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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 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 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]+ 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 fesc at z > 6 with the James Webb Space Telescope (JWST). The leakers comprise half of our sample, have a median LyC$f esc ≈ 50 per cent (conservative range: 20-55 per cent), and an ionizing production efficiency (xi 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 ζion and prolific fesc occurring in sync.