The Synchrony of Production & Escape: Half the Bright Ly$\hat{I}\pm$ Emitters at $z\approx 2$ have Lyman Continuum Escape Fractions $\approx 50\%$

Abstract

The ionizing photon escape fraction (LyC $f_{\mathrm{esc}}$) 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 $f_{\mathrm{esc}}$ with resolved (R>4000) LyA profiles from the X-SHOOTER LyA survey at z=2 (XLS-z2). We select leakers ($f_{\mathrm{esc}}>20$%) and non-leakers ($f_{\mathrm{esc}}<5$%) from a representative sample of $>0.2L^{\ast }$ LyA emitters (LAEs). With median stacked spectra of these subsets covering 1000-8000 Ã (rest-frame) we investigate the conditions for LyC $f_{\mathrm{esc}}$. 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 $f_{\mathrm{esc}}$ of ~50% vs. ~10%. The extreme EWs in leakers (O3+Hb~1100 Ã ) 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 $f_{\mathrm{esc}}>20\%$ stack, suggesting they are causally connected, and motivating why indicators like O32 may suffice to constrain $f_{\mathrm{esc}}$ at z>6 with JWST. The leakers comprise half our sample, have a median LyC $f_{\mathrm{esc}}$~50%, and an ionising production efficiency $\log (\hat{I}{¾}_{\mathrm{ion}}/\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1})$~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 $\hat{I}{¾}_{\mathrm{ion}}$ and prolific $f_{\mathrm{esc}}$ occurring in sync. (ABRIDGED)

Submitted to MNRAS. Figure 4 and Table 2 show key results from the stacks. Figure 8 presents a unifying scheme for the LyC duty cycle. Our companion paper (Matthee & Naidu et al. 2021) works out implications of these results for the cosmic UV background at $z\approx 2-8$. Comments warmly welcomed and greatly appreciated!