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-$\alpha$ flux observed from galaxies. One luminous galaxy, COLA1, stands out because of its unique double-peaked Ly$\alpha$ 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 [OIII]($\lambda4969,5008$) doublet at $5.35<z<6.95$, with 40 of these sources showing H$\beta$. For COLA1 we additionally detect [OIII]$_{4363}$ and H$\gamma$. We measure a systemic redshift of $z=6.5917$ for COLA1, confirming the double-peak nature of the Ly$\alpha$ profile. This implies that it resides in a highly ionized bubble and that it is leaking ionizing photons with a high escape fraction $f_{\rm esc}{\rm (LyC)}=20$-$50$%, 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$\alpha$ escape fraction of $81\pm5\%$, Balmer decrement indicating no dust, a steep UV slope ($\beta_{\rm UV}=-3.2\pm 0.4$), and a star-formation surface density $\gtrsim 10\times$ that of typical galaxies at similar redshift. We detect 5 galaxies in COLA1's close environment ($\Delta z<0.02$). 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 ($M_{\rm{UV}}\sim-21.3$) 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 Ly$\alpha$ profile ($R_{\rm ion}\approx0.7$ pMpc), with minor contribution from detected neighbours ($-17.5>M_{\rm UV}>-19.5$).