2019MNRAS.486..769K

We present new measurements of the flux power-spectrum P(k) of the z < 0.5 H I Lyman-α Forest spanning scales k ∼0.001-0.1 s km^–1. These results were derived from 65 far-ultraviolet quasar spectra (resolution R ∼18 000) observed with the Cosmic Origin Spectrograph (COS) on board the Hubble Space Telescope. The analysis required careful masking of all contaminating, coincident absorption from H I and metal-line transitions of the Galactic interstellar medium and intervening absorbers as well as proper treatment of the complex COS line-spread function. From the P(k) measurements, we estimate the H I photoionization rate (Γ_ H I_) in the z < 0.5 intergalactic medium. Our results confirm most of the previous Γ_ H I_ estimates. We conclude that previous concerns of a photon underproduction crisis are now resolved by demonstrating that the measured Γ_ H I_ can be accounted for by ultraviolet emission from quasars alone. In a companion paper, we will present constraints on the thermal state of the z < 0.5 intergalactic medium from the P(k) measurements presented here.