[论文解读] CEERS: Diversity of Lyman-Alpha Emitters during the Epoch of Reionization

Intae Jung, Steven L. Finkelstein+30 more|arXiv (Cornell University)|Apr 11, 2023

Galaxies: Formation, Evolution, Phenomena被引用 23

一句话总结

本文分析 JWST/NIRSpec 光谱的三个 z~7.5 的 LAEs，表征它们的 ISM 属性、Lyα 发射和推断的电离气泡大小，揭示再电离时期 Lyα 逃逸情景的多样性。

ABSTRACT

We analyze rest-frame ultraviolet to optical spectra of three $z\simeq7.47$ - $7.75$ galaxies whose Ly$α$-emission lines were previously detected with Keck/MOSFIRE observations, using the JWST/NIRSpec observations from the Cosmic Evolution Early Release Science (CEERS) survey. From NIRSpec data, we confirm the systemic redshifts of these Ly$α$ emitters, and emission-line ratio diagnostics indicate these galaxies were highly ionized and metal poor. We investigate Ly$α$ line properties, including the line flux, velocity offset, and spatial extension. For the one galaxy where we have both NIRSpec and MOSFIRE measurements, we find a significant offset in their flux measurements ($\sim5 imes$ greater in MOSFIRE) and a marginal difference in the velocity shifts. The simplest interpretation is that the Ly$α$ emission is extended and not entirely encompassed by the NIRSpec slit. The cross-dispersion profiles in NIRSpec reveal that Ly$α$ in one galaxy is significantly more extended than the non-resonant emission lines. We also compute the expected sizes of ionized bubbles that can be generated by the Ly$α$ sources, discussing viable scenarios for the creation of sizable ionized bubbles ($>$1 physical Mpc). The source with the highest-ionization condition is possibly capable of ionizing its own bubble, while the other two do not appear to be capable of ionizing such a large region, requiring additional sources of ionizing photons. Therefore, the fact that we detect Ly$α$ from these galaxies suggests diverse scenarios on escape of Ly$α$ during the epoch of reionization. High spectral resolution spectra with JWST/NIRSpec will be extremely useful for constraining the physics of patchy reionization.

研究动机与目标

• Investigate rest-frame UV to optical spectra of z~7.47–7.75 Lyα emitters (LAEs) detected by Keck/MOSFIRE to understand ISM conditions and Lyα escape during reionization.
• Assess emission-line diagnostics to infer ionization, metallicity, and dust properties of these early galaxies.
• Estimate ionized bubble sizes around Lyα sources to evaluate their role in creating channels for Lyα transmission through the IGM.
• Compare Lyα measurements between JWST/NIRSpec and ground-based MOSFIRE to assess spatial/extinction effects on flux and velocity offsets.

提出的方法

• Use JWST/NIRSpec MR and PRISM spectra to detect emission lines such as Hβ, [O III], and [O II], and measure systemic redshifts.
• Fit emission lines with Gaussian or asymmetric Gaussian profiles; derive line fluxes after correcting for instrumental broadening.
• Compute line-ratio diagnostics (O3, O32, RO3, R23, Ne3O2) to infer ionization state, electron temperature, and metallicity.
• Derive Te-based metallicities using [O III] 4363 and RO3, and estimate Z/Z⊙ and 12+log(O/H).
• Estimate ionizing photon production efficiency (ξion) from Hβ and UV luminosity; infer LyC escape implications via indirect indicators.
• Compare Lyα fluxes and velocity offsets between NIRSpec and MOSFIRE, accounting for slit losses and spatial extension of Lyα emission.
• Model potential ionized bubble sizes around LAEs to discuss whether individual sources can ionize >1 phys Mpc regions.

实验结果

研究问题

• RQ1 What are the ISM properties (ionization, metallicity, temperature, dust) of z~7.5 LAEs as revealed by JWST/NIRSpec emission lines?
• RQ2 How does Lyα emission behave in these galaxies when observed with JWST versus ground-based spectroscopy, including fluxes and velocity offsets?
• RQ3 What are the implied sizes of ionized bubbles around these LAEs, and can the sources sustain self-driven ionized regions during reionization?
• RQ4 Do these LAEs indicate diverse scenarios for Lyα escape and reionization patchiness at z>7?

主要发现

• Two LAEs (z8_13573 and z8_69492) show high O32 ratios (6.84 and 19.30) indicative of extreme ionization conditions.
• Ionization parameters are log U (relative to c) of −2.15+0.13−0.12 and −1.76+0.22−0.19, at the high end for z≲6 galaxies and consistent with z>7 objects.
• Te-based oxygen abundances are 12+log(O/H)=7.50+0.09−0.09 and 7.77+0.01−0.01, corresponding to Z/Z⊙≈0.07 and 0.12.
• Measured Ne3O2 values are 0.49+0.09−0.08 and 1.03+0.35−0.24, placing the galaxies in high-ionization, metal-poor regimes in OHNO-style diagrams.
• OIII] 4363 detections yield electron temperatures consistent with low metallicities for the two MR-grating sources.
• Lyα fluxes from MOSFIRE are 1.23±0.18 and 3.13±0.65 ×10−17 erg s−1 cm−2 with velocity offsets from systemic redshifts of −165±72 km s−1 and 142±142 km s−1 (and −̄, 424+104−290 km s−1 from NIRSpec for z8_69492).
• NIRSpec Lyα fluxes are significantly smaller than MOSFIRE measurements, suggesting slit losses due to extended Lyα emission.
• The Lyα emission appears spatially extended in at least one galaxy, and the three LAEs show diverse implications for Lyα escape and ionized-bubble requirements during reionization.