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

ABSTRACT Strong $\text{Ly}\, \alpha$ 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 $\text{Ly}\, \alpha$ through the intergalactic medium (IGM). In this work, we use deep JWST$\text{Ly}\, \alpha$ spectroscopy to constrain the size of a candidate ionized bubble at $z\sim 8.7$ in the Extended Groth Strip (EGS) field, with a potential radius of $R_\mathrm{ b}=2$ physical Mpc (pMpc) or larger. We measure a photometric galaxy density and find that the volume is a factor of ${\sim} 2.5\text{-}3.6$ overdense, suggesting that there may be a large population of galaxies capable of creating an $R_\mathrm{ b}\sim 2$ pMpc bubble. Then, we infer the $\text{Ly}\, \alpha$ transmission through the IGM for galaxies in the EGS volume using our deep spectroscopy, finding $\mathcal {T}_{\small IGM}=0.26_{-0.14}^{+0.25}$. This transmission is consistent with the average at $z\sim 9$ and is mildly inconsistent with the transmission expected for an $R_\mathrm{ b}\sim 2$ pMpc bubble ($\mathcal {T}_{{\small IGM},2\, \text{pMpc}}=0.53\text{-}0.63$), 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 $\text{Ly}\, \alpha$ transmission may be consistent with smaller, $R_\mathrm{ b}\sim 0.5\text{-}1$ pMpc bubbles in EGS. This additionally motivates the need for future wider area $\text{Ly}\, \alpha$ 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.