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

Top: False-color JWST NIRCam image of the COLA1 field (C1F) coverage based on the F115W/F200W/F356W images. The vertical and horizontal axes represent the angular separation relative to COLA1, which is located in the center of the middle tile. Bottom: A slice of the error cube of the C1F in the horizontal direction of the top panel. This illustrates how the field of view depends on wavelength, and that it is maximal around 3.8 µm, which is the undispersed wavelength of the NIRCam grism. We note that the [O III] doublet at the COLA1 redshift falls exactly at this wavelength. The units are relative to the minimum flux error across the field. 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).