[Paper Review] JWST/NIRSpec Spectroscopy of $z=7-9$ Star Forming Galaxies with CEERS: New Insight into Bright Ly$α$ Emitters in Ionized Bubbles

Mengtao Tang, Daniel P. Stark+10 more|arXiv (Cornell University)|Jan 17, 2023

Galaxies: Formation, Evolution, Phenomena18 citations

TL;DR

The study analyzes JWST/NIRSpec spectra of 21 galaxies at z>7 from CEERS, revealing extreme ionization, very low metallicity, and large Lyα velocity offsets; Lyα is significantly weaker at z>7 compared to lower redshifts, implying small ionized bubbles or other attenuating factors.

ABSTRACT

We describe new JWST/NIRSpec observations of galaxies at $z\gtrsim7$ taken from the CEERS survey. Previous observations of this area have revealed associations of Ly$α$ emitters at redshifts ($z=7.5$, $7.7$, $8.7$) where the intergalactic medium (IGM) is thought to be mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 $z\gtrsim7$ galaxies with robust redshifts in the CEERS dataset, including 10 in the Ly$α$ associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 $=17.84$ and Ne3O2 $=0.89$, linear scale) and metallicity ($12+\log{( m{O/H})}=7.84$) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six $z\gtrsim7$ Ly$α$ emitters in the sample. Each has a hard ionizing spectrum indicating that their visibility is likely enhanced by efficient ionizing photon production. Ly$α$ velocity offsets are found to be very large ($\gtrsim300$ km s$^{-1}$), likely also contributing to their detectability. We find that Ly$α$ in $z\gtrsim7$ galaxies is $6-12 imes$ weaker than in lower redshift samples with matched rest-optical spectral properties. If the bubbles around the Ly$α$ emitters are relatively small ($\lesssim0.5-1$ pMpc), we may expect such significant attenuation of Ly$α$ in these ionized regions. We discuss several other effects that may contribute to weaker Ly$α$ emission at $z\gtrsim7$. Deep spectroscopy of fainter galaxies in the vicinity of the Ly$α$ emitters will better characterize the physical scale of the ionized bubbles in this field.

Motivation & Objective

• Characterize the rest-frame optical and UV emission lines of z>7 galaxies in CEERS to derive ionization, metallicity, and stellar population properties.
• Assess how Lyα visibility at z>7 relates to surrounding ionized bubbles and the ionizing photon production efficiency of bright galaxies.
• Determine whether bright Lyα emitters at z>7 reside in large ionized regions and identify physical processes driving Lyα detectability in the reionization era.
• Quantify ionizing photon production efficiency (ξion), Lyα velocity offsets, and Lyα escape fractions for a spectroscopically confirmed sample at z>7.

Proposed method

• Utilize JWST/NIRSpec spectroscopy from the CEERS program in EGS with MR (R~1000) and prism (R~100) modes to detect rest-frame optical lines (e.g., [O III], Hβ, [O II], [Ne III]).
• Reduce data with the JWST pipeline, perform 1D extraction, and measure emission-line fluxes (Gaussian fits for S/N>5; direct integration for lower S/N).
• Compute rest-frame line ratios (O3, O32, Ne3O2, R23) and electron temperature indicators ([O III] 4363/[O III] 5007) to diagnose gas conditions.
• Fit rest-UV to optical SEDs with beagle (BC03 + CLOUDY-based photoionization models) to derive stellar masses, ages, dust content, and EW([O III]+Hβ).
• Cross-check slit-loss corrections via SED-based fluxes and compare line fluxes across MR bands to ensure consistent flux calibration.
• Identify robust z>7 galaxies (21 objects) and isolate 10 within three Lyα associations for targeted analysis of ionized bubble environments.

Experimental results

Research questions

• RQ1 What are the ionization states, metallicities, and stellar populations of bright z>7 galaxies in CEERS as inferred from rest-frame optical lines?
• RQ2 How does Lyα visibility at z>7 depend on the properties of the ionized bubbles and the intrinsic production/escape of Lyα photons?
• RQ3 Do z>7 Lyα emitters preferentially reside in large ionized regions, and what mechanisms (ionizing photon production, outflows) boost Lyα detection?
• RQ4 What is the relation between ξion, Lyα velocity offsets, and Lyα escape fraction in these extreme galaxies?

Key findings

• The sample shows very high ionization and low metallicity gas, with O32 ≈ 17.84 and Ne3O2 ≈ 0.89, and 12+log(O/H) ≈ 7.84, uncommon at lower redshifts.
• Six of the z>7 Lyα emitters exhibit the most extreme spectral properties, including hard ionizing spectra and strong emission-line EWs.
• Lyα velocity offsets in these z>7 galaxies are large, ≳300 km/s, enhancing the likelihood of Lyα transmission through the IGM.
• Lyα emission in z>7 galaxies is 6–12× weaker than in lower-redshift samples with matched rest-frame optical properties, implying significant IGM attenuation or bubble-scale effects.
• SED modeling indicates extremely young ages (median ~5 Myr) and negligible dust, with very large rest-frame [O III]+Hβ EWs (median ≈ 1994 Å, range ~879–3276 Å).
• The study suggests that if Lyα-emitting bubbles are small (≲0.5–1 pMpc), the observed attenuation could explain the weaker Lyα visibility at z>7.