The Low-z Intergalactic Medium. III. H I and Metal Absorbers at z < 0.4

By C. Danforth, J. Michael Shull, O. Vi, C. Iv, C. Iii, Si Iv, S. Iii. 2007

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We conduct an ultraviolet (HST and FUSE) spectroscopic survey of H I (Lyman lines) and seven metal ions (O VI, N V, C IV, C III, Si IV, Si III, Fe III) in the low-redshift IGM at z < 0.4. We analyzed 650 Lyα absorbers over redshift path length Δ z = 5.27, detecting numerous absorbers: 83 O VI systems, 39 C III, 53 Si III, 24 C IV, 24 N V, and so on. In the low-z IGM, we have accounted for ~40% of the baryons: 30% in the photoionized Lyα forest and 10% in the (T = 105–106) WHIM traced by O VI. Statistical metallicities are consistent with the canonical value of 10% solar, with considerable scatter. Improved statistics for weak absorbers allows us to estimate ΩWHIM/Ωb = 0.073 ± 0.008 down to log NO VI = 13.4 and 0.086 ± 0.008 down to log NO VI = 13.0. The O VI absorber line frequency, d𝒩/dz = 40−8+14, down to 10 mÅ equivalent width suggests a 250-300 kpc extent of metals around dwarf galaxies. Many absorbers appear to contain multiphase gas, with both collisional ionization and photoionization determining the ionization state. N V absorption is well correlated with O VI, and both ions show similarly steep power-law indices d𝒩/dz ∝ N−β with βO VI ≈ βN V ≈ 2 while βH I. We conclude that O VI and N V are reliable tracers of the portion of the WHIM at T ≈ 105–106 K. C IV may be present in both collisional and photoionized phases; NC IV correlates poorly with both NH I and NO VI and βH I < βC IV < βO VI. The ions C III, Si III, and Si IV are well correlated with H I and show patterns typical of photoionization. Adjacent ion stages of the same element (C III/IV and Si III/IV) provide useful constraints on the photoionization parameter, log U ≈ − 1.5 ± 0.5.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/fd70a2c419bc4ef2187c39401325216bc0a51072

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