The low-z intergalactic medium.: III.: HI 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 Ly alpha absorbers over redshift path length Delta 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 similar to 40% of the baryons: 30% in the photoionized Ly alpha forest and 10% in the (T = 10(5)-10(6)) 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 Omega(WHIM)/Omega(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, dN/dz = 40(-8)(+14), down to 10 m angstrom 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 proportional to N-beta with beta(O) (VI) approximate to beta(N v) approximate to 2 while beta(H) (I) = 1.7. We conclude that O VI and N V are reliable tracers of the portion of the WHIM at T approximate to 10(5)-10(6) 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 beta(H) (I) < beta(C) (IV) < beta(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 approximate to -1.5 +/- 0.5.