597 research outputs found
Was the Universe Reionized by Massive Population-III Stars?
The WMAP satellite has measured a large optical depth to electron scattering
after cosmological recombination of 0.17+-0.04, implying significant
reionization of the primordial gas only ~200 million years after the big bang.
However, the most recent overlap of intergalactic HII regions must have occured
at z<9 based on the Lyman-alpha forest constraint on the thermal history of the
intergalactic medium. Here we argue that a first generation of metal-free stars
with a heavy (rather than Salpeter) mass function is therefore required to
account for much of the inferred optical depth. This conclusion holds if
feedback regulates star formation in early dwarf galaxies as observed in
present-day dwarfs.Comment: 4 pages, 1 figure, replaced to match version accepted by ApJ Letter
The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks
As stars evolve along the Asymptotic Giant Branch, strong winds are driven
from the outer envelope. These winds form a shell, which may ultimately become
a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at
the presence of a binary companion. Some post-Asymptotic Giant Branch objects
are surrounded by torii of crystalline dust, but there is no generally accepted
mechanism for annealing the amorphous grains in the wind to crystals. In this
Letter, we show that the shaping of the wind by a binary companion is likely to
lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ
The metallicity of galactic winds
The abundance evolution of galaxies depends critically on the balance between the mixing of metals in their interstellar medium (ISM), the inflow of new gas and the outflow of enriched gas. We study these processes in gas columns perpendicular to a galactic disc using sub-parsec resolution simulations that track stellar ejecta with the flash code. We model a simplified ISM stirred and enriched by supernovae and their progenitors. We vary the density distribution of the gas column and integrate our results over an exponential disc to predict wind and ISM enrichment properties for disc galaxies. We find that winds from more massive galaxies are hotter and more highly enriched, in stark contrast to that which is often assumed in galaxy formation models. We use these findings in a simple model of galactic enrichment evolution, in which the metallicity of forming galaxies is the result of accretion of nearly pristine gas and outflow of enriched gas along an equilibrium sequence. We compare these predictions to the observed massâmetallicity relation, and demonstrate how the galaxy's gas fraction is a key controlling parameter. This explains the observed flattening of the massâmetallicity relation at higher stellar masses
Constraining reionization using the thermal history of the baryons
The thermal evolution of the intergalactic medium (IGM) depends on the
reionization history of the universe. Numerical simulations indicate that the
low density IGM, which is responsible for the low column density Ly-alpha
forest, follows a well defined temperature-density relation. This results in a
cut-off in the distribution of line widths as a function of column density. We
use hydrodynamic simulations to calibrate the relation between the cut-off and
the temperature-density relation and apply this relation to Keck spectra
spanning a redshift range z=2-4.5. We find that the temperature peaks at z~3
and interpret this as evidence for reheating due to the reionization of helium.Comment: 4 pages, 2 figures, to appear in "Cosmic evolution and galaxy
formation: Structure, interactions, and feedback", eds. J. Franco et a
Targeted incremental economic responses to democratic backsliding in the EU
NWOVI.Veni.201R.061Institutions, Decisions and Collective Behaviou
Radio Afterglows of Gamma-Ray Bursts and Hypernovae at High Redshift, and their Potential for 21-cm Absorption Studies
We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae
(HNe) at high redshifts and quantify their detectability, as well as their
potential usefulness for 21 cm absorption line studies of the intergalactic
medium (IGM) and intervening structures. We examine several sets of source and
environment model parameters that are physically plausible at high redshifts.
The radio afterglows of GRBs would be detectable out to z ~ 30, while the
energetic HNe could be detectable out to z ~ 20 even by the current Very Large
Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral
IGM is difficult to detect even by the proposed Square Kilometer Array (SKA),
except for highly energetic sources. We also find that the 21 cm line due to
collapsed gas clouds with high optical depth may be detected on rare occasions.Comment: 36 pages, 12 figures, 1 table, accepted for publication in Ap
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
The Legitimacy of Free Trade Agreements as Tools of EU Democracy Promotion
This article questions whether the European Union (EU) strategy of using free trade agreements (FTAs) as tools of democracy promotion is, currently, normatively coherent and legitimate. It focuses on FTAs with proximate autocracies and makes four main claims. First, FTAs raise significant legitimacy concerns in that they can ordinarily be expected to generate both economic âwinnersâ and âlosersâ in the target country without democratic processes in place to legitimate these costs. Second, the EU risks empowering autocrats (rather than catalysing democratic transition) in the way it negotiates FTAs. Third, âleverageâ strategies of withholding or suspending cooperation as a result of violations of democratic and human rights norms are applied inconsistently by the EU, undermining leverage credibility. Fourth, the best-case impact of regulatory convergence with the EU acquis communautaire on the democratic character of sector-level policymaking is mixed: increased transparency and accountability can improve democratic credentials, while, paradoxically, increased stakeholder participation is normatively suspect in the absence of a democratic framework.All LEI research area
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
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