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  / dz = 4.95 ± 0.39 over path length Δ z  = 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 Δ z  ≈ 48, we find four damped Ly α absorbers (DLA) with ( d  / dz ) DLA = 0.083 − 0.040 + 0.066 at 〈 z 〉 = 0.18 . The mean LLS frequency in z  = 0.24–0.48 is ( d  / 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  / dz ) pLLS = 1.69 ± 0.23 at 〈 z 〉 = 0.39 , a frequency consistent with gaseous halo sizes R  ≈ 100 h −1 kpc for (0.3–3 L *) 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.