Deep JWST spectroscopy of galaxies in a candidate ionized bubble at $z = 8.7$: probing reionization at pMpc scales with Ly$α$ emission

TL;DR

This study uses deep JWST Lyα spectroscopy of four galaxies at $z\sim8.7$ in the EGS field to probe the topology of reionization via the transmission of Lyα through the IGM. By combining rest-UV/optical spectroscopy with NIRCam photometry and BEAGLE SED modeling, the authors derive the Lyα transmission $T_{IGM}$ and assess the presence of a large ionized bubble, finding $T_{IGM}=0.26^{+0.25}_{-0.14}$, consistent with field values and mildly inconsistent with a $R_b=2$ pMpc bubble. They identify a modest photometric overdensity near EGSY8p7, suggesting a larger galaxy population that could contribute to ionization, but the Lyα constraints argue against a bubble of the largest expected size at $z\sim9$, though a smaller bubble ($\sim0.5-1$ pMpc) remains possible. The paper highlights the need for wider-area Lyα surveys and outlines forecasted capabilities to constrain bubble sizes with JWST in upcoming programs, paving the way to map reionization topology through Lyα transmission.

Abstract

Strong Ly$α$ emission observed from galaxies when the Universe is expected to be highly neutral is thought to trace large ionized regions that facilitate the transmission of Ly$α$ through the IGM. In this work, we use deep JWST Ly$α$ spectroscopy to constrain the size of a candidate ionized bubble at $z\sim8.7$ in the EGS field, with a potential radius of $R_b=2$ physical Mpc (pMpc) or larger. We measure a photometric galaxy density and find that the volume is a factor of $\sim2.5-3.6$ overdense, suggesting that there may be a large population of galaxies capable of creating an $R_b\sim2$ pMpc bubble. Then, we infer the Ly$α$ transmission through the IGM for galaxies in the EGS volume using our deep spectroscopy, finding $\mathcal{T}_{\rm IGM}=0.26_{-0.14}^{+0.25}$. This transmission is consistent with the average at $z\sim9$ and is mildly inconsistent with the transmission expected for an $R_b\sim2$ pMpc bubble ($\mathcal{T}_{\rm IGM, 2{\rm pMpc}}=0.53-0.63$), implying that such a large bubble is unlikely to be present. However, the photometric galaxy density in the EGS field is larger than in several other deep fields. This overdensity and the moderate Ly$α$ transmission may be consistent with smaller, $R_b\sim0.5-1$ pMpc bubbles in EGS. This additionally motivates the need for future wider area Ly$α$ spectroscopy in EGS and other fields to obtain a more representative understanding of the sizes of ionized bubbles in the early stages of reionization, and the properties of the galaxies that create them.

Figures (11)

• Figure 1: The NIRCam SEDs of the four $z = 8.7$ objects observed by GO 4287. We show the observed photometry as blue circles, the model photometry as open black diamonds, and the model spectra with uncertainies as black lines and grey shaded regions. All four galaxies are moderately to extremely bright, with absolute UV magnitudes ranging from $-22.1 \lesssim M_\textsc{uv} \lesssim -20.7$ and are inferred to have high ionizing photon production efficiencies of $\xi_\text{ion} = 10^{25.5 - 26.2}$ Hz erg$^{-1}$.
• Figure 2: The G395M spectra, probing the rest-frame optical, of the four objects at $z = 8.7$ that were observed by GO 4287. All show strong [Oiii] emission, and several have multiple additional Balmer line detections. Of these four objects, two are observed to have $\text{Ly}\,\alpha$ emission (EGSY8p7 and CEERS-1025, see Figures \ref{['fig:69787_uv_lines']} and \ref{['fig:87873_uv_lines']}).
• Figure 3: The rest-frame UV lines detected in the G140H spectrum of EGSY8p7 from GO 4287. We detect $\text{Ly}\,\alpha$ and the red components of the Niv]$\lambda\lambda$1483,1486 and Civ$\lambda\lambda$1548,1550 doublets. We measure an equivalent width for $\text{Ly}\,\alpha$ of $\text{EW}_{0, \text{Ly}\,\alpha} \sim 8$ Å and a velocity offset of $\Delta v = 164$ km s$^{-1}$.
• Figure 4: The rest-frame UV lines detected in the G140H spectrum of CEERS-1025 from GO 4287. We detect $\text{Ly}\,\alpha$ and the blue component of the high-ionization Nv$\lambda\lambda$1239,1243 doublet. We measure an equivalent width for $\text{Ly}\,\alpha$ of $\text{EW}_{0, \text{Ly}\,\alpha} = 3$ Å redshifted from systemic by $\Delta v = 251$ km s$^{-1}$. Tang2025 provides a detailed discussion of the implications of the Nv detection for this object.
• Figure 5: The 3D distribution of objects that have been spectroscopically confirmed in the EGS field at $z = 8.7 \pm 0.1$. LAEs are shown as yellow stars and objects that are spectroscopically confirmed but have no $\text{Ly}\,\alpha$ detections are shown as black circles. One LAE (EGS_ z910_ 44164) has been observed with NIRSpec as part of the CEERS program (ID CEERS-1029) but does not fall in the CEERS imaging footprint and is shown as the open yellow star. There are ten objects with UV luminosities $M_\textsc{uv} \lesssim -19$ that lie within the imaging or in the close vicinity in a narrow redshift range of $\Delta z \sim 0.2$, corresponding to $\sim 5.3$ pMpc along the line of sight. In comparison, measurements of the $z \sim 9$ UV luminosity function suggest that no more than six objects at $z = 8.6 - 8.8$ and $M_\textsc{uv} \lesssim -19$ are expected in the CEERS imaging area in an average field, hinting at a galaxy overdensity at $z \sim 8.7$ in the volume.