712 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
Cosmological HII Bubble Growth During Reionization
We present general properties of ionized hydrogen (HII) bubbles and their
growth based on a state-of-the-art large-scale (100 Mpc/h) cosmological
radiative transfer simulation. The simulation resolves all halos with atomic
cooling at the relevant redshifts and simultaneously performs radiative
transfer and dynamical evolution of structure formation. Our major conclusions
include: (1) for significant HII bubbles, the number distribution is peaked at
a volume of at all redshifts. But, at ,
one large, connected network of bubbles dominates the entire HII volume. (2)
HII bubbles are highly non-spherical. (3) The HII regions are highly biased
with respect to the underlying matter distribution with the bias decreasing
with time. (4) The non-gaussianity of the HII region is small when the universe
becomes 50% ionized. The non-gaussianity reaches its maximal near the end of
the reionization epoch . But at all redshifts of interest there is a
significant non-gaussianity in the HII field. (5) Population III galaxies may
play a significant role in the reionization process. Small bubbles are
initially largely produced by Pop III stars. At even the largest HII
bubbles have a balanced ionizing photon contribution from Pop II and Pop III
stars, while at Pop II stars start to dominate the overall ionizing
photon production for large bubbles, although Pop III stars continue to make a
non-negligible contribution. (6) The relationship between halo number density
and bubble size is complicated but a strong correlation is found between halo
number density and bubble size for large bubbles.Comment: 10 pages, 14 figures; accepted version; higher resolution figures and
supplementary material can be found at
http://www.astro.princeton.edu/~msshin/reionization/web.ht
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
Tramp Novae Between Galaxies in the Fornax Cluster: Tracers of Intracluster Light
We report the results of a survey for novae in and between the galaxies of
the Fornax cluster. Our survey provides strong evidence that intracluster novae
exist and that they provide a useful, independent measure of the intracluster
light in Fornax. We discovered six strong nova candidates in six distinct
epochs spanning eleven years from 1993 to 2004. The data were taken with the 4m
and the 1.5m telescopes at CTIO. The spatial distribution of the nova
candidates is consistent with 16-41% of the total light in the cluster
being in the intracluster light, based on the ratio of the number of novae we
discovered in intracluster space over the total number of novae discovered plus
a simple completeness correction factor. This estimate is consistent with
independent measures of intracluster light in Fornax and Virgo using
intracluster planetary nebulae. The accuracy of the intracluster light
measurement improves with each survey epoch as more novae are discovered.Comment: 30 pages, 10 figures, accepted for publication in the Astrophysical
Journal (Sep 9, 2004). Version 2: Added references. Full resolution versions
of figures 1-7 and 10 can be found at
http://astrowww.phys.uvic.ca/~neill/fnx
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
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