2020MNRAS.499.1395M

Identifying and characterizing reionized bubbles enables us to track both their size distribution, which depends on the primary ionizing sources, and the relationship between reionization and galaxy evolution. We demonstrate that spectrally resolved z >= 6 Lyman-alpha (Lyα) emission can constrain properties of reionized regions. Specifically, the distance from a source to a neutral region sets the minimum observable Lyα velocity offset from systemic. Detection of flux on the blue side of the Lyα resonance implies the source resides in a large, sufficiently ionized region that photons can escape without significant resonant absorption, and thus constrains both the sizes of and the residual neutral fractions within ionized bubbles. We estimate the extent of the region around galaxies which is optically thin to blue Lyα photons, analogous to quasar proximity zones, as a function of the source's ionizing photon output and surrounding gas density. This optically thin region is typically <= 0.3 pMpc in radius (allowing transmission of flux >= -250 km s^–1), <= 20 per cent of the distance to the neutral region. In a proof-of-concept, we demonstrate the z ≃ 6.6 galaxy COLA1 - with a blue Lyα peak - likely resides in an ionized region >0.7 pMpc, with residual neutral fraction <10^–5.5. To ionize its own proximity zone we infer COLA1 has a high ionizing photon escape fraction (f_esc > 0.50), relatively steep UV slope (β < -1.79), and low line-of-sight gas density (∼0.5 times the cosmic mean), suggesting it is a rare, underdense line-of-sight.