The Statistical and Physical Properties of the Low Redshift Lyman Alpha Forest Observed with HST/STIS

We examine the Lyα absorber population at z < 0.3 detected in spectra of the QSOs PG0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph aboard 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 ΛCDM universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with NHI ≈ 10 cm. The observed absorber population is complete for NHI & 10 cm, with a column density distribution slope of β = 2.04 ± 0.23 and a median b-parameter of 21 km/s 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 shockheated warm gas. The density, temperature, and column density of these absorbers follow similar relationships to those at high redshift, though with substantially larger scatter due to the shockheated gas. The b-parameters typically have a significant contribution from thermal broadening, which facilitates a measurement of the low-z IGM temperature as traced by Lyα absorbers. From our simulation we estimate TIGM ∼ 5000 K, with an upper limit of 10 4 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 ΓHI = 10 s (estimated modeling uncertainty), close to predictions based on quasar properties. Subject headings: cosmology: observations — cosmology: theory — intergalactic medium — quasars: absorption lines — quasars: individual (PG0953+415,H1821+643)