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

Abstract

Strong Ly$\alpha$ 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$\alpha$ through the IGM. In this work, we use deep JWST Ly$\alpha$ 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$\alpha$ 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$\alpha$ 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$\alpha$ 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.