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

TL;DR

This study leverages JWST/NIRSpec CEERS data to obtain rest-frame optical and UV spectra for 21 galaxies at z>7, including 10 within Lyα-emitter associations. The galaxies exhibit extreme ionization (high O32/Ne3O2) and metal-poor gas (12+log(O/H) ~7.6–7.9) with ionizing photon production efficiencies up to log10(xi_ion) ≈ 26.0, and several show broad [O III] components indicative of strong outflows. Despite large Lyα velocity offsets that should aid IGM transmission, the Lyα emission is 6–12× weaker than lower-z analogs with similar rest-frame optical properties, suggesting small ionized bubbles (≲0.5–1 pMpc) or other attenuation mechanisms in these reionization-era environments. The results imply that bright LAEs at z>7 are powered by very efficient ionizing sources but remain limited by IGM opacity, highlighting the need for deeper surveys of fainter neighbors to map bubble scales and Lyα visibility during reionization.

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 ($12+\log{(\rm{O/H})}=7.84$) 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 $6-12\times$ 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.

Figures (22)

• Figure 1: Medium resolution ($R\sim1000$) 2D NIRSpec spectra of the sixteen galaxies at $z\gtrsim7$ at rest-frame optical wavelengths. Blue lines show the rest-frame optical emission lines detected on 2D spectra. The two tentative detections of [O III]$\lambda4363$ in CEERS-689 and CEERS-698 are shown by blue dotted lines.
• Figure 2: Low resolution ($R\sim100$) 2D (top of each panel) and 1D (bottom of each panel) NIRSpec prism spectra of the five galaxies at $z>7$ with prism observations. Blue lines mark the position of emission lines detected in the spectrum of each object. The grey shaded regions show the uncertainty of flux.
• Figure 3: Medium resolution ($R\sim1000$) NIRSpec 1D spectra of rest-frame optical emission lines of $z\gtrsim7$ galaxies with more than three detected lines ([O III], H$\beta$, and [O II] or [Ne III] or H$\gamma$; eleven objects). The two tentative detections of [O III]$\lambda4363$ in CEERS-689 and CEERS-698 are shown by blue dotted lines. The spectra have been shifted to the rest frame. The grey shaded regions show the flux uncertainty. Spectra of the remaining five galaxies with three and fewer rest-frame optical line detections are shown in Fig. \ref{['fig:spectra_opt_1d_less']} in Appendix \ref{['sec:appendix']}.
• Figure 4: Spectral energy distributions (blue circles) with the best-fit beagle stellar population synthesis models (black lines) of the 12 galaxies at $z>7$ with broadband photometry measurements spanning from rest-frame UV to optical in the CEERS NIRSpec sample. The best-fit spectra and synthetic photometry (red squares) are derived from the posterior median. For CEERS-24, CEERS-23, and CEERS-3, their F115W fluxes are non-detections. To avoid introducing uncertainties from Lyman series emission and absorption, we do not fit F115W flux for galaxies at $z>7.5$. The SEDs show large flux excess at NIRCam F410M or F444W, or IRAC $[4.5]$, indicating intense [O III]+H$\beta$ emission lines.
• Figure 5: Correlation between ionizing photon production efficiency $\xi_{\rm{ion}}$ and [O III]+H$\beta$ EW for CEERS NIRSpec galaxies at $z\gtrsim7$. The EW and $\xi_{\rm{ion}}$ are derived from SED fitting using beagleChevallard2016, and the posterior median and $1\sigma$ confidence interval are plotted. Ly$\alpha$ emitters (LAEs; EW$_{\rm{Ly}\alpha}>9$ Å) and non-LAEs at $z>7$ are shown by red stars and blue circles, respectively. The median [O III]+H$\beta$ EW of typical $z\sim7-8$ star-forming galaxies ($\approx780$ Å; Endsley2023a) is shown by the vertical orange dotted line. We also overplot EELGs (i.e., systems with similarly large [O III]+H$\beta$ EWs as $z>7$ galaxies) at $z\sim2$Tang2019 in cyan squares as comparison. The $\xi_{\rm{ion}}-$ EW$_{\rm{[OIII]+H}\beta}$ relation at $z\sim2$ from Tang2019 is presented as black dashed line.