3,110 research outputs found
Kurtosis in Large-Scale Structure as a Constraint on Non-Gaussian Initial Conditions
We calculate the kurtosis of a large-scale density field which has undergone
weakly non-linear gravitational evolution from arbitrary non-Gaussian initial
conditions. It is well known that the weakly evolved {\twelveit skewness} is
equal to its initial value plus the term induced by gravity, which scales with
the rms density fluctuation in precisely the same way as for Gaussian initial
conditions. As in the case of skewness, the evolved {\twelveit kurtosis} is
equal to its initial value plus the contribution induced by gravity. The
scaling of this induced contribution, however, turns out to be qualitatively
different for Gaussian versus non-Gaussian initial conditions. Therefore,
measurements of the kurtosis can serve as a powerful discriminating test
between the hypotheses of Gaussian and non-Gaussian nature of primordial
density fluctuations.Comment: uuencoded compressed tar file including postscript text (17 pages)
and 2 postscript figures, submitted to MNRA
Simulations of the Microwave Sky and of its ``Observations''
Here follows a preliminary report on the construction of fake millimeter and
sub-millimeter skies, as observed by virtual instruments, e.g. the COBRA/SAMBA
mission, using theoretical modeling and data extrapolations. Our goal is to
create maps as realistic as possible of the relevant physical contributions
which may contribute to the detected signals. This astrophysical modeling is
followed by simulations of the measurement process itself by a given
instrumental configuration. This will enable a precise determination of what
can and cannot be achieved with a particular experimental configuration, and
provide a feedback on how to improve the overall design. It is a key step on
the way to define procedures for the separation of the different physical
processes in the future observed maps. Note that this tool will also prove
useful in preparing and analyzing current (\eg\ balloon borne) Microwave
Background experiments. Keywords: Cosmology -- Microwave Background
Anisotropies.Comment: 6 pages of uuencoded compressed postscript (1.2 Mb uncompressed), to
appear in the proceedings of the meeting "Far Infrared and Sub-millimeter
Space Missions in the Next Decade'', Paris, France, Eds. M. Sauvage, Space
Science Revie
The FIR/submm window on galaxy formation
Our view on the deep universe has been so far biased towards optically bright
galaxies. Now, the measurement of the Cosmic Infrared Background in FIRAS and
DIRBE residuals, and the observations of FIR/submm sources by the ISOPHOT and
SCUBA instruments begin unveiling the ``optically dark side'' of galaxy
formation. Though the origin of dust heating is still unsolved, it appears very
likely that a large fraction of the FIR/submm emission is due to
heavily-extinguished star formation. Consequently, the level of the CIRB
implies that about 2/3 of galaxy/star formation in the universe is hidden by
dust shrouds. In this review, we introduce a new modeling of galaxy formation
and evolution that provides us with specific predictions in FIR/submm
wavebands. These predictions are compared with the current status of the
observations. Finally, the capabilities of current and forthcoming instruments
for all-sky and deep surveys of FIR/submm sources are briefly described.Comment: 10 pages, Latex, 5 postscript figures, to appear in ``The Birth of
Galaxies'', 1999, B. Guiderdoni, F.R. Bouchet, T.X. Thuan & J. Tran Thanh Van
(eds), Editions Frontiere
All sky CMB map from cosmic strings integrated Sachs-Wolfe effect
By actively distorting the Cosmic Microwave Background (CMB) over our past
light cone, cosmic strings are unavoidable sources of non-Gaussianity.
Developing optimal estimators able to disambiguate a string signal from the
primordial type of non-Gaussianity requires calibration over synthetic full sky
CMB maps, which till now had been numerically unachievable at the resolution of
modern experiments. In this paper, we provide the first high resolution full
sky CMB map of the temperature anisotropies induced by a network of cosmic
strings since the recombination. The map has about 200 million sub-arcminute
pixels in the healpix format which is the standard in use for CMB analyses
(Nside=4096). This premiere required about 800,000 cpu hours; it has been
generated by using a massively parallel ray tracing method piercing through a
thousands of state of art Nambu-Goto cosmic string numerical simulations which
pave the comoving volume between the observer and the last scattering surface.
