2,677 research outputs found
The Bispectrum of IRAS Galaxies
We compute the bispectrum for the galaxy distribution in the IRAS QDOT, 2Jy,
and 1.2Jy redshift catalogs for wavenumbers 0.05<k<0.2 h/Mpc and compare the
results with predictions from gravitational instability in perturbation theory.
Taking into account redshift space distortions, nonlinear evolution, the survey
selection function, and discreteness and finite volume effects, all three
catalogs show evidence for the dependence of the bispectrum on configuration
shape predicted by gravitational instability. Assuming Gaussian initial
conditions and local biasing parametrized by linear and non-linear bias
parameters b_1 and b_2, a likelihood analysis yields 1/b_1 =
1.32^{+0.36}_{-0.58}, 1.15^{+0.39}_{-0.39} and b_2/b_1^2=-0.57^{+0.45}_{-0.30},
-0.50^{+0.31}_{-0.51}, for the for the 2Jy and 1.2Jy samples, respectively.
This implies that IRAS galaxies trace dark matter increasingly weakly as the
density contrast increases, consistent with their being under-represented in
clusters. In a model with chi^2 non-Gaussian initial conditions, the bispectrum
displays an amplitude and scale dependence different than that found in the
Gaussian case; if IRAS galaxies do not have bias b_1> 1 at large scales, \chi^2
non-Gaussian initial conditions are ruled out at the 95% confidence level. The
IRAS data do not distinguish between Lagrangian or Eulerian local bias.Comment: 30 pages, 11 figure
Search for Outbursts in the Narrow 511-keV Line from Compact Sources Based on INTEGRAL Data
We present the results of a systematic search for outbursts in the narrow
positron annihilation line on various time scales (5x10^4 - 10^6 s) based on
the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts
were detected with a statistical significance higher than ~6 sigma for any of
the time scales considered over the entire period of observations. We also show
that, given the large number of independent trials, all of the observed spikes
could be associated with purely statistical flux fluctuations and, in part,
with a small systematic prediction error of the telescope's instrumental
background. Based on the exposure achieved in ~6 yr of INTEGRAL operation, we
provide conservative upper limits on the rate of outbursts with a given
duration and flux in different parts of the sky.Comment: 16 pages, 8 figures. To be published in Astronomy Letters, 2010, Vol.
36, No 4, p. 23
CMB Polarization Data and Galactic Foregrounds: Estimation of Cosmological Parameters
We estimate the accuracy with which various cosmological parameters can be
determined from the CMB temperature and polarization data when various galactic
unpolarized and polarized foregrounds are included and marginalized using the
multi-frequency Wiener filtering technique. We use the specifications of the
future CMB missions MAP and PLANCK for our study. Our results are in
qualitative agreement with earlier results obtained without foregrounds, though
the errors in most parameters are higher because of degradation of the
extraction of polarization signal in the presence of foregrounds.Comment: 6 pages, submitted to MNRA
Correlations in the Far Infrared Background
We compute the expected angular power spectrum of the cosmic Far Infrared
Background (FIRB). We find that the signal due to source correlations dominates
the shot--noise for \ell \la 1000 and results in anisotropies with rms
amplitudes between 5% and 10% of the mean
for l \ga 150. The angular power spectrum depends on several unknown
quantities, such as the UV flux density evolution, optical properties of the
dust, biasing of the sources of the FIRB, and cosmological parameters. However,
when we require our models to reproduce the observed DC level of the FIRB, we
find that the anisotropy is at least a few percent in all cases. This
anisotropy is detectable with proposed instruments, and its measurement will
provide strong constraints on models of galaxy evolution and large-scale
structure at redshifts up to at least .Comment: 7 pages, 4 figures included, uses emulateapj.sty. More models
explored than in original version. Accepted for publication in Ap
The Mid-Infrared Instrument for the James Webb Space Telescope, VII: The MIRI Detectors
The MIRI Si:As IBC detector arrays extend the heritage technology from the
Spitzer IRAC arrays to a 1024 x 1024 pixel format. We provide a short
discussion of the principles of operation, design, and performance of the
individual MIRI detectors, in support of a description of their operation in
arrays provided in an accompanying paper (Ressler et al. (2015)). We then
describe modeling of their response. We find that electron diffusion is an
important component of their performance, although it was omitted in previous
models. Our new model will let us optimize the bias voltage while avoiding
avalanche gain. It also predicts the fraction of the IR-active layer that is
depleted (and thus contributes to the quantum efficiency) as signal is
accumulated on the array amplifier. Another set of models accurately predicts
the nonlinearity of the detector-amplifier unit and has guided determination of
the corrections for nonlinearity. Finally, we discuss how diffraction at the
interpixel gaps and total internal reflection can produce the extended
cross-like artifacts around images with these arrays at short wavelengths, ~ 5
microns. The modeling of the behavior of these devices is helping optimize how
we operate them and also providing inputs to the development of the data
pipeline
Fundamental Discreteness Limitations of Cosmological N-Body Clustering Simulations
We explore some of the effects that discreteness and two-body scattering may
have on N-body simulations with ``realistic'' cosmological initial conditions.
