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

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 Ω_WHIM/Ω_b = 0.073 +/- 0.008 down to log N_{O VI} = 13.4 and 0.086 +/- 0.008 down to log N_{O VI} = 13.0. The O VI absorber line frequency, dScript N/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 dScript N/dz propto N^-β with β_{O VI} ≈ β_{N V} ≈ 2 while β_{H 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; N_{C IV} correlates poorly with both N_{H I} and N_{O VI} and β_{H I} < β_{C IV} < β_{O 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.