A unique window into the epoch of reionisation: A double-peaked Lyman-<i>α</i> emitter in the proximity zone of a quasar at <i>z</i> ∼ 6.6

We present a detailed study of a double-peaked Ly α emitter, named LAE-11, found in the proximity zone of quasar J0910-0414 at z ∼ 6.6 at a proper distance of d QSO ∼ 0.3 pMpc from the quasar. We use a combination of deep photometric data from the Subaru Telescope, Hubble Space Telescope , and James Webb Space Telescope with spectroscopic data from Keck/DEIMOS, JWST /NIRCam WFSS, and JWST /NIRSpec MSA to characterise the ionising and general properties of the galaxy, as well as the quasar environment surrounding it. Apart from Ly α , we detect H β , [OIII] λλ 4960, 5008 doublet, and H α emission lines in the various spectral datasets. The presence of a double-peaked Ly α profile in the galaxy spectrum allows us to characterise the opening angle and lifetime of the central quasar as θ Q &gt; 49.17° and t Q &gt; 3.8 × 10 5 years, and probe the effect of the quasar’s environment on the star formation of the galaxy. LAE-11 is a fairly bright (M UV = −19.90 −0.12 +0.13 ), blue galaxy with a UV slope of β = −2.61 −0.08 +0.06 and a moderate ongoing star formation rate (SFR UV = 5.55 ± 0.65 M ⊙ yr −1 and SFR H α = 12.93 ± 1.20 M ⊙ yr −1 ). Since the galaxy is located in a quasar-ionised region, we have a unique opportunity to measure the escape fraction of Lyman continuum photons using the un-attenuated double-peaked Ly α emission profile and its equivalent width at such high redshift. Moreover, we employ diagnostics which do not rely on the detection of Ly α for comparison, and find that all tracers of ionising photon leakage agree within 1 σ uncertainty. We measure a moderate escape of Lyman continuum photons from LAE-11 of f esc LyC = (9 − 43)%. Detections of both H α and H β emission lines allow for separate measurements of the ionising photon production efficiency, resulting in log( ξ ion /Hz erg −1 ) = 25.57 −0.12 +0.16 and 25.63 −0.11 +0.17 for H α and H β , respectively, when using the median f esc LyC . The total ionising output of LAE-11, log( f esc LyC ξ ion,H α /Hz erg −1 ) = 24.25 +0.26 −0.29 , is higher than the value of 24.3 − 24.8 that is traditionally assumed to be needed to drive reionisation forward.