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 (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) Lyα profiles from the X-SHOOTER Lyα survey at z = 2 (XLS-z2). With empirically motivated criteria, we use Lyα profiles to select leakers (fesc$>20\%$) and non-leakers (fesc$<5\%$) from a representative sample of >0.2L* Lyman-α emitters (LAEs). We use median stacked spectra of these subsets over λrest ≈ 1000 − 8000Å to investigate the conditions for LyC fesc. Our stacks show similar mass, metallicity, MUV, and βUV. We find the following differences between leakers vs. non-leakers: (i) strong nebular CIV and HeII emission vs. non-detections, (ii) [O iii]/[O ii]≈8.5 vs. ≈3, (iii) Hα/Hβ indicating no dust vs. E(B − V) ≈ 0.3, (iv) MgII emission close to the systemic velocity vs. redshifted, optically thick MgII, (v) Lyα fesc of $\approx 50\%$ vs. $\approx 10\%$. The extreme EWs in leakers ([O iii]+Hβ ≈ 1100 Å rest-frame) 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 fesc$>20\%$ stack, suggesting they are causally connected, and motivating why indicators like [O iii]/[O ii] may suffice to constrain fesc at z > 6 with JWST. The leakers comprise half our sample, have a median LyC fesc$\approx 50\%$ (conservative range: $20-55\%$), and an ionising production efficiency $\log ({\xi _{\rm {ion}}/\rm {Hz\ erg^{-1}}})\approx 25.9$ (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.

High Equivalent Width of Hα+[N II] Emission in z~8 Lyman-break Galaxies from IRAC 5.8μm Observations: Evidence for Efficient Lyman-continuum Photon production in the Epoch of Re-ionization

“…where L UV is the UV-continuum luminosity computed from the stacked SED. The application of the above relations to our measurements yields ξ ion,0 = 10 25.97 +0.18 −0.28 Hz/erg, assuming negligible dust attenuation, as expected for L < L * LBGs at z > 4 (e.g., Dunlop et al 2017, Bouwens et al 2021b, Casey et al 2021 and from the extrapolation of the results for LAE at lower redshifts (e.g., Naidu et al 2022). If instead we consider a case with a small amount of dust attenuation, we obtain ξ ion,0 = 10 25.84 +0.18 −0.28 Hz/erg.…”

“…We only considered measurements corresponding to MUV ∼ −19.9 mag or log(M⋆/M⊙) ∼ 8.1 ± 1.0 when the MUV information was missing. The red open circles flag those results at z < 4 whose sample was explicitly selected to have rest-optical lines with EW0 1000 Å (Nakajima et al 2016, Tang et al 2019 or with likely a hard ionizing spectra (Naidu et al 2022). We arbitrarily shifted by ∆z = −0.06 our measurement for AV = 0.2 mag (open star) to improve the readability.…”

The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics

“…First, orientation could cause the observed scatter since f esc LyC likely depends on line of sight in an anisotropic, radiationbounded escape scenario (e.g., Zastrow et al 2013;Cen & Kimm 2015). Alternatively, the observed scatter might be due to a time delay between the onset of star formation and the escape of the LyC since feedback requires sufficient time to clear out escape channels, thus decoupling properties from f esc LyC to some extent (e.g., Trebitsch et al 2017; Barrow et al 2020; Naidu et al 2022). Whatever might cause this scatter, it is likely physical because an upper envelope in f esc LyC persists across several diagnostics.…”

The Low-Redshift Lyman Continuum Survey II: New Insights into LyC Diagnostics

“…First, orientation could cause the observed scatter since f LyC esc likely depends on line of sight in an anisotropic, radiation bounded escape scenario (e.g., Zastrow et al 2013;Cen & Kimm 2015). Alternatively, the observed scatter might be due to a time delay between the onset of star formation and the escape of the LyC since feedback requires sufficient time to clear out escape channels, thus decoupling properties from f LyC esc to some extent (e.g., Trebitsch et al 2017;Barrow et al 2020;Naidu et al 2022) Investigation of two-dimensional diagnostics sheds light on these possible explanations. At higher mass and higher metallicity, the lower O 32 , lower Hβ EW LCEs likely clear out their birth cloud by mechanical feedback from early type stars.…”