The statistical and physical properties of the low-redshift Lyα forest observed with the Hubble Space Telescope/STIS

We examine the Lyα absorber population at z < 0.3 detected in spectra of the quasars PG 0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We compare their statistical properties to those in carefully constructed mock quasar spectra drawn from a cosmological hydrodynamic simulation of a Λ-dominated cold dark matter universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with N_HI ≈ 10^12.3 cm^-2. The observed absorber population is complete for N_HI ≳ 10¹³ cm^-2, with a column density distribution slope of β = 2.04 ± 0.23 and a median b-parameter of 21 km s^-1 above this limit. The intergalactic gas giving rise to these weak absorbers is analogous to that at high redshift, located in diffuse large-scale structures that are highly photoionized by the metagalactic UV flux, though a greater number arise within shock-heated warm gas. The density, temperature, and column density of these absorbers follow similar relationships of those at high redshift, though with substantially larger scatter due to the shock-heated gas. The b-parameters typically have a significant contribution from thermal broadening, which facilitates a measurement of the low-z intergalactic medium temperature as traced by Lyα absorbers. From our simulation we estimate T_IGM ∼ 5000 K, with an upper limit of 10⁴ K, at the mean density. The agreement in predicted and observed amplitude of the column density distributions allows us to measure the H I photoionization rate at z̄ = 0.17 to be Γ_HI = 10^-13.3±0.7 s^-1 (estimated modeling uncertainty), close to predictions based on quasar properties.