We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity δ. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. For the median metallicity this technique was described and tested in Paper I of this series. Here we generalize it to reconstruct the full probability distribution of the carbon abundance and apply it to 19 high-quality quasar absorption spectra. We find that the carbon abundance is spatially highly inhomogeneous and is well-described by a lognormal distribution for fixed δ and z. Using data in the range log δ = −0.5 − 1.8 and z = 1.8 − 4.1, and a renormalized version of the Haardt & Madau (2001) model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = −3.47 +0.07 −0.06 + 0.08 +0.09 −0.10 (z − 3) + 0.65 +0.10 −0.14 (log δ − 0.5) and a lognormal scatter of σ([C/H]) = 0.76 +0.05 −0.08 + 0.02 +0.08 −0.12 (z − 3) − 0.23 +0.09 −0.07 (log δ − 0.5). Thus, we find significant trends with overdensity, but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = −2.80 ± 0.13 (Ω C ≈ 2 × 10 −7 ), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for δ < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out temperatures high enough for collisional ionization to be important for the observed C IV) and of the evolution of the effective Lyα optical depth.

We identify 31 dimensionless physical constants required by particle physics and cosmology, and emphasize that both microphysical constraints and selection effects might help elucidate their origin. Axion cosmology provides an instructive example, in which these two kinds of arguments must both be taken into account, and work well together. If a Peccei-Quinn phase transition occurred before or during inflation, then the axion dark matter density will vary from place to place with a probability distribution. By calculating the net dark matter halo formation rate as a function of all four relevant cosmological parameters and assessing other constraints, we find that this probability distribution, computed at stable solar systems, is arguably peaked near the observed dark matter density. If cosmologically relevant WIMP dark matter is discovered, then one naturally expects comparable densities of WIMPs and axions, making it important to follow up with precision measurements to determine whether WIMPs account for all of the dark matter or merely part of it.

Observations have established that the di †use intergalactic medium (IGM) at z D 3 is enriched to D10~2.5 solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to 1 3 È1 2 Z _ at z \ 0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressureÈdriven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations we add dust and (gaseous) metals, assuming instantaneous recycling and distributing the dust and metals in the gas according to three simple parameterized prescriptions, one for each enrichment mechanism. These prescriptions are formulated to capture the basic ejection physics, and calibrated when possible with empirical data. Our method allows exploration of a large number of models, yet for each model yields a speciÐc (not statistical) realization of the cosmic metal distribution that can be compared in detail to observations. Our results indicate that dynamical removal of metals from Z108.5 M _ galaxies cannot account for the observed metallicity of low column density Lya absorbers and that dynamical removal from galaxies cannot account for the ICM metallicities. Dynamical Z1010.5 M _ removal also fails to produce a strong enough mass-metallicity relation in galaxies. In contrast, either wind or radiation-pressure ejection of metals from relatively large galaxies can plausibly account for all three sets of observations (though it is unclear whether metals can be distributed uniformly enough in the low-density regions without overly disturbing the IGM and whether clusters can be enriched quite as much as observed). We investigate in detail how our results change with variations in our assumed parameters and how results for the di †erent ejection processes compare.

We study the abundance of silicon in the intergalactic medium by analyzing the statistics of Si IV, C IV, and H I pixel optical depths in a sample of 19 high-quality quasar absorption spectra, which we compare with realistic spectra drawn from a hydrodynamical simulation. Simulations with a constant and uniform Si/C ratio, a C distribution as derived in Paper II of this series, and a UV background (UVB) model from Haardt & Madau reproduce the observed trends in the ratio of Si IV and C IV optical depths, τ SiIV /τ CIV . The ratio τ SiIV /τ CIV depends strongly on τ CIV , but it is nearly independent of redshift for fixed τ CIV , and is inconsistent with a sharp change in the hardness of the UVB at z ≈ 3. Scaling the simulated optical depth ratios gives a measurement of the global Si/C ratio (using our fiducial UVB, which includes both galaxy and quasar contributions) of [Si/C]= 0.77 ± 0.05, with a possible systematic error of ∼ 0.1 dex. The inferred [Si/C] depends on the shape of the UVB (harder backgrounds leading to higher [Si/C]), ranging from [Si/C]≃ 1.5 for a quasar-only UVB, to [Si/C]≃ 0.25 for a UVB including both galaxies and artificial softening; this provides the dominant uncertainty in the overall [Si/C]. Examination of the full τ SiIV /τ CIV distribution yields no evidence for inhomogeneity in [Si/C] and constrains the width of a lognormal probability distribution in [Si/C] to be much smaller than that of [C/H]; this implies a common origin for Si and C. Since the inferred [Si/C] depends on the UVB shape, this also suggests that inhomogeneities in the hardness of the UVB are small. There is no evidence for evolution in [Si/C]. Variation in the inferred [Si/C] with density depends on the UVB and rules out the quasar-only model unless [Si/C] increases sharply at low density. Comparisons with low-metallicity halo stars and nucleosynthetic yields suggest either that our fiducial UVB is too hard or that supermassive Population III stars might have to be included. The inferred [Si/C], if extrapolated to low density, corresponds to a contribution to the cosmic Si abundance of [Si/H]= −2.0, or Ω Si ≃ 3.2 × 10 −7 , a significant fraction of all Si production expected by z ≈ 3.