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

The escaping ionising efficiency from galaxies, fescξion, is a crucial ingredient for understanding their contribution to hydrogen reionisation, but both of its components, fesc and ξion, are extremely difficult to measure. We measured the average escaping ionising efficiency ⟨fescξion⟩ 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 Nion = ρUV fesc ξion, the product of the ionising output and the UV density ρUV, 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 ρUV, are robustly measured at z ≤ 6. We calculated the missing factor of ⟨fescξion⟩ 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⟨fescξion⟩/erg Hz-1 = 24.28+0.21-0.20 at z = 5 when integrating the ρUV down to a limiting magnitude Mlim = −11. Our measurement of the escaping ionising efficiency of galaxies is in rough agreement with both observations of early galaxies and with most models.