An Ultraviolet Survey of Low-redshift Partial Lyman-limit Systems with the HST Cosmic Origins Spectrograph

Abstract

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)={C}₀{N}^-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.