Measuring the properties of reionised bubbles with resolved Lyman alpha spectra
Abstract
Identifying and characterising reionised bubbles enables us to track both their size distribution, which depends on the primary ionising sources, and the relationship between reionisation and galaxy evolution. We demonstrate that spectrally resolved z6 Lyman-alpha (Lyα) emission can constrain properties of reionised regions. Specifically, the distant 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 ionised region that photons can escape without significant resonant absorption, and thus constrains both the sizes of and the residual neutral fractions within ionised 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 ionising 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% 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 ionised region >0.7 pMpc, with residual neutral fraction <10-5.5. To ionise its own proximity zone we infer COLA1 has a high ionising photon escape fraction (fesc>0.50), relatively steep UV slope (β < -1.79), and low line-of-sight gas density (0.5× the cosmic mean), suggesting it is a rare, underdense line-of-sight.