James Webb Space Telescope Feed Post

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

Abstract: The increasingly neutral intergalactic gas at z > 6 impacts the Lyman-a flux observed from galaxies. One luminous galaxy, COLA1, stands out because of its unique double-peaked Lya 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](?4969,5008) doublet at 5.35 < z < 6.95, with 40 of these sources showing Hß. For COLA1 we additionally detect [OIII]4363 and H?. We measure a systemic redshift of z=6.5917 for COLA1, confirming the double-peak nature of the Lya 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 Lya escape fraction of 81±5%, Balmer decrement indicating no dust, a steep UV slope (ßUV=-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 (?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 Lya profile (Rion˜0.7 pMpc), with minor contribution from detected neighbours (-17.5 > MUV > -19.5).