Zusammenfassung
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 $0.24 z 0.84$. 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 \AA). For 158 absorbers with $N_HI 15$, the
mean frequency is $d N/dz = 4.95 0.39$ over pathlength $\Delta z =
31.94$ ($0.24 z 0.84)$. We identify 8 Lyman Limit Systems (LLS, $łog
N_HI 17.2$) and 54 partial systems (pLLS) with $16.0 łog
N_HI < 17.2$. Toward 159 AGN between $0.01 < z_abs < 0.84$ with
$\Delta z 48$, we find four damped Ly$\alpha$ absorbers (DLA) with $(d
N/dz)_DLA = 0.083^+0.066_-0.040$ at $z =
0.18$. The mean LLS frequency between $z = 0.24-0.48$ is $(d N/dz)_\rm
LLS = 0.36^+0.20_-0.13$ fitted to $N(z) =
(0.25^+0.13_-0.09)(1+z)^1.14$. For 54 pLLS we find $(d N/dz)_\rm
pLLS = 1.69\pm0.23$ at $z = 0.39$, a frequency consistent with
gaseous halo sizes $R 100 h^-1~kpc$ for ($0.3-3L^*$) galaxies.
A maximum-likelihood analysis yields a distribution $f(N,z) = C_0 N^-\beta
(1+z)^\gamma$ with $= 1.48 0.05$ and $=
1.14^+0.88_-0.89$ for $15 N_HI 17.5$. The far-UV
opacity gradient is $d \tau_eff / dz (0.444)(1+z)^1.14$ over
the range $15 N_HI 17$, implying mean LyC optical depth
$\tau_eff 0.3-0.5$ toward sources at $z = 1-2$.
Nutzer