We present the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [O iii] 88 µm line and rest-frame 90 µm dust continuum emission in a Y -dropout Lyman break galaxy (LBG), MACS0416 Y1, lying behind the Frontier Field cluster MACS J0416.1−2403. This [O iii] detection confirms the LBG with a spectroscopic redshift of z = 8.3118 ± 0.0003, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 µm flux density of 137 ± 26 µJy corresponds to a de-lensed total infrared (IR) luminosity of L IR = (1.7±0.3)×10 11 L if assuming a dust temperature of T dust = 50 K and an emissivity index of β = 1.5, yielding a large dust mass of 4×10 6 M . The ultraviolet-to-far IR spectral energy distribution modeling where the [O iii] emissivity model is incorporated suggests the presence of a young (τ age ≈ 4 Myr), star-forming (SFR ≈ 60 M yr −1 ), moderately metal-polluted (Z ≈ 0.2Z ) stellar component with a mass of M star = 3 × 10 8 M . An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in τ age ≈ 4 Myr, suggesting a pre-existing evolved stellar component with M star ∼ 3 × 10 9 M and τ age ∼ 0.3 Gyr as the origin of the dust mass.

Using four different suites of cosmological simulations, we generate synthetic spectra for galaxies with different Lyman continuum escape fractions (f esc ) at redshifts z ≈ 7-9, in the rest-frame wavelength range relevant for the James Webb Space Telescope (JWST) NIRSpec instrument. By investigating the effects of realistic star formation histories and metallicity distributions on the EW(Hβ)-β diagram (previously proposed as a tool for identifying galaxies with very high f esc ), we find that neither of these effects are likely to jeopardize the identification of galaxies with extreme Lyman continuum leakage. Based on our models, we expect that essentially all z ≈ 7-9 galaxies that exhibit restframe EW(Hβ) 30Å to have f esc > 0.5. Incorrect assumptions concerning the ionizing fluxes of stellar populations or the dust properties of z > 6 galaxies can in principle bias the selection, but substantial model deficiencies of this type should at the same time be evident from offsets in the observed distribution of z > 6 galaxies in the EW(Hβ)-β diagram compared to the simulated one. Such offsets would thereby allow JWST/NIRSpec measurements of these observables to serve as input for further model refinement.

Recent observations of galaxies at z 7, along with the low value of the electron scattering optical depth measured by the Planck mission, make galaxies plausible as dominant sources of ionizing photons during the epoch of reionization. However, scenarios of galaxy-driven reionization hinge on the assumption that the average escape fraction of ionizing photons is significantly higher for galaxies in the reionization epoch than in the local Universe. The NIRSpec instrument on the James Webb Space Telescope (JWST) will enable spectroscopic observations of large samples of reionization-epoch galaxies. While the leakage of ionizing photons will not be directly measurable from these spectra, the leakage is predicted to have an indirect effect on the spectral slope and the strength of nebular emission lines in the rest-frame ultraviolet and optical. Here, we apply a machine learning technique known as lasso regression on mock JWST/NIRSpec observations of simulated z = 7 galaxies in order to obtain a model that can predict the escape fraction from JWST/NIRSpec data. Barring systematic biases in the simulated spectra, our method is able to retrieve the escape fraction with a mean absolute error of ∆f esc ≈ 0.12 for spectra with S/N ≈ 5 at a rest-frame wavelength of 1500Å for our fiducial simulation. This prediction accuracy represents a significant improvement over previous similar approaches.

We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman-break galaxy at z = 7.15, B14-65666 (“Big Three Dragons”), which is an object detected in [O iii] 88 μm, [C ii] 158 μm, and dust continuum emission during the epoch of reionization. Our targets are the [N ii] 122 μm fine-structure emission line and the underlying 120 μm dust continuum. The dust continuum is detected with a ∼19σ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 μm, we obtain a best-fit dust temperature of 40 K (79 K) and an infrared luminosity of log 10 ( L IR / L ⊙ ) = 11.6 (12.1) at the emissivity index β = 2.0 (1.0). The [N ii] 122 μm line is not detected. The 3σ upper limit of the [N ii] luminosity is 8.1 × 107 L ⊙. From the [N ii], [O iii], and [C ii] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high (Z > 0.4 Z ⊙), the ionization parameter and hydrogen density are log 10 U ≃ − 2.7 ± 0.1 and n H ≃ 50–250 cm−3, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, N/O, is constrained to be subsolar. At Z < 0.4 Z ⊙, the allowed U drastically increases as the assumed metallicity decreases. For high ionization parameters, the N/O constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.

Resonant inelastic x-ray scattering spectra excited in the immediate vicinity of the core-level ionization thresholds of N2 have been recorded. Final states of well-resolved symmetry-selected Rydberg series converging to valence-level ionization thresholds with vibrational excitations are observed. The results are well described by a quasi-two-step model which assumes that the excited electron is unaffected by the radiative decay. This threshold dynamics simplifies the interpretation of resonant inelastic x-ray scattering spectra considerably and facilitates characterization of low-energy excited final states in molecular systems.