The synchrony of production and escape: half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50

ABSTRACT The ionizing photon escape fraction Lyman continuum (LyC) fesc 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 fesc 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 ₄ₒ₂ 20{\ per\ cent}) and non-leakers (f ₄ₒ₂ 5{\ per\ cent}) from a representative sample of 0. 2L* Lyman α emitters (LAEs). We use median stacked spectra of these subsets over λrest ≈ 1000–8000 Å to investigate the conditions for LyC fesc. Our stacks show similar mass, metallicity, MUV, and βUV. 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 fesc of 50{\ per\ cent} versus 10{\ per\ cent}. The extreme equivalent widths (EWs) in leakers (O iii+ H 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 ₄ₒ₂ 20{\ per\ cent} stack, suggesting they are causally connected, and motivating why indicators like O iii/O ii may suffice to constrain fesc at z 6 with the James Webb Space Telescope (JWST). The leakers comprise half of our sample, have a median LyCf ₄ₒ₂ 50{\ per\ cent} (conservative range: 20\!-\!55{\ per\ cent}), and an ionizing production efficiency ({₈₎₍/ 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 ξion and prolific fesc occurring in sync.