We use an identical subset of particles from the initial conditions for a
Particle-Mesh (PM) calculation as the initial conditions for a variety
PM and Tree code runs. We investigate the effect of mass resolution (the
mean interparticle separation) since most ``high resolution'' codes only have
high resolution in gravitational force. The phase-insensitive two--point
statistics, such as the power spectrum (autocorrelation) are somewhat affected
by these variations, but phase-sensitive statistics show greater differences.
Results converge at the mean interparticle separation scale of the lowest
mass-resolution code. As more particles are added, but the force resolution is
held constant, the PM and the Tree runs agree more and more strongly with
each other and with the PM run which had the same initial conditions. This
shows high particle density is necessary for correct time evolution, since many
different results cannot all be correct. However, they do not so converge to a
PM run which continued the fluctuations to small scales. Our results show that
ignoring them is a major source of error on comoving scales of the missing
wavelengths. This can be resolved by putting in a high particle density. Since
the codes never agree well on scales below the mean comoving interparticle
separation, we find little justification for quantitative predictions on this
scale. Some measures vary by 50%, but others can be off by a factor of three or
more. Our results suggest possible problems with the density of galaxy halos,
formation of early generation objects such as QSO absorber clouds, etc.Comment: Revised version to be published in Astrophysical Journal. One figure
changed; expanded discussion, more information on code parameters. Latex, 44
pages, including 19 figures. Higher resolution versions of Figures 10-15
available at: ftp://kusmos.phsx.ukans.edu/preprints/nbod
The INTEGRAL/SPI response and the Crab observations
The Crab region was observed several times by INTEGRAL for calibration
purposes. This paper aims at underlining the systematic interactions between
(i) observations of this reference source, (ii) in-flight calibration of the
instrumental response and (iii) the development and validation of the analysis
tools of the SPI spectrometer. It first describes the way the response is
produced and how studies of the Crab spectrum lead to improvements and
corrections in the initial response. Then, we present the tools which were
developed to extract spectra from the SPI observation data and finally a Crab
spectrum obtained with one of these methods, to show the agreement with
previous experiments. We conclude with the work still ahead to understand
residual uncertainties in the response.Comment: 4 pages, 4 figures, Proc. of the 5th INTEGRAL Workshop (Feb. 16-20
2004), to be published by ES
Relaxation times of unstable states in systems with long range interactions
We consider several models with long-range interactions evolving via
Hamiltonian dynamics. The microcanonical dynamics of the basic Hamiltonian Mean
Field (HMF) model and perturbed HMF models with either global anisotropy or an
on-site potential are studied both analytically and numerically. We find that
in the magnetic phase, the initial zero magnetization state remains stable
above a critical energy and is unstable below it. In the dynamically stable
state, these models exhibit relaxation time scales that increase algebraically
with the number of particles, indicating the robustness of the
quasistationary state seen in previous studies. In the unstable state, the
corresponding time scale increases logarithmically in .Comment: Minor change
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