870 research outputs found
Three-Point Statistics from a New Perspective
Multipole expansion of spatial three-point statistics is introduced as a tool
for investigating and displaying configuration dependence. The novel
parametrization renders the relation between bi-spectrum and three-point
correlation function especially transparent as a set of two-dimensional Hankel
transforms. It is expected on theoretical grounds, that three-point statistics
can be described accurately with only a few multipoles. In particular, we show
that in the weakly non-linear regime, the multipoles of the reduced bispectrum,
, are significant only up to quadrupole. Moreover, the non-linear bias in
the weakly non-linear regime only affects the monopole order of these
statistics. As a consequence, a simple, novel set of estimators can be
constructed to constrain galaxy bias. In addition, the quadrupole to dipole
ratio is independent of the bias, thus it becomes a novel diagnostic of the
underlying theoretical assumptions: weakly non-linear gravity and perturbative
local bias. To illustrate the use of our approach, we present predictions based
on both power law, and CDM models. We show that the presently favoured
SDSS-WMAP concordance model displays strong ``baryon bumps'' in the 's.
Finally, we sketch out three practical techniques estimate these novel
quantities: they amount to new, and for the first time edge corrected,
estimators for the bispectrum.Comment: 5 pages 6 figures, ApL accepte
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
The Omega dependence of the velocity divergence distribution
Analytical studies based on perturbative theory have shown that the moments
of the Probability Distribution Function (PDF) of the local smoothed velocity
divergence are expected to have a very specific dependence on the density
parameter Omega in the quasi-linear regime. This dependence is particularly
interesting as it does not involve the possible bias between the galaxy spatial
distribution and the underlying mass distribution. This implies a new and
promising method for determining a bias-independent value of Omega based on a
reliable determination of the velocity divergence PDF.
In this paper we study the Omega dependence of the velocity divergence PDF
and its first moments in a set of N-body simulations, using the so-called
Voronoi and Delaunay methods. We show that this dependence is in agreement with
the theoretical prediction, even while the number density of velocity field
tracers has been diluted to a value comparable to that available in current
galaxy catalogues.
In addition, we demonstrate that a sufficiently reliable determination of
these statistical quantities is also possible when the measurement of the
galaxy peculiar velocities is restricted to the one component along the
line-of-sight. Under ideal, noise-free circumstances we can successfully
discriminate between low and high Omega.Comment: 12 pages, LaTex file, 5 figures. Revised version with significant
extensions of the numerical studies, accepted for publication in MNRA
Previrialization
We propose a method to solve the "previrialization" problem of whether the non-linear interactions between perturbations at different scales increase or decrease the rate of growth of structure. As a measure of this effect we calculate the weakly non-linear corrections to the variance of the probability distribution function of the density field. We assume Gaussian initial conditions and use perturbative expansions to calculate these corrections for scale-free initial power spectra. As a realistic example, we also compute the corrections for the spectrum proposed by Peacock \& Dodds (1994). The calculations are performed for both a Gaussian and a top-hat smoothing of the evolved fields. We show that the effect of weakly non-linear interactions depends strongly on the spectral index; they increase the variance for the spectral index n=-2, but decrease it for n \ge -1. Finally, we compare our perturbative calculations to N-body simulations and a formula of a type proposed by Hamilton et al. (1991)
Semi-analytic modelling of galaxy evolution in the IR/submm range
This paper proposes a new semi-analytic modelling of galaxy properties in the
IR/submm wavelength range, which is explicitly set in a cosmological framework.
This type of approach has had some success in reproducing the optical
properties of galaxies. We hereafter propose a simple extension to the IR/submm
range. We estimate the IR/submm luminosities of ``luminous UV/IR galaxies'',
and we explore how much star formation could be hidden in
heavily--extinguished, ``ultraluminous IR galaxies'' by designing a family of
evolutionary scenarios which are consistent with the current status of the
``cosmic constraints'', as well as with the IRAS luminosity function and faint
counts, but with different high-z IR luminosity densities. However, these
scenarios generate a Cosmic Infrared Background whose spectrum falls within the
range of the isotropic IR component detected by Puget et al. (1996) and
revisited by Guiderdoni et al. (1997). We give predictions for the faint galaxy
counts and redshift distributions at IR and submm wavelengths. The submm range
is very sensitive to the details of the evolutionary scenarios. As a result,
the on-going and forthcoming observations with ISO and SCUBA (and later with
SIRTF, SOFIA, FIRST and PLANCK) will put strong constraints on the evolution of
galaxies at z=1 and beyond.Comment: 21 pages, Latex, 20 postscript figures, accepted for publication in
Month. Not. Roy. Astron. So
Numerical Analyses of Weakly Nonlinear Velocity-Density Coupling
We study evolution of various statistical quantities of smoothed cosmic
density and velocity fields using N-body simulations. The parameter
characterizes nonlinear coupling of
these two fields and determines behavior of bulk velocity dispersion as a
function of local density contrast.
It is found that this parameter depends strongly on the smoothing scale even
in quasi-linear regimes where the skewness parameter
is nearly constant and close to the predicted value by the second-order
perturbation theory. We also analyze weakly nonlinear effects caused by an
adaptive smoothing known as the gather approach.Comment: 22 pages, 4 figures, to appear in ApJ (558, Sep 10
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
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
Reconstructing baryon oscillations
The baryon acoustic oscillation (BAO) method for constraining the expansion
history is adversely affected by non-linear structure formation, which washes
out the correlation function peak created at decoupling. To increase the
constraining power of low z BAO experiments, it has been proposed that one use
the observed distribution of galaxies to "reconstruct'' the acoustic peak.
Recently Padmanabhan, White and Cohn provided an analytic formalism for
understanding how reconstruction works within the context of Lagrangian
perturbation theory. We extend that formalism to include the case of biased
tracers of the mass and, because the quantitative validity of LPT is
questionable, we investigate reconstruction in N-body simulations. We find that
LPT does a good job of explaining the trends seen in simulations for both the
mass and for biased tracers and comment upon the implications this has for
reconstruction.Comment: 9 pages, 8 figure
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