James Webb Space Telescope Feed Post

Harvard ADS: Mapping reionization bubbles in the JWST era I: empirical edge detection with Lyman alpha emission from galaxies

Paper abstract: Ionized bubble sizes during reionization trace physical properties of the first galaxies. JWST's ability to spectroscopically confirm and measure Lyman-alpha (Ly\alpha) emission in sub-L* galaxies opens the door to mapping ionized bubbles in 3D. However, existing Lya-based bubble measurement strategies rely on constraints from single galaxies, which are limited by the large variability in intrinsic Ly\alpha emission. As a first step, we present two bubble size estimation methods using Lya spectroscopy of ensembles of galaxies, enabling us to map ionized structures and marginalize over Ly\alpha emission variability. We test our methods using Gpc-scale reionization simulations of the intergalactic medium (IGM). To map bubbles in the plane of the sky, we develop an edge detection method based on the asymmetry of Ly\alpha 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\alpha transmission and the line-of-sight distance from galaxies to the nearest neutral IGM patch. Both methods can robustly recover bubbles with radius \gtrsim10 comoving Mpc, sufficient for mapping bubbles even in the early phases of reionization, when the IGM is ~70-90\% neutral. These methods require \gtrsim0.002-0.004 galaxies/cMpc^3, a 5\sigma Ly\alpha equivalent width upper limit of <~30\r{A} for the faintest targets, and redshift precision \Delta z <~ 0.015, feasible with JWST spectroscopy. Shallower observations will provide robust lower limits on bubble sizes. Additional constraints on IGM transmission from Ly\alpha escape fractions and line profiles will further refine these methods, paving the way to our first direct understanding of ionized bubble growth.