The Synchrony of Production & Escape:Half the Bright Ly$α$ Emitters at $z\approx2$ have Lyman Continuum Escape Fractions $\approx50\%$

Naidu, Rohan P. and Matthee, Jorryt and Oesch, Pascal A. and Conroy, Charlie and Sobral, David and Pezzulli, Gabriele and Hayes, Matthew and Erb, Dawn and Amorín, Ricardo and Gronke, Max and Schaerer, Daniel and Tacchella, Sandro and Kerutt, Josephine and Paulino-Afonso, Ana and Calhau, João and Llerena, Mario and Röttgering, Huub (2022) The Synchrony of Production & Escape:Half the Bright Ly$α$ Emitters at $z\approx2$ have Lyman Continuum Escape Fractions $\approx50\%$. Monthly Notices of the Royal Astronomical Society, 510 (3). pp. 4582-4607. ISSN 0035-8711

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Abstract

The ionizing photon escape fraction (LyC $f_{\rm{esc}}$) 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 redshift. Here we circumvent this uncertainty by inferring $f_{\rm{esc}}$ with resolved (R>4000) LyA profiles from the X-SHOOTER LyA survey at z=2 (XLS-z2). We select leakers ($f_{\rm{esc}}>20$%) and non-leakers ($f_{\rm{esc}}0.2 L^{*}$ LyA emitters (LAEs). With median stacked spectra of these subsets covering 1000-8000 {\AA} (rest-frame) we investigate the conditions for LyC $f_{\rm{esc}}$. We find the following differences between leakers vs. non-leakers: (i) strong nebular CIV and HeII emission vs. non-detections, (ii) O32~8.5 vs. ~3, (iii) Ha/Hb indicating no dust vs. E(B-V)~0.3, (iv) MgII emission close to the systemic velocity vs. redshifted, optically thick MgII, (v) LyA $f_{\rm{esc}}$ of ~50% vs. ~10%. The extreme EWs in leakers (O3+Hb~1100 {\AA}) constrain the characteristic timescale 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 ISM -- occur simultaneously in the $f_{\rm{esc}}>20\%$ stack, suggesting they are causally connected, and motivating why indicators like O32 may suffice to constrain $f_{\rm{esc}}$ at z>6 with JWST. The leakers comprise half our sample, have a median LyC $f_{\rm{esc}}$~50%, and an ionising production efficiency $\log({\xi_{\rm{ion}}/\rm{Hz\ erg^{-1}}})$~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 $\xi_{\rm{ion}}$ and prolific $f_{\rm{esc}}$ occurring in sync. (ABRIDGED)