Mapping reionization bubbles in JWST era

II. Inferring the position and characteristic size of individual bubbles

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56125 Pisa, PI, Italy
² Cosmic Dawn Center (DAWN), Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁴ Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, 34136 Trieste, Italy
⁵ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA

^⋆ Corresponding author.

Received: 14 January 2025
Accepted: 27 May 2025

Abstract

The James Webb Space Telescope (JWST) is discovering an increasing number of galaxies well into the early stages of the epoch of reionization (EoR). Many of these galaxies are clustered with strong Lyman-alpha (Lyα) emission, which indicates surrounding cosmic HII regions that would facilitate Lyα transmission through the intergalactic medium (IGM). Detecting these HII bubbles would allow us to connect their growth to the properties of the galaxies inside them. We developed a new forward-modeling framework to estimate the size of the local HII region and its location based on Lyα spectra of galaxy groups in the early stages of the EoR. Our model uses the complementary information provided by neighboring sightlines through the IGM. Our forward models sample the main sources of uncertainty, including (i) the global neutral fraction, (ii) the EoR morphology, (iii) emergent Lyα emission, and (iv) NIRSpec instrument noise. Depending on the availability of complementary nebular lines, ∼0.006–0.01 galaxies per cMpc³ are required for a confidence of ≳95% that the location and size of the HII bubble as recovered by our method is accurate to within ∼1 comoving Mpc. This approximately corresponds to some dozens of galaxies at z∼7−8 in an ∼2×2 tiled pointing with JWST NIRSpec. A sample like this can be achieved with a targeted survey that is complete down to M_UV^min ≲ − 19–−17, depending on the overdensity of the field. We tested our method on 3D EoR simulations and on mispecified equivalent-width distributions. We accurately recovered the HII region that surrounded the targeted galaxy groups in both cases.