395 research outputs found
The Enrichment History of Baryons in the Universe
We present predictions for the cosmic metal budget in various phases of
baryons from redshift z=6-0, taken from a cosmological hydrodynamic simulation
that includes a well-constrained model for enriched galactic outflows. We find
that substantial amounts of metals are found in every baryonic phase at all
epochs, with diffuse intergalactic gas dominating the metal budget at early
epochs and stars and halo gas dominating at recent epochs. We provide a full
accounting of metals in the context of the missing metals problem at z~2.5,
showing that ~40% of the metals are in galaxies, and the remainder is divided
between diffuse IGM gas and shocked gas in halos and filamentary structures.
Comparisons with available observations of metallicity and metal mass fraction
evolution show broad agreement. We predict stars have a mean metallicity of
one-tenth solar already at z=6, which increases slowly to one-half solar today,
while stars just forming today have typically solar metallicity. Our HI column
density-weighted mean metallicity (comparable to Damped Ly-alpha system
metallicities) slowly increases from one-tenth to one-third solar from z=6-1,
then falls to one-quarter solar at z=0. The global mean metallicity of the
universe tracks ~50% higher than that of the diffuse phase down to z~1, and by
z=0 it has a value around one-tenth solar. Metals move towards higher densities
and temperatures with time, peaking around the mean cosmic density at z=2 and
an overdensity of 100 at z=0. We study how carbon and oxygen ions trace the
path of metals in phase space, and show that OIII-OVII lines provide the most
practical option for constraining intergalactic medium metals at z<2.Comment: 10 pages, MNRAS accepted. Minor changes, Figure 1c fixe
When Does the Intergalactic Medium Become Enriched?
We use cosmological hydrodynamic simulations including galactic feedback
based on observations of local starbursts to find a self-consistent
evolutionary model capable of fitting the observations of the intergalactic
metallicity history as traced by C IV between z=6.0->1.5. Our main finding is
that despite the relative invariance in the measurement of Omega(C IV) as well
as the column density and linewidth distributions over this range, continual
feedback from star formation-driven winds are able to reproduce the
observations, while an early enrichment scenario where a majority of the metals
are injected into the IGM at z>6 is disfavored. The constancy of the C IV
observations results from a rising IGM metallicity content balanced by a
declining C IV ionization fraction due to a 1) decreasing physical densities,
2) increasing ionization background strength, and 3) metals becoming more
shock-heated at lower redshift. Our models predict that ~20x more metals are
injected into the IGM between z=6->2 than at z>6. We show that the median C IV
absorber at z=2 traces metals injected 1 Gyr earlier indicating that the
typical metals traced by C IV are neither from very early times nor from very
recent feedback.Comment: 6 pages, 3 figures, to appear in the proceedings of "Chemodynamics:
from the First Stars to Local Galaxies", Lyon, France, July 10-14, 200
Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime
An accurate treatment of the multiphase interstellar medium (ISM) in
hydrodynamic galaxy simulations requires that we follow not only the thermal
evolution of the gas, but also the evolution of its chemical state, including
its molecular chemistry, without assuming chemical (including ionisation)
equilibrium. We present a reaction network that can be used to solve for this
thermo-chemical evolution. Our model follows the evolution of all ionisation
states of the 11 elements that dominate the cooling rate, along with important
molecules such as H2 and CO, and the intermediate molecular species that are
involved in their formation (20 molecules in total). We include chemical
reactions on dust grains, thermal processes involving dust, cosmic ray
ionisation and heating and photochemical reactions. We focus on conditions
typical for the diffuse ISM, with densities of 10^-2 cm^-3 < nH < 10^4 cm^-3
and temperatures of 10^2 K < T < 10^4 K, and we consider a range of radiation
fields, including no UV radiation. In this paper we consider only gas that is
optically thin, while paper II considers gas that becomes shielded from the
radiation field. We verify the accuracy of our model by comparing chemical
abundances and cooling functions in chemical equilibrium with the
photoionisation code Cloudy. We identify the major coolants in diffuse
interstellar gas to be CII, SiII and FeII, along with OI and H2 at densities nH
> 10^2 cm^-3. Finally, we investigate the impact of non-equilibrium chemistry
on the cooling functions of isochorically or isobarically cooling gas. We find
that, at T < 10^4 K, recombination lags increase the electron abundance above
its equilibrium value at a given temperature, which can enhance the cooling
rate by up to two orders of magnitude. The cooling gas also shows lower H2
abundances than in equilibrium, by up to an order of magnitude.Comment: 26 pages, 13 figures, accepted for publication in MNRAS. Corrected an
error in figure 2. Supplementary material can be found at
http://noneqism.strw.leidenuniv.n
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