An Ultraviolet Survey of Low-Redshift Partial Lyman-Limit Systems with
  the HST Cosmic Origins Spectrograph
  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 $N_{\rm HI} \,(
{\rm cm}^{-2})$ 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 \AA). For 158 absorbers with $\log N_{\rm HI} \geq 15$, 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, $\log
N_{\rm HI} \geq 17.2$) and 54 partial systems (pLLS) with $16.0 \leq \log
N_{\rm HI} < 17.2$. Toward 159 AGN between $0.01 < z_{\rm abs} < 0.84$ with
$\Delta z \approx 48$, we find four damped Ly$\alpha$ absorbers (DLA) with $(d
{\cal N}/dz)_{\rm DLA} = 0.083^{+0.066}_{-0.040}$ at $\langle z \rangle =
0.18$. The mean LLS frequency between $z = 0.24-0.48$ is $(d {\cal N}/dz)_{\rm
LLS} = 0.36^{+0.20}_{-0.13}$ fitted to $N(z) =
(0.25^{+0.13}_{-0.09})(1+z)^{1.14}$. For 54 pLLS we find $(d {\cal N}/dz)_{\rm
pLLS} = 1.69\pm0.23$ at $\langle z \rangle = 0.39$, a frequency consistent with
gaseous halo sizes $R \approx 100 h^{-1}~{\rm kpc}$ for ($0.3-3L^*$) 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 $15 \leq \log N_{\rm HI} \leq 17.5$. The far-UV
opacity gradient is $d \tau_{\rm eff} / dz \approx (0.444)(1+z)^{1.14}$ over
the range $15 \leq \log N_{\rm HI} \leq 17$, implying mean LyC optical depth
$\tau_{\rm eff} \approx 0.3-0.5$ toward sources at $z = 1-2$.
