A measurement of the escaping ionising efficiency of galaxies at redshift 5

¹ Institute for Theoretical Physics, Heidelberg University, Philosophenweg 12, D-69120 Heidelberg, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

^⋆ Corresponding author; bosman@thphys.uni-heidelberg.de

Received: 11 July 2024
Accepted: 3 September 2024

Abstract

The escaping ionising efficiency from galaxies, f_escξ_ion, is a crucial ingredient for understanding their contribution to hydrogen reionisation, but both of its components, f_esc and ξ_ion, are extremely difficult to measure. We measured the average escaping ionising efficiency ⟨f_escξ_ion⟩ of galaxies at z = 5 implied by the mean level of ionisation in the intergalactic medium via the Lyman-α forest. We used the fact that N_ion = ρ_UV f_esc ξ_ion, the product of the ionising output and the UV density ρ_UV, can be calculated from the known average strength of the UV background and the mean free path of ionising photons. These quantities, as well as ρ_UV, are robustly measured at z ≤ 6. We calculated the missing factor of ⟨f_escξ_ion⟩ at z = 5 during a convenient epoch after hydrogen reionisation had been completed and the intergalactic medium had reached ionisation equilibrium but before bright quasars began to dominate the ionising photon production. Intuitively, our constraint corresponds to the required escaping ionising production from galaxies in order to avoid over- or under-ionising the Lyman-α forest. We obtained a measurement of log⟨f_escξ_ion⟩/erg Hz^-1 = 24.28^+0.21_-0.20 at z = 5 when integrating the ρ_UV down to a limiting magnitude M_lim = −11. Our measurement of the escaping ionising efficiency of galaxies is in rough agreement with both observations of early galaxies and with most models.