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

Abstract

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 ~ 8.7 in the EGS field, with a potential radius of Rb = 2 physical Mpc (pMpc) or larger. We measure a photometric galaxy density and find that the volume is a factor of ~2.5 − 3.6 overdense, suggesting that there may be a large population of galaxies capable of creating an Rb ~ 2 pMpc bubble. Then, we infer the Ly α transmission through the IGM for galaxies in the EGS volume using our deep spectroscopy, finding $\mathcal {T}{\rm {\small IGM}}=0.26{-0.14}^{+0.25}$. This transmission is consistent with the average at z ~ 9 and is mildly inconsistent with the transmission expected for an Rb ~ 2 pMpc bubble ($\mathcal {T}_{\rm {\small IGM},2, \text{pMpc}}=0.53-0.63$), 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, Rb ~ 0.5 − 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.