The synchrony of production and escape: half the bright Ly? emitters at <i>z</i> ? 2 have Lyman continuum escape fractions ?50

The ionizing photon escape fraction (LyC $f_{\rm{esc}}$) of star-forming galaxies is the single greatest unknown in reionization budget. Stochastic sightline effects prohibit direct separation LyC leakers from non-leakers at significant redshift. Here we circumvent this uncertainty by inferring $f_{\rm{esc}}$ with resolved (R>4000) LyA profiles X-SHOOTER survey z=2 (XLS-z2). We select ($f_{\rm{esc}}>20$%) and ($f_{\rm{esc}}<5$%) a representative sample $>0.2 L^{*}$ emitters (LAEs). With median stacked spectra these subsets covering 1000-8000 {\AA} (rest-frame) investigate conditions for $f_{\rm{esc}}$. find following differences between vs. non-leakers: (i) strong nebular CIV HeII emission non-detections, (ii) O32~8.5 ~3, (iii) Ha/Hb indicating no dust E(B-V)~0.3, (iv) MgII close to systemic velocity redshifted, optically thick MgII, (v) ~50% ~10%. extreme EWs (O3+Hb~1100 {\AA}) constrain characteristic timescale ~3-10 Myr bursts when short-lived stars hardest shine. defining traits -- extremely stellar populations, low column densities, dust-free, high ionization state ISM occur simultaneously $f_{\rm{esc}}>20\%$ stack, suggesting they are causally connected, motivating why indicators like O32 may suffice z>6 JWST. comprise half our sample, have $f_{\rm{esc}}$~50%, an ionising production efficiency $\log({\xi_{\rm{ion}}/\rm{Hz\ erg^{-1}}})$~25.9. These results show LAEs type rare z=2, but that become norm higher redshift highly efficient ionizers, $\xi_{\rm{ion}}$ prolific occurring sync. (ABRIDGED)