Affiliation: Institute for Theoretical Physics, University of Heidelberg

Contribution: Oral

Title: Strong contender for a true reionisation-driving galaxy: a double peaked proximate LAE at z~7

Abstract: Early faint galaxies are thought to be behind the Epoch of Reionisation (EoR), however the amount of ionising photons which they output into their environment remains debated. One excellent diagnostic of ionising photon escape is Lyα emission line morphology. Occasionally the line displays two maxima whose separation is an indication of Lyman Continuum (LyC) leakage (f_esc(LyC)). The two peaks directly trace the scattering of Lyα photons on outflowing gas surrounding the Lyman α emitter (LAE). However, detections of the blue peak are incredibly rare at z > 5 due to increased absorption of λ < 1215 ̊A photons by the mostly neutral IGM. The only way to observe the blue peak with no attenuation is if the galaxy is proximate to a bright quasar which has ionised the surrounding gas; until now only one such “proximate LAE” had been discovered. Here we introduce a potential representative driver of the EoR: a second double-peaked proximate LAE at z ∼ 7 near quasar J0910-0424, named “LAE-11”. We use a combination of photometric and spectroscopic data from JWST/NIRCam, HST/WFC3, Subaru/HSC and Keck/DEIMOS. In addition to Lyα, we detect Hβ and [OIII] doublet emission lines. LAE-11 has a Lyα peak separation of ∆v_sep = 286 ± 42 km/s, indicating an f_esc(LyC) ∼ 10%. This value is in agreement with a separate f_esc(LyC) tracer based on UV slope. Through extensive spectral modelling we find that LAE-11 is relatively UV-faint (M_UV ∼ −18.6), but is nevertheless a powerful ionising source with log ξ_ion = 25.4 ± 0.3. We measure for the first time the total ionising output of a galaxy at cosmic dawn: log <ξ_ion f_esc(LyC)> = 24.4 /(erg s). This is a relatively high output for a faint galaxy compared to low-z analogues. If LAE-11 is representative of early galaxies, then comparably faint objects would be sufficient to reionise the bulk of neutral hydrogen in the Early Universe.

This contribution can be found here (pdf).