SIMBAD references

Using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, we measured the abundances of six ions (C III, C IV, Si III, Si IV, N V, and O VI) in the low-redshift (z ≤ 0.4) intergalactic medium (IGM). Both C IV and Si IV have increased in abundance by a factor of ∼10 from z ~ 5.5 to the present. We derive ion mass densities, ρ_ion≡ Ω_ionρ_cr, with Ω_ion expressed relative to the closure density. Our models of mass-abundance ratios, (Si III/Si IV) = 0.67_–0.19^+0.35, (C III/C IV) 0.70_–0.20^+0.43, and (Ω_CIII + Ω_CIV)/ (Ω_SiIII + Ω_SiIV) = 4.9_–1.1^+2.2 , are consistent with the photoionization parameter log U = -1.5±0.4, hydrogen photoionization rate Γ_H= (8±2)x10^–14/s at z < 0.4, and specific intensity I₀= (3±1)x10^–23 erg/cm2/s/Hz/sr at the Lyman limit. Consistent ionization corrections for C and Si are scaled to an ionizing photon flux Φ₀= 10⁴/cm2/s, baryon overdensity Δ_b~ 200±50, and "alpha-enhancement" (Si/C enhanced to three times its solar ratio). We compare these metal abundances to the expected IGM enrichment and abundances in higher photoionized states of carbon (C V) and silicon (Si V, Si VI, and Si VII). Our ionization modeling infers IGM metal densities of (5.4±0.5)x10⁵ M_☉/Mpc3 in the photoionized Lyα forest traced by the C and Si ions and (9.1±0.6)x10⁵ M_☉/Mpc3 in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density ρ_Z= (1.5±0.8)x10⁶ M_☉/Mpc3 or Ω_Z~ 10^–5. This represents 10%±5% of the metals produced by (6±2)x10⁸ M_☉/Mpc3 of integrated star formation with yield y_m= 0.025±0.010. The missing metals at low redshift may reside within galaxies and in undetected ionized gas in galactic halos and circumgalactic medium.