2017ApJ...849..106S

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_H I_ (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 (AGNs) taken by the Cosmic Origins Spectrograph on board the Hubble Space Telescope with G130M/G160M gratings (1134-1795 Å). For 158 absorbers with log N_H I_ >= 15, the mean frequency is d N/dz=4.95±0.39 over path length Δz = 31.94 (0.24 <= z <= 0.84). We identify eight Lyman limit systems (LLS, log N_H I_ >= 17.2) and 54 partial systems (pLLS) with 16.0 <= log N_H I_ < 17.2. Toward 159 AGNs in the range 0.01 < z_abs < 0.84 with Δz ≃ 48, we find four damped Lyα absorbers (DLA) with (d N/dz)_DLA=0.083_–0.040^+0.066 at <z>=0.18. The mean LLS frequency in z = 0.24-0.48 is (d N/dz)_LLS=0.36_–0.13^+0.20 fitted to N(z)=(0.25_–0.09^+0.13)(1+z)^1.14. For 54 pLLSs, we find (d N/dz)_pLLS=1.69±0.23 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)=C₀N^-β^(1+z)γ^ with β = 1.48 ± 0.05 and γ=1.14_–0.89^+0.88 for 15 <= log N_H I_ <= 17.5. The far-UV opacity gradient is d τ_eff/dz ≃ (0.444)(1 + z)^1.14 over the range 15 <= log N_H I_ <= 17, implying mean LyC optical depth τ_eff ≃ 0.3-0.5 toward sources at z = 1-2.