The power spectrum of the Lyman-α Forest at z < 0.5

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