Tracing the Cosmic Metal Evolution in the Low-redshift Intergalactic Medium

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^+0.35_-0.19, (C III/C IV) = 0.70^+0.43_-0.20, and (Ω _{C \scriptsize{III}} + Ω _C \scriptsize{IV}) / (Ω _Si \scriptsize{III} + Ω _Si \scriptsize{IV}) = 4.9^+2.2_-1.1, are consistent with the photoionization parameter log U = -1.5 ± 0.4, hydrogen photoionization rate Γ_H = (8 ± 2) × 10^-14 s^-1 at z < 0.4, and specific intensity I ₀ = (3 ± 1) × 10^-23 erg cm^-2 s^-1 Hz^-1 sr^-1 at the Lyman limit. Consistent ionization corrections for C and Si are scaled to an ionizing photon flux Φ₀ = 10⁴ cm^-2 s^-1, 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) × 10⁵ M _⊙ Mpc^-3 in the photoionized Lyα forest traced by the C and Si ions and (9.1 ± 0.6) × 10⁵ M _⊙ Mpc^-3 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) × 10⁶ M _⊙ Mpc^-3 or Ω _Z ≈ 10^-5. This represents 10% ± 5% of the metals produced by (6 ± 2) × 10⁸ M _⊙ Mpc^-3 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA Contract NAS5-26555.