6,164 research outputs found
Radiative transfer simulations of cosmic reionization I: methodology and initial results
We present a new hybrid code for large volume, high resolution simulations of
cosmic reionization, which utilizes a N-body algorithm for dark matter,
physically motivated prescriptions for baryons and star formation, and an
adaptive ray tracing algorithm for radiative transfer of ionizing photons. Two
test simulations each with 3 billion particles and 400 million rays in a 50
Mpc/h box have been run to give initial results. Halos are resolved down to
virial temperatures of 10^4 K for the redshift range of interest in order to
robustly model star formation and clumping factors. This is essential to
correctly account for ionization and recombination processes. We find that the
halos and sources are strongly biased with respect to the underlying dark
matter, re-enforcing the requirement of large simulation boxes to minimize
cosmic variance and to obtain a qualitatively correct picture of reionization.
We model the stellar initial mass function (IMF), by following the spatially
dependent gas metallicity evolution, and distinguish between the first
generation, Population III (PopIII) stars and the second generation, Population
II (PopII) stars. The PopIII stars with a top-heavy IMF produce an order of
magnitude more ionizing photons at high redshifts z>10, resulting in a more
extended reionization. In our simulations, complete overlap of HII regions
occurrs at z~6.5 and the computed mass and volume weighted residual HI
fractions at 5<z<6.5 are both in good agreement with high redshift quasar
absorption measurements from SDSS. The values for the Thomson optical depth are
consistent within 1-sigma of the current best-fit value from third-year WMAP.Comment: 15 pages, 13 figures; accepted by ApJ; higher resolution figures can
be found at http://www.astro.princeton.edu/~htrac/reionization.htm
Environmental Effects on Real-Space and Redshift-Space Galaxy Clustering
Galaxy formation inside dark matter halos, as well as the halo formation
itself, can be affected by large-scale environments. Evaluating the imprints of
environmental effects on galaxy clustering is crucial for precise cosmological
constraints with data from galaxy redshift surveys. We investigate such an
environmental impact on both real-space and redshift-space galaxy clustering
statistics using a semi-analytic model derived from the Millennium Simulation.
We compare clustering statistics from original SAM galaxy samples and shuffled
ones with environmental influence on galaxy properties eliminated. Among the
luminosity-threshold samples examined, the one with the lowest threshold
luminosity (~0.2L_*) is affected by environmental effects the most, which has a
~10% decrease in the real-space two-point correlation function (2PCF) after
shuffling. By decomposing the 2PCF into five different components based on the
source of pairs, we show that the change in the 2PCF can be explained by the
age and richness dependence of halo clustering. The 2PCFs in redshift space are
found to change in a similar manner after shuffling. If the environmental
effects are neglected, halo occupation distribution modeling of the real-space
and redshift-space clustering may have a less than 6.5% systematic uncertainty
in constraining beta from the most affected SAM sample and have substantially
smaller uncertainties from the other, more luminous samples. We argue that the
effect could be even smaller in reality. In the Appendix, we present a method
to decompose the 2PCF, which can be applied to measure the two-point
auto-correlation functions of galaxy sub-samples in a volume-limited galaxy
sample and their two-point cross-correlation functions in a single run
utilizing only one random catalog.Comment: 13 pages, 6 figures, Accepted by AP
The Axis-Ratio Distribution of Galaxy Clusters in the SDSS-C4 Catalog as a New Cosmological Probe
We analyze the C4 catalog of galaxy clusters from the Sloan Digital Sky
Survey (SDSS) to investigate the axis-ratio distribution of the projected two
dimensional cluster profiles. We consider only those objects in the catalog
whose virial mass is close to 10^{14}h^{-1}M_{sun}, with member galaxies within
the scale radius 1000 kpc. The total number of such objects turns out to be
336. We also derive a theoretical distribution by incorporating the effect of
projection onto the sky into the analytic formalism proposed recently by Lee,
Jing, & Suto. The theoretical distribution of the cluster axis-ratios is shown
to depend on the amplitude of the linear power spectrum (sigma_8) as well as
the density parameter (Omega_{m}). Finally, fitting the observational data to
the analytic distribution with Omega_{m} and sigma_{8} as two adjustable free
parameters, we find the best-fitting value of sigma_{8}=(1.01 +/-
0.09)(Omega_{m}/0.6)^{(0.07 +/- 0.02) +0.1 Omega_{m}}$. It is a new
sigma_{8}-Omega_{m} relation, different from the previous one derived from the
local abundance of X-ray clusters. We expect that the axis-ratio distribution
of galaxy clusters, if combined with the local abundance of clusters, may put
simultaneous constraints on sigma_{8} and Omega_{m}.Comment: accepted for publication in ApJ, 17 pages, 3 figures, improved
analysis, more discussion on the validity and the caveats of the mode
Radial Alignment in Simulated Clusters
Observational evidence for the radial alignment of satellites with their dark
matter host has been accumulating steadily in the past few years. The effect is
seen over a wide range of scales, from massive clusters of galaxies down to
galaxy-sized systems, yet the underlying physical mechanism has still not been
established. To this end, we have carried out a detailed analysis of the shapes
and orientations of dark matter substructures in high-resolution N-body
cosmological simulations. We find a strong tendency for radial alignment of the
substructure with its host halo: the distribution of halo major axes is very
anisotropic, with the majority pointing towards the center of mass of the host.
The alignment peaks once the sub-halo has passed the virial radius of the host
for the first time, but is not subsequently diluted, even after the halos have
gone through as many as four pericentric passages. This evidence points to the
existence of a very rapid dynamical mechanism acting on these systems and we
argue that tidal torquing throughout their orbits is the most likely candidate.Comment: v2: 13 pages, 10 figures, ApJ in press. Revisions include a new
section (4.2) comparing our results with observations, and a few added
reference
Where can we really find the First Stars' Remnants today?
A number of recent numerical investigations concluded that the remnants of
rare structures formed at very high redshift, such as the very first stars and
bright redshift z~6 QSOs, are preferentially located at the center of the most
massive galaxy clusters at redshift z=0. In this paper we readdress this
question using a combination of cosmological simulations of structure formation
and extended Press-Schechter formalism and we show that the typical remnants of
Population III stars are instead more likely to be found in a group
environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun.
Similarly, the descendants of the brightest z~6 QSOs are expected to be in
medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the
most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1
Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims
that the most massive clusters preferentially host these remnants is rooted in
the numerical method used to initialize their numerical simulations: Only a
small region of the cosmological volume of interest was simulated with
sufficient resolution to identify low-mass halos at early times, and this
region was chosen to host the most massive halo in the cosmological volume at
late times. The conclusion that the earliest structures formed in the entire
cosmological volume evolve into the most massive halo at late times was thus
arrived at by construction. We demonstrate that, to the contrary, the first
structures to form in a cosmological region evolve into relatively typical
objects at later times. We propose alternative numerical methods for simulating
the earliest structures in cosmological volumes.Comment: 18 pages, 5 figures, ApJ accepted, high resolution version of the
paper available at http://www.stsci.edu/~trenti/papers/halo_evolution.pd
The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum
We explore the impact of spatial fluctuations in the intergalactic medium
temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We
develop a semianalytic model to examine temperature fluctuations resulting from
inhomogeneous HI and incomplete HeII reionizations. Detection of these
fluctuations might provide insight into the reionization histories of hydrogen
and helium. Furthermore, these fluctuations, neglected in previous analyses,
could bias constraints on cosmological parameters from the Lyman-alpha forest.
We find that the temperature fluctuations resulting from inhomogeneous HI
reionization are likely to be very small, with an rms amplitude of < 5%,
. More important are the temperature fluctuations
that arise from incomplete HeII reionization, which might plausibly be as large
as 50%, . In practice, however, these temperature
fluctuations have only a small effect on flux power spectrum predictions. The
smallness of the effect is possibly due to density fluctuations dominating over
temperature fluctuations on the scales probed by current measurements. On the
largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the
flux power spectrum may be as large as ~10% in extreme models. The effect is
larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening.
Our results suggest that the omission of temperature fluctuations effects from
previous analyses does not significantly bias constraints on cosmological
parameters.Comment: 11 pages, 5 figures, ApJ accepte
The Clustering of Massive Halos
The clustering properties of dark matter halos are a firm prediction of
modern theories of structure formation. We use two large volume,
high-resolution N-body simulations to study how the correlation function of
massive dark matter halos depends upon their mass and formation history. We
find that halos with the lowest concentrations are presently more clustered
than those of higher concentration, the size of the effect increasing with halo
mass; this agrees with trends found in studies of lower mass halos. The
clustering dependence on other characterizations of the full mass accretion
history appears weaker than the effect with concentration. Using the integrated
correlation function, marked correlation functions, and a power-law fit to the
correlation function, we find evidence that halos which have recently undergone
a major merger or a large mass gain have slightly enhanced clustering relative
to a randomly chosen population with the same mass distribution.Comment: 10 pages, 8 figures; text improved, references and one figure added;
accepted for publication in Ap
Leading Order Calculation of Shear Viscosity in Hot Quantum Electrodynamics from Diagrammatic Methods
We compute the shear viscosity at leading order in hot Quantum
Electrodynamics. Starting from the Kubo relation for shear viscosity, we use
diagrammatic methods to write down the appropriate integral equations for
bosonic and fermionic effective vertices. We also show how Ward identities can
be used to put constraints on these integral equations. One of our main results
is an equation relating the kernels of the integral equations with functional
derivatives of the full self-energy; it is similar to what is obtained with
two-particle-irreducible effective action methods. However, since we use Ward
identities as our starting point, gauge invariance is preserved. Using these
constraints obtained from Ward identities and also power counting arguments, we
select the necessary diagrams that must be resummed at leading order. This
includes all non-collinear (corresponding to 2 to 2 scatterings) and collinear
(corresponding to 1+N to 2+N collinear scatterings) rungs responsible for the
Landau-Pomeranchuk-Migdal effect. We also show the equivalence between our
integral equations obtained from quantum field theory and the linearized
Boltzmann equations of Arnold, Moore and Yaffe obtained using effective kinetic
theory.Comment: 45 pages, 22 figures (note that figures 7 and 14 are downgraded in
resolution to keep this submission under 1000kb, zoom to see them correctly
The non-Gaussian tail of cosmic-shear statistics
Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass () statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA
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