arXiv subjects

F. R. Bouchet

Publications and source records attributed to F. R. Bouchet.

251 records · Page 14Linked to original sources

arXiv1998

Galactic dust polarized emission at high latitudes and CMB polarization

We estimate the dust polarized emission in our galaxy at high galactic latitudes, which is the dominant foreground for measuring CMB polarization using the high frequency instrument (HFI) aboard Planck surveyor. We compare it with the level of CMB polarization and conclude that, for angular scales $\le 1^{\circ}$, the scalar-induced CMB polarization and temperature-polarization cross-correlation are much larger than the foreground level at $ν\simeq 100 GHz$. The tensor-induced signals seem to be at best comparable to the foreground level.}

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arXiv1998

Moving Gravitational Lenses: imprints on the CMB

With the new generation of instruments for Cosmic Microwave Background (CMB) observations aiming at an accuracy level of a few percent in the measurement of the angular power spectrum of the anisotropies, the study of the contributions due to secondary effects has gained impetus. Furthermore, a reinvestigation of the main secondary effects is crucial in order to predict and quantify their effects on the CMB and the errors that they induce in the measurements. In this paper, we investigate the contribution, to the CMB, of secondary anisotropies induced by the transverse motions of clusters of galaxies. This effect is similar to the Kaiser-Stebbins effect. In order to address this problem, we model the gravitational potential well of an individual structure using the Navarro, Frenk & White profile. We generalise the effect of one structure to a population of objects predicted using the Press-Schechter formalism. We simulate maps of these secondary fluctuations, compute the angular power spectrum and derive the average contributions for three cosmological models. We then investigate a simple method to separate this new contribution from the primary anisotropies and from the main secondary effect, the Sunyaev-Zel'dovich kinetic effect from the lensing clusters.

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arXiv1997

The optically-dark side of galaxy formation

In this letter, we estimate how much starlight was absorbed by dust and released in the infrared during the formation of galaxies. New constraints are set on this so-far hidden history by the recent detection of the Cosmic Infrared Background (CIB) built up from the accumulated light of faint sources (and revisited here). By means of a semi-analytical model of hierarchical galaxy formation, we estimate the population of dust-enshrouded starburst galaxies fitting the CIB. It is argued that a deep ISO survey at 175 micron could already detect these sources and help close the gap in our understanding of the history of light in the Universe.

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arXiv1997

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.

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arXiv1997

The contribution of galaxies to the IR background

A new semi-analytic modelling of galaxy evolution in the IR/submm is hereafter outlined. This type of approach successfully reproduces the optical properties of galaxies. We illustrate a simple extension to the IR/submm wavelength range by taking the case of the SCDM model. We design a family of evolutionary scenarios with star formation histories reproducing the evolution of the overall gas and luminosity densities in the universe. These scenarios are also consistent with IRAS data and preliminary ISO counts, and we use them to disentangle the ``Cosmic Infared Background'' detected by Puget et al. (1996) into discrete sources. We finally give predictions for the faint galaxy counts and redshift distributions in various submm wavelengths, which seem to be very sensitive to the details of the evolutionary scenarios. As a consequence, it is easy to anticipate that the current and forthcoming instruments, such as SCUBA, FIRST and PLANCK, will strongly constrain the evