Mapping reionization bubbles in JWST era

Context. Ionized bubble sizes during reionization trace physical properties of the first galaxies. JWST's ability to spectroscopically confirm and measure Lyman-alpha (Ly α ) emission in sub-L ★ galaxies makes it possible to map to map ionized bubbles in 3D. However, existing Ly α -based bubble measurement strategies rely on constraints from single galaxies, which are limited by the large variability in intrinsic Ly α emission. Aims. As a first step, we present two bubble-size-estimation methods using Ly α spectroscopy of ensembles of galaxies, enabling us to map ionized structures and marginalize over Ly α 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 Ly α transmission as a function of spatial position. To map bubbles along the line of sight, we develop an algorithm using the tight relation between Ly α 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/cMpc 3 , a 5 σ Ly α equivalent width upper limit of ≲30 Å for the faintest targets, and redshift precision Δ z ≲0.015, which is feasible with JWST spectroscopy. Shallower observations will provide robust lower limits on bubble sizes. Additional constraints on IGM transmission from Ly α escape fractions and line profiles will further refine these methods, paving the way to our first direct understanding of ionized bubble growth.