Anthony Aguirre

Metallicity of the Intergalactic Medium Using Pixel Statistics. II. The Distribution of Metals as Traced by C IV

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 2001 Haardt & Madau model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = -3.47 + 0.08(z - 3) + 0.65(log δ - 0.5) and a lognormal scatter of σ([C/H]) = 0.76 + 0.02(z - 3) - 0.23(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 (ΩC ≈ 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.

Dimensionless constants, cosmology and other dark matters

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 weakly interacting massive particle (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.

Metal Enrichment of the Intergalactic Medium in Cosmological Simulations

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~10-2.5 solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to - 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-pressure-driven 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 specific (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 108.5 M☉ galaxies cannot account for the observed metallicity of low column density Lyα absorbers and that dynamical removal from 1010.5 M☉ galaxies cannot account for the ICM metallicities. Dynamical 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 different ejection processes compare.

Intergalactic Dust and Observations of Type Ia Supernovae

Estimates of the cosmic star formation rate and of cluster metallicities independently imply that at z 0.5 the gas in the universe has substantial average metallicity: 1/10 Z/Z☉ 1/3 for Ωgas = 0.05. This metal density probably cannot be contained in known solar-metallicity galaxies of density parameter Ω* ≈ 0.004, implying significant enrichment of the intergalactic medium (IGM) by ejection of metals and dust from galaxies via winds, in mergers or in dust efflux driven by radiation pressure. Galaxies have a dust-to-metal ratio of ~0.5 in their interstellar media, but some fraction (1 - f) > 0 of this must be destroyed in the IGM or during the ejection process. Assuming the Draine & Lee dust model and preferential destruction of small grains (as destruction by sputtering would provide), I calculate the reddening and extinction of a uniform cosmological dust component in terms of f and the minimum grain size amin. Very small grains provide most of the reddening but less than half of the opacity for optical extinction. For f 0.3 and amin 0.1 μm, the intergalactic dust would be too gray to have been detected by its reddening, yet dense enough to be cosmologically important: it could account for the recently observed Type Ia supernova dimming at z ~ 0.5 without cosmic acceleration. It would also have implications for galaxy counts and evolutionary studies and would contribute significantly to the cosmic infrared background (CIB). The importance of gray intergalactic dust of the described type can be tested by observations of z = 0.5 supernovae in (rest) R-band or longer wavelengths and by the fluxes of a large sample of supernovae at z > 1.

Metal enrichment of the intergalactic medium at z=3 by galactic winds

Studies of quasar absorption lines reveal that the low-density intergalactic medium (IGM) at z ~ 3 is enriched to between 10-3 and 10-2 solar metallicity. This enrichment may have occurred in an early generation of Population III stars at redshift z 10, by protogalaxies at 6 z 10, or by larger galaxies at 3 z 6. This paper addresses the third possibility by calculating the enrichment of the IGM at z 3 by galaxies of baryonic mass 108.5 M☉. We use already completed cosmological simulations, to which we add a prescription for chemical evolution and metal ejection by winds, assuming that the winds have properties similar to those observed in local starbursts and Lyman break galaxies. Results are given for a number of representative models, and we also examine the properties of the galaxies responsible for the enrichment as well as the physical effects responsible for wind escape and propagation. We find that winds of velocity 200-300 km s-1 are capable of enriching the IGM to the mean level observed, although many low-density regions would remain metal free. Calibrated by observations of Lyman break galaxies, our calculations suggest that most galaxies at z 3 should drive winds that can escape and propagate to large radii. The primary effect limiting the enrichment of low-density intergalactic gas in our scenario is then the travel time from high- to low-density regions, implying that the metallicity of low-density gas is a strong function of redshift.

Dust versus Cosmic Acceleration

Two groups have recently discovered a statistically significant deviation in the fluxes of high-redshift Type Ia supernovae from the predictions of a Friedmann model with a zero cosmological constant. In this Letter, I argue that bright, dusty, starburst galaxies would preferentially eject a dust component with a shallower opacity curve (hence less reddening) and a higher opacity/mass than the observed galactic dust that is left behind. Such dust could cause the falloff in flux at high redshift without violating constraints on reddening or metallicity. The specific model presented is of needle-like dust, which is expected from the theory of crystal growth and has been detected in samples of interstellar dust. Carbon needles with conservative properties can supply the necessary opacity and would very likely be ejected from galaxies as required. The model is not subject to the arguments given in the literature against gray dust but may be constrained by future data from supernova searches done at higher redshift, in clusters, or over a larger frequency range.

