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

¹ Institute for Theoretical Physics, Heidelberg University, Philosophenweg 12, D–69120 Heidelberg, Germany
² Max-Planck-Institut für Astronomie, streetKönigstuhl 17 69117 Heidelberg, Germany
³ Department of Astronomy, University of Michigan, street1085 S. University Ann Arbor MI 48109, USA
⁴ Département d’Astronomie, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204 Tucson, AZ 85721-0065, USA
⁶ Department of Physics, Massachusetts Institute of Technology, Cambridge MA 02139, USA
⁷ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge MA 02139, USA
⁸ Department of Physics, University of California, Santa Barbara CA 93106-9530, USA
⁹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
¹⁰ Department of Physics, Northwestern College, 101 7th Street SW, Orange City IA 51041, USA
¹¹ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark

^⋆ Corresponding author.

Received: 16 December 2024
Accepted: 22 June 2025

Abstract

We present a detailed study of a double-peaked Lyα 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 Lyα, we detect Hβ, [OIII]_{λλ4960, 5008} doublet, and Hα emission lines in the various spectral datasets. The presence of a double-peaked Lyα profile in the galaxy spectrum allows us to characterise the opening angle and lifetime of the central quasar as θ_Q > 49.17° and t_Q > 3.8 × 10⁵ 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_Hα = 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 Lyα emission profile and its equivalent width at such high redshift. Moreover, we employ diagnostics which do not rely on the detection of Lyα for comparison, and find that all tracers of ionising photon leakage agree within 1σ uncertainty. We measure a moderate escape of Lyman continuum photons from LAE-11 of f_esc^LyC = (9 − 43)%. Detections of both Hα and Hβ 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 Hα and Hβ, respectively, when using the median f_esc^LyC. The total ionising output of LAE-11, log(f_esc^LyCξ_ion,Hα/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.