We examine the Ly \alpha 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 \Lambda CDM universe .
We find very good agreement in the column density and b -parameter distributions , down to the smallest observable absorbers with N _ { HI } \approx 10 ^ { 12.3 } { cm } ^ { -2 } .
The observed absorber population is complete for N _ { HI } \gtrsim 10 ^ { 13 } { cm } ^ { -2 } , with a column density distribution slope of \beta = 2.04 \pm 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 shock-heated 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 shock-heated 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 Ly \alpha absorbers .
From our simulation we estimate T _ { IGM } \sim 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 \bar { z } = 0.17 to be { \Gamma _ { HI } } = 10 ^ { -13.3 \pm 0.7 } { s } ^ { -1 } ( estimated modeling uncertainty ) , close to predictions based on quasar properties .