Astrophys. J., 796, 49 (2014/November-3)
Tracing the cosmic metal evolution in the low-redshift intergalactic medium.
SHULL J.M., DANFORTH C.W. and TILTON E.M.
Abstract (from CDS):
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, ρion≡ Ωionρcr, with Ωion 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 (ΩCIII + ΩCIV)/ (ΩSiIII + ΩSiIV) = 4.9–1.1+2.2 , are consistent with the photoionization parameter log U = -1.5±0.4, hydrogen photoionization rate ΓH= (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 Δb~ 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 ρZ= (1.5±0.8)x106 M☉/Mpc3 or ΩZ~ 10–5. This represents 10%±5% of the metals produced by (6±2)x108 M☉/Mpc3 of integrated star formation with yield ym= 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.
intergalactic medium - quasars: absorption lines - ultraviolet: galaxies
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