We explicitly show how this map corrects previous results derived in the flat
sky approximation, while remaining completely compatible at the smallest
scales.Comment: 8 pages, 4 figures, uses RevTeX. References added, matches published
versio
Probing CMB Non-Gaussianity Using Local Curvature
It is possible to classify pixels of a smoothed cosmic microwave background
(CMB) fluctuation map according to their local curvature in ``hill'', ``lake''
and ``saddle'' regions. In the Gaussian case, fractional areas occupied by
pixels of each kind can be computed analytically for families of excursion sets
as functions of threshold and moments of the fluctuation power spectrum. We
show how the shape of these functions can be used to constrain accurately the
level of non-Gaussianity in the data by applying these new statistics to an
hypothetical mixed model suggested by Bouchet et al. (2001). According to our
simple test, with only one 12.5x12.5 deg^2 map, Planck should be able to detect
with a high significance a non-Gaussian level as weak as 10% in temperature
standard deviation (rms) (5% in C_l), whereas a marginal detection would be
possible for MAP with a non-Gaussian level around 30% in temperature (15% in
C_l).Comment: 11 pages, 13 figures, submitted to MNRA
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
The Power Spectrum, Bias Evolution, and the Spatial Three-Point Correlation Function
We calculate perturbatively the normalized spatial skewness, , and full
three-point correlation function (3PCF), , induced by gravitational
instability of Gaussian primordial fluctuations for a biased tracer-mass
distribution in flat and open cold-dark-matter (CDM) models. We take into
account the dependence on the shape and evolution of the CDM power spectrum,
and allow the bias to be nonlinear and/or evolving in time, using an extension
of Fry's (1996) bias-evolution model. We derive a scale-dependent,
leading-order correction to the standard perturbative expression for in
the case of nonlinear biasing, as defined for the unsmoothed galaxy and
dark-matter fields, and find that this correction becomes large when probing
positive effective power-spectrum indices. This term implies that the inferred
nonlinear-bias parameter, as usually defined in terms of the smoothed density
fields, might depend on the chosen smoothing scale. In general, we find that
the dependence of on the biasing scheme can substantially outweigh that
on the adopted cosmology. We demonstrate that the normalized 3PCF, , is an
ill-behaved quantity, and instead investigate , the variance-normalized
3PCF. The configuration dependence of shows similarly strong
sensitivities to the bias scheme as , but also exhibits significant
dependence on the form of the CDM power spectrum. Though the degeneracy of
with respect to the cosmological parameters and constant linear- and
nonlinear-bias parameters can be broken by the full configuration dependence of
, neither statistic can distinguish well between evolving and non-evolving
bias scenarios. We show that this can be resolved, in principle, by considering
the redshift dependence of .Comment: 41 pages, including 12 Figures. To appear in The Astrophysical
Journal, Vol. 521, #
The effect of point sources on satellite observations of the cosmic microwave background
We study the effect of extragalactic point sources on satellite observations
of the cosmic microwave background (CMB). In order to separate the
contributions due to different foreground components, a maximum-entropy method
is applied to simulated observations by the Planck Surveyor satellite. In
addition to point sources, the simulations include emission from the CMB and
the kinetic and thermal Sunyaev-Zel'dovich (SZ) effects from galaxy clusters,
as well as Galactic dust, free-free and synchrotron emission. We find that the
main input components are faithfully recovered and, in particular, that the
quality of the CMB reconstruction is only slightly reduced by the presence of
point sources. In addition, we find that it is possible to recover accurate
point source catalogues at each of the Planck Surveyor observing frequencies.Comment: 12 pages, 9 figures, submitted to MNRA
SPI Measurements of the Diffuse Galactic Hard X-ray Continuum
INTEGRAL Spectrometer SPI data from the first year of the Galactic Centre
Deep Exposure has been analysed for the diffuse continuum from the Galactic
ridge. A new catalogue of sources from the INTEGRAL Imager IBIS has been used
to account for their contribution to the celestial signal. Apparently diffuse
emission is detected at a level ~10% of the total source flux. A comparison of
the spectrum of diffuse emission with that from an analysis of IBIS data alone
shows that they are consistent. The question of the contribution of unresolved
sources to this ridge emission is still open.Comment: Proceedings of the 5th INTEGRAL Workshop, Munich 16-20 February 2004.
ESA SP-552. Reference to Terrier et al. (2004) updated to include astro-ph
versio
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
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