We present an ultraviolet spectroscopic survey of strong H i absorbers in the intergalactic medium , probing their evolution over the last 6-7 Gyr at redshifts 0.24 \leq z \leq 0.84 .
We measure column densities N _ { HI } ( { cm } ^ { -2 } ) from the pattern of Lyman-series absorption lines and flux decrement at the Lyman limit ( LL ) when available .
We analyzed 220 H i absorbers in ultraviolet spectra of 102 active galactic nuclei ( AGN ) taken by the Cosmic Origins Spectrograph aboard the Hubble Space Telescope with G130M/G160M gratings ( 1134-1795 Å ) .
For 158 absorbers with \log N _ { HI } \geq 15 , the mean frequency is d { \cal N } / dz = 4.95 \pm 0.39 over pathlength \Delta z = 31.94 ( 0.24 \leq z \leq 0.84 ) .
We identify 8 Lyman Limit Systems ( LLS , \log N _ { HI } \geq 17.2 ) and 54 partial systems ( pLLS ) with 16.0 \leq \log N _ { HI } < 17.2 .
Toward 159 AGN between 0.01 < z _ { abs } < 0.84 with \Delta z \approx 48 , we find four damped Ly \alpha absorbers ( DLA ) with ( d { \cal N } / dz ) _ { DLA } = 0.083 ^ { +0.066 } _ { -0.040 } at \langle z \rangle = 0.18 .
The mean LLS frequency between z = 0.24 - 0.48 is ( d { \cal N } / dz ) _ { LLS } = 0.36 ^ { +0.20 } _ { -0.13 } fitted to N ( z ) = ( 0.25 ^ { +0.13 } _ { -0.09 } ) ( 1 + z ) ^ { 1.14 } .
For 54 pLLS we find ( d { \cal N } / dz ) _ { pLLS } = 1.69 \pm 0.23 at \langle z \rangle = 0.39 , a frequency consistent with gaseous halo sizes R \approx 100 h ^ { -1 } ~ { } { kpc } for ( 0.3 - 3 L ^ { * } ) galaxies .
A maximum-likelihood analysis yields a distribution f ( N,z ) = C _ { 0 } N ^ { - \beta } ( 1 + z ) ^ { \gamma } with \beta = 1.48 \pm 0.05 and \gamma = 1.14 ^ { +0.88 } _ { -0.89 } for 15 \leq \log N _ { HI } \leq 17.5 .
The far-UV opacity gradient is d \tau _ { eff } / dz \approx ( 0.444 ) ( 1 + z ) ^ { 1.14 } over the range 15 \leq \log N _ { HI } \leq 17 , implying mean LyC optical depth \tau _ { eff } \approx 0.3 - 0.5 toward sources at z = 1 - 2 .