Mapping reionization bubbles in JWST era: I. Empirical edge detection with Lyman alpha emission from galaxies

Context. Ionized bubble sizes during reionization trace physical properties of the first galaxies. JWST's ability to spectroscopically confirm and measure Lyman-alpha (Lyα) emission in sub-L* galaxies makes it possible to map to map ionized bubbles in 3D. However, existing Lyα-based bubble measurement strategies rely on constraints from single galaxies, which are limited by the large variability in intrinsic Lyα emission. Aims. As a first step, we present two bubble-size-estimation methods using Lyα spectroscopy of ensembles of galaxies, enabling us to map ionized structures and marginalize over Lyα emission variability. We tested our methods using gigaparsec-scale reionization simulations of the intergalactic medium (IGM). Methods. To map bubbles in the plane of the sky, we developed an edge detection method based on the asymmetry of Lyα transmission as a function of spatial position. To map bubbles along the line of sight, we develop an algorithm using the tight relation between Lyα transmission and the line-of-sight distance from galaxies to the nearest neutral IGM patch. Results. Both methods can robustly recover bubbles with a radius ≳10 comoving Mpc, sufficient for mapping bubbles even in the early phases of reionization, when the IGM is ∼70-90% neutral. These methods require ≳0.002-0.004 galaxies/cMpc3, a 5σ Lyα equivalent width upper limit of ≲30 A for the faintest targets, and redshift precision Δz≲0.015, which is feasible with JWST spectroscopy. Shallower observations will provide robust lower limits on bubble sizes. Additional constraints on IGM transmission from Lyα escape fractions and line profiles will further refine these methods, paving the way to our first direct understanding of ionized bubble growth.