Problems for Modified Newtonian Dynamics in Clusters and the Lyα Forest

The observed dynamics of gas and stars on galactic and larger scales cannot be accounted for by self-gravity, indicating that there are large quantities of unseen matter, or that gravity is non-Newtonian in these regimes. Milgroms MOdified Newtonian Dynamics (MOND) postulates that Newtons laws are modified at very low acceleration, and can account for the rotation curves of galaxies and some other astrophysical observations, without dark matter. Here we apply MOND to two independent physical systems: Ly-alpha absorbers and galaxy clusters. While physically distinct, both are simple hydrodynamical systems with characteristic accelerations in the MOND regime. We find that Ly-alpha absorbers are somewhat smaller than in Newtonian gravity with dark matter, but the result depends crucially on the (unknown) background acceleration field in which they are embedded. In clusters MOND appears to explain the observed (baryonic) mass-temperature relation. However, given observed gas density and enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND predicts radial temperature profiles which disagree badly with observations. We show this explicitly for the Virgo, Abell 2199 and Coma clusters, but the results are general, and seem very difficult to avoid. If this discrepancy is to be resolved by positing additional (presumably baryonic) dark matter, then this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc. This result strongly disfavors MOND as an alternative to dark matter (Abridged).The observed dynamics of gas and stars on galactic and larger scales cannot be accounted for by self-gravity, indicating that there are large quantities of unseen matter or that gravity is non-Newtonian in these regimes. Milgroms modified Newtonian dynamics (MOND) postulates that Newtons laws are modified at very low acceleration, and can account for the rotation curves of galaxies and some other astrophysical observations, without dark matter. Here we apply MOND to two independent physical systems: Lyα absorbers and galaxy clusters. While physically distinct, both are simple hydrodynamical systems with characteristic accelerations in the MOND regime. We find that, because MOND violates the strong equivalence principle, the properties of Lyα absorbers depend strongly on the (unknown) background acceleration field in which they are embedded. If this field is small compared to their internal accelerations, then the absorbers are more dense and about 10 times smaller than in Newtonian gravity with dark matter, in conflict with sizes inferred from quasar pair studies. If, however, the background field is rather large, then the absorbers take on properties similar to those predicted in the cold dark matter picture. In clusters MOND appears to explain the observed (baryonic) mass-temperature relation. However, given observed gas density and enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND predicts radial temperature profiles that disagree badly with observations. We show this explicitly for the Virgo, Abell 2199, and Coma Clusters, but the results are general and seem very difficult to avoid. If this discrepancy is to be resolved by positing additional (presumably baryonic) dark matter, then this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc and about 10 times the gas mass within 200 kpc. This result strongly disfavors MOND as an alternative to dark matter.

Cosmological Constant or Intergalactic Dust? Constraints from the Cosmic Far-Infrared Background

Recent observations of Type Ia supernovae (SNe) at redshifts 0 < z < 1 reveal a progressive dimming that has been interpreted as evidence for a cosmological constant of ΩΛ ~ 0.7. An alternative explanation of the SN results is an open universe with ΩΛ = 0 and the presence of 0.1 μm dust grains with a mass density of Ωdust ~ a few × 10-5 in the intergalactic (IG) medium. The same dust that dims the SNe absorbs the cosmic UV/optical background radiation around ~1 μm, and reemits it at far-infrared (FIR) wavelengths. Here we compare the FIR emission from IG dust with observations of the cosmic microwave (CMB) and cosmic far-infrared backgrounds (FIRB) by the DIRBE/FIRAS instruments. We find that the emission would not lead to measurable distortion of the CMB, but would represent a substantial fraction (75%) of the measured value of the FIRB in the 300-1000 μm range. This contribution would be marginally consistent with the present unresolved fraction of the observed FIRB in an open universe. However, we find that IG dust probably could not reconcile the standard Ω = 1 CDM model with the SN observations, even if the necessary quantity of dust existed. Future observations, capable of reliably resolving the FIRB to a flux limit of ~0.5 mJy, along with a more precise measure of the coarse-grained FIRB, will provide a definitive test of the IG dust hypothesis in all cosmologies.

The enrichment history of cosmic metals

We use a suite of cosmological, hydrodynamical simulations to investigate the chemical enrichment history of the Universe. Specifically, we trace the origin of the metals back in time to investigate when various gas phases were enriched and by what halo masses. We find that the age of the metals decreases strongly with the density of the gas in which they end up. At least half of the metals that reside in the diffuse intergalactic medium (IGM) at redshift zero (two) were ejected from galaxies above redshift two (three). The mass of the haloes that last contained the metals increases rapidly with the gas density. More than half of the mass in intergalactic metals was ejected by haloes with total masses less than 10 11 M⊙ and stellar masses less than 10 9 M⊙. The range of halo masses that contributes to the enrichment is wider for the hotter part of the IGM. By combining the ‘when’ and ‘by what’ aspects of the enrichment history, we show that metals residing in lower density gas were typically ejected earlier and by lower mass haloes.

Confronting cosmological simulations with observations of intergalactic metals

Using the statistics of pixel optical depths, we compare H I, C IV, and C III absorption in a set of six high-quality z ~ 3-4 quasar absorption spectra to that in spectra drawn from two different state-of-the-art cosmological simulations that include galactic outflows. We find that the simulations predict far too little C IV absorption, unless the UV background is extremely soft, and always predict far too small C III/C IV ratios. We note, however, that much of the enriched gas is in a phase (T ~ 105-107 K, ρ/ρ ~ 0.1-10, Z 0.1 Z☉) that should cool by metal line emission—which was not included in our simulations. When the effect of cooling is modeled, the predicted C IV absorption increases substantially, but the C III/C IV ratios are still far too small because the density of the enriched gas is too low. Finally, we find that the predicted metal distribution is much too inhomogeneous to reproduce the observed probability distribution of C IV absorption. These findings suggest that strong z 6 winds cannot fully explain the observed enrichment and that an additional (perhaps higher z) contribution is required.