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

• Published in: The Astrophysical journal 2017-11, Vol.849 (2), p.106
• Main Authors: Shull, J. Michael, Danforth, Charles W., Tilton, Evan M., Moloney, Joshua, Stevans, Matthew L.
• Format: Article
• Language: English
• Subjects: Absorbers; Active galactic nuclei; Astrophysics; cosmological parameters; cosmology: observations; Galaxies; Galaxy distribution; Hubble Space Telescope; Intergalactic media; intergalactic medium; Opacity; Optical analysis; Optical thickness; Origins; Polls & surveys; quasars: absorption lines; Red shift; Space telescopes; Ultraviolet spectra; ultraviolet: galaxies
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa9229

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 NH 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 ). For 158 absorbers with log NH i ≥ 15, the mean frequency is d  dz = 4.95 0.39 over path length Δz = 31.94 (0.24 ≤ z ≤ 0.84). We identify eight Lyman limit systems (LLS, log NH i ≥ 17.2) and 54 partial systems (pLLS) with 16.0 ≤ log NH i < 17.2. Toward 159 AGNs in the range 0.01 < zabs < 0.84 with Δz 48, we find four damped Ly 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 (0.3-3L*) 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 NH i ≤ 17.5. The far-UV opacity gradient is d τeff/dz (0.444)(1 + z)1.14 over the range 15 ≤ log NH i ≤ 17, implying mean LyC optical depth τeff 0.3-0.5 toward sources at z = 1-2.