Anatomy of an ionized bubble: NIRCam grism spectroscopy of the z = 6.6 double-peaked Lyman-α emitter COLA1 and its environment

The increasingly neutral intergalactic gas at z > 6 impacts the Lyman- α (Ly α ) flux observed from galaxies. One luminous galaxy, COLA1, stands out because of its unique double-peaked Ly α line at z = 6.6, unseen in any simulation of reionization. Here, we present JWST/NIRCam wide-field slitless spectroscopy in a 21 arcmin 2 field centered on COLA1. We find 141 galaxies spectroscopically selected through the [O III ] doublet at 5.35 < z < 6.95, with 40 of these sources showing H β . For COLA1, we additionally detect [O III ] 4363 as well as H γ . We measure a systemic redshift of z = 6.5917 for COLA1, confirming the classical double-peak nature of the Ly α profile. This implies that it resides in a highly ionized bubble and that it is leaking ionizing photons with a high escape fraction of f esc (LyC) = 20–50%, making it a prime laboratory to study Lyman continuum escape in the Epoch of Reionization. COLA1 shows all the signs of a prolific ionizer with a Ly α escape fraction of 81 ± 5%, Balmer decrement indicating no dust, a steep UV slope ( β UV = −3.2 ± 0.4), and a star-formation surface density ≳10× that of typical galaxies at similar redshift. We detect five galaxies in COLA1’s close environment (Δ z < 0.02). Exploiting the high spectroscopic completeness inherent to grism surveys, and using mock simulations that fully mimic the selection function, we show that the number of detected companions is very typical for a normal similarly UV-bright ( M UV ∼ −21.3) galaxy – that is, the ionized bubble around COLA1 is unlikely to be due to an excessively large over-density. Instead, the measured ionizing properties suggest that COLA1 by itself might be powering the bubble required to explain its double-peaked Ly α profile ( R ion ≈ 0.7 pMpc), with only minor contributions from detected neighbors (−19.5 ≲ M UV ≲ −17.5).