The Power Spectrum of the Lyman-$\hat{I}\pm$ Forest at z < 0.5

Abstract

We present new measurements of the flux power-spectrum P(k) of the $z<0.5$ HI Lyman-$\hat{I}\pm$ forest spanning scales k ~ 0.001-0.1 s/km. These results were derived from 65 far ultraviolet quasar spectra (resolution R~18000) observed with the Cosmic Origin Spectrograph (COS) on board the Hubble Space Telescope. The analysis required careful masking of all contaminating, coincident absorption from HI 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 HI photoionization rate ($\hat{I}{“}_{\mathrm{HI}}$) in the z<0.5 intergalactic medium. Our results confirm most of the previous $\hat{I}{“}_{\mathrm{HI}}$ estimates. We conclude that previous concerns of a photon underproduction crisis are now resolved by demonstrating that the measured $\hat{I}{“}_{\mathrm{HI}}$ 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.

15 pages, 8 figures, Accepted for publication in MNRAS