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, O VI) in
the low-redshift (z < 0.4) intergalactic medium and explored C and Si
ionization corrections from adjacent ion stages. 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, (rho_ion) = (Omega_ion)(rho_cr) with Omega_ion
expressed relative to closure density. Our models of the mass-abundance ratios,
(Si III / Si IV) = 0.67(+0.35,-0.19), (C III / C IV) = 0.70(+0.43,-0.20), and
(Omega_CIII + Omega_CIV) / (Omega_SiIII + Omega_SiIV) = 4.9(+2.2,-1.1), are
consistent with a 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
Hz sr) at the Lyman limit. We find mean photoionization parameter log U = -1.5
+/- 0.4, baryon overdensity Delta_b = 200 +/- 50, and Si/C enhanced to three
times its solar ratio (enhancement of alpha-process elements). 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, Si VII). Our
ionization modeling infers IGM metal densities of (5.4 +/- 0.5) x 10^5 M_sun /
Mpc^3 in the photoionized Lya forest traced by the C and Si ions and (9.1 +/-
0.6) x 10^5 M_sun / 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_sun / Mpc^3 or Omega_Z = 10^{-5}. This represents 10 +/- 5 percent of the
metals produced by (6 +/- 2) x 10^8 M_sun / 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 galaxy halos and
circumgalactic medium.
