2022MNRAS.510.4582N

The ionizing photon escape fraction [Lyman continuum (LyC) f_esc] of star-forming galaxies is the single greatest unknown in the reionization budget. Stochastic sightline effects prohibit the direct separation of LyC leakers from non-leakers at significant redshifts. Here we circumvent this uncertainty by inferring f_esc using resolved (R > 4000) Lyman α (Lyα) profiles from the X-SHOOTER Lyα survey at z = 2 (XLS-z2). With empirically motivated criteria, we use Lyα profiles to select leakers (f_esc > 20 per cent) and non-leakers (f_esc < 5 per cent) from a representative sample of >0.2L* Lyman α emitters (LAEs). We use median stacked spectra of these subsets over λ_rest ≃ 1000-8000 Å to investigate the conditions for LyC f_esc. Our stacks show similar mass, metallicity, M_UV, and β_UV. We find the following differences between leakers versus non-leakers: (i) strong nebular C IV and He II emission versus non-detections; (ii) [O III]/[O II] ≃ 8.5 versus ≃3; (iii) Hα/Hβ indicating no dust versus E(B - V) ≃ 0.3; (iv) Mg II emission close to the systemic velocity versus redshifted, optically thick Mg II; and (v) Lyα f_esc of ~50 per cent versus ~10 per cent. The extreme equivalent widths (EWs) in leakers ([O III]+ Hβ~1100 Å rest frame) constrain the characteristic time-scale of LyC escape to ≃3-10 Myr bursts when short-lived stars with the hardest ionizing spectra shine. The defining traits of leakers - extremely ionizing stellar populations, low column densities, a dust-free, high-ionization state interstellar medium (ISM) - occur simultaneously in the f_esc > 20 per cent stack, suggesting they are causally connected, and motivating why indicators like [O III]/[O II] may suffice to constrain f_esc at z > 6 with the James Webb Space Telescope (JWST). The leakers comprise half of our sample, have a median LyCf_esc ~50 per cent (conservative range: 20-55 per cent), and an ionizing production efficiency log(ξ_ion/ Hz erg^–1)~25.9 (conservative range: 25.7-25.9). These results show LAEs - the type of galaxies rare at z ≃ 2, but that become the norm at higher redshift - are highly efficient ionizers, with extreme ξ_ion and prolific f_esc occurring in sync.