The Power Spectrum of the Lyman-$\alpha$ Forest at z < 0.5

TL;DR

This study measures the flux power-spectrum of the low-redshift Lyman-alpha forest using Hubble data, providing insights into the intergalactic medium's ionization rate and resolving previous photon underproduction concerns.

Contribution

It presents the first detailed flux power-spectrum measurements at z<0.5 from HST/COS data, refining the intergalactic HI photoionization rate estimates.

Findings

Measured P(k) over scales 0.001-0.1 s/km

Confirmed previous estimates of the HI photoionization rate

Resolved the photon underproduction crisis with quasar UV emission

Abstract

We present new measurements of the flux power-spectrum P(k) of the $z<0.5$ HI Lyman-$\alpha$ 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 ($\Gamma_{\rm HI}$) in the z<0.5 intergalactic medium. Our results confirm most of the previous $\Gamma_{\rm HI}$ estimates. We conclude that previous concerns of a photon underproduction crisis are now resolved by demonstrating that the measured $\Gamma_{\rm HI}$ 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.