy+BPASS photoionization models, we estimate that the high population has a lower stellar metallicity (i.e. harder ionizing spectrum) but slightly higher nebular metallicity and higher ionization parameter compared to the low population. While the high population is more α -enhanced (i.e. higher α /Fe) than the low population, both samples are significantly more α -enhanced compared to local star-forming galaxies with similar rest-optical line ratios. These differences must be accounted for in all high-redshift star-forming galaxies – not only those 'offset' from local excitation sequences. [ABSTRACT FROM AUTHOR]

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z ∼ 5. We combine new z ∼ 5 measurements of H  i Ly α and low- and high-ionization interstellar metal absorption features with comparable measurements at z ∼ 2–4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Ly α equivalent width, performing corrections to Ly α strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Ly α emission strength over the redshift range z ∼ 2–5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Ly α equivalent widths, we observe evolution at z ∼ 5 towards greater Ly α emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly α emission strength and low-ionization line strength at z ∼ 2−4 as primarily reflecting the radiative transfer of Ly α photons, this evolution at z ∼ 5 suggests a higher intrinsic production rate of Ly α photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly α emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Ly α emission evolve towards higher redshift. We conclude that increasing intrinsic Ly α photon production and strong detection of nebular C  iv emission (signalling lower metallicity) at z ∼ 5 indicate an elevated ionized photon production efficiency (ξion). [ABSTRACT FROM AUTHOR]

We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (λ ⩽ 912 Å), He I (λ ⩽ 504 Å), and He II (λ ⩽ 228 Å) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed WM-basic model atmospheres. The median LyC production efficiency is QLyC = (6 ± 2) × 1060 LyC photons per M☼ of star formation (range [3.1-9.4] × 1060), corresponding to a revised calibration of 1053.3 ± 0.2 photons s–1 per M☼ yr–1. Efficiencies in the helium continua are photons and photons at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z ≈ 7 is for fiducial values of the intergalactic medium (IGM) clumping factor CH ≈ 3 and LyC escape fraction fesc ≈ 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, CH, and fesc, in assessing whether IGM reionization was complete by z ≈ 7. Monte Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) ≈ 0.4-0.5 and F(912–)/F(912+) ≈ 0.4-0.7 in the far-UV (1500 Å), the LyC (900 Å), and at the Lyman edge (912 Å). These ratios can be used to calibrate the LyC escape fractions in starbursts. [ABSTRACT FROM AUTHOR]