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 flux observed from galaxies. One luminous galaxy, COLA1, stands out because of its unique double-peaked Lyα line at $z=6.6$, unseen in any simulation of reionization. Here we present JWST/NIRCam wide-field slitless spectroscopy in a 21 arcmin² field centered on COLA1. We find 141 galaxies spectroscopically-selected through the [OIII](λ4969,5008) doublet at $5.35<z<6.95$, with 40 of these sources showing Hβ. For COLA1 we additionally detect [OIII]₄₃₆₃ and Hγ. We measure a systemic redshift of $z=6.5917$ for COLA1, confirming the double-peak nature of the Lyα profile. This implies that it resides in a highly ionized bubble and that it is leaking ionizing photons with a high escape fraction fesc (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 Lyα escape fraction of 81±5%, Balmer decrement indicating no dust, a steep UV slope (βUV=-3.2± 0.4), and a star-formation surface density 10× that of typical galaxies at similar redshift. We detect 5 galaxies in COLA1's close environment (Δ z<0.02). Exploiting the high spectroscopic completeness inherent to grism surveys, and using mock simulations that mimic the selection function, we show the that number of detected companions is very typical for a similarly UV-bright (MUV~-21.3) galaxy; that is, the ionized bubble around COLA1 is unlikely 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 Lyα profile (Rᵢₒₙ≈0.7 pMpc), with minor contribution from detected neighbours (-17.5>MUV>-19.5).