We explore the production and escape of ionizing photons in young galaxies by investigating the ultraviolet and optical emission-line properties of models of ionizationbounded and density-bounded H regions, active-galactic-nucleus (AGN) narrow-line regions and radiative shocks computed all using the same physically-consistent description of element abundances and depletion on to dust grains down to very low metallicities. We compare these models with a reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC) leakers at various redshifts, which allows the simultaneous exploration of more spectral diagnostics than typically available at once for individual subsamples. We confirm that current single-and binary-star population synthesis models do not produce hard-enough radiation to account for the high-ionization emission of the most metal-poor galaxies. Introducing either an AGN or radiative-shock component brings models into agreement with observations. A published model including X-ray binaries is an attractive alternative to reproduce the observed rise in He λ4686/Hβ ratio with decreasing oxygen abundance in metal-poor star-forming galaxies, but not the high observed He λ4686/Hβ ratios of galaxies with large EW(Hβ). A source of harder ionizing radiation appears to be required in these extreme objects, such as an AGN or radiative-shock component, perhaps linked to an initial-mass-function bias toward massive stars at low metallicity. This would also account for the surprisingly high [O ]/[O ] ratios of confirmed LyC leakers relative to ionization-bounded models. We find no simple by-eye diagnostic of the nature of ionizing sources and the escape of LyC photon, which require proper simultaneous fits of several lines to be discriminated against. galactic nuclei (AGN), arising from gas accretion onto primordial black holes; radiative shocks, induced by large-scale gas flows; and the leakage of Lyman-continuum (LyC) photons through a porous interstellar medium (ISM), contributing to reionization (see, e.g., the review by Stark 2016). In waiting for advent of the James Webb Space Telescope (JWST), which will enable deep rest-frame ultraviolet and optical emission-line spectroscopy of galaxies into the reionization era at redshifts z ∼ 10-15, more nearby metal-poor galaxies approaching the properties of primeval galaxies offer a useful laboratory in which to test our ability to interpret emission-line spectra.

Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N1500 Å ≳ 5/resel), high-resolution (R ∼ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M ⋆(M ⊙) < 10.1), star formation rate (−2.0 < log SFR (M ⊙ yr−1) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O32 < 38.0), reddening (0.02 < E(B − V) < 0.67), and nebular density (10 < n e (cm−3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

We describe new JWST/NIRSpec observations of galaxies at 𝑧 7 taken as part of the CEERS survey of the EGS field. Previous observations of this area have revealed associations of Ly𝛼 emitters at redshifts (𝑧 = 7.5, 7.7, 8.7) where the IGM is thought mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 𝑧 7 galaxies with robust redshifts in the CEERS dataset, including 10 in the Ly𝛼 associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 = 17.84 and Ne3O2 = 0.89) and metallicity (12 + log [O/H] = 7.84) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six 𝑧 7 Ly𝛼 emitting galaxies in the sample. Each have hard ionizing spectra indicating that their visibility is likely enhanced by efficient ionizing photon production. Ly𝛼 velocity offsets are found to be very large ( 300 km s −1 ), likely also contributing to their detectability. We find that Ly𝛼 in 𝑧 7 galaxies is 6 − 12× weaker than in lower redshift samples with matched rest-frame optical spectral properties. If the bubbles around the Ly𝛼 emitters are relatively small ( 0.5 − 1 pMpc), we may expect such significant attenuation of Ly𝛼 in these ionized regions. We discuss several other effects that may contribute to weaker Ly𝛼 emission at 𝑧 7. Deep spectroscopy of fainter galaxies in the vicinity of the Ly𝛼 emitters will better characterize the physical scale of the ionized bubbles in this field.