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 similar to 10 from z approximate to 5.5 to the present. We derive ion mass densities, rho(ion) = Omega(ion)rho(cr), with Omega(ion) expressed relative to the closure density. Our models of mass-abundance ratios, (Si III/ Si IV) = 0.67(-0.19)(+0.35), (CIII/ C IV) = 0.70(-0.20)(+0.43), and (Omega(C) (III) + Omega(C) (IV))/(Omega(Si III) + Omega(Si IV)) = 4.9(-1.1)(+2.2), are consistent with the photoionization parameter log U = -1.5 +/- 0.4, hydrogen photoionization rate Gamma(H) = (8 +/- 2) x 10(-14) s(-1) at z < 0.4, and specific intensity I-0 = (3 +/- 1) x 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 Phi(0) = 10(4) cm(-2) s(-1), baryon overdensity Delta(b) approximate to 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) x 10(5) M-circle dot Mpc(-3) in the photoionized Ly alpha forest traced by the C and Si ions and (9.1 +/- 0.6) x 10(5) M-circle dot Mpc(-3) in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density rho(Z) = (1.5 +/- 0.8) x 10(6) M-circle dot Mpc(-3) or Omega(Z) approximate to 10(-5). This represents 10% +/- 5% of the metals produced by (6 +/- 2) x 10(8) M-circle dot 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.