Julien Devriendt

Abstract:

We use the SPHINX²⁰ cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR₁₀) or 100 Myr (SFR₁₀₀). Both have f_esc>20 per cent and experienced an extreme burst of star formation, but Bursty Leakers have SFR₁₀ > SFR₁₀₀, while Remnant Leakers have SFR₁₀ < SFR₁₀₀. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H II region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H II region densities. Both types of leaker exhibit [C II]_158μm deficits on the [C II]–SFR₁₀₀ relation, while only Bursty Leakers show deficits when ₁₀ is used. We predict that [C II] luminosity and SFR indicators such as Hα and M_1500Å can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C II] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with f_esc>20 per cent dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with f_esc<5 per cent becomes significant at the tail-end of reionization.