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

We examine the Ly alpha 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 <Lambda>-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(H I)approximate to cm(-2). The observed absorber population is complete for N-H I greater than or similar to 10(13) cm(-2), with a column density distribution slope of beta = 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 Ly alpha absorbers. From our simulation we estimate T(IGM)similar to 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 to be (z) over bar =0.17 to be Gamma (H I) = 10(-13.3+0.7) s(-1) (estimated modeling uncertainty), close to predictions based on quasar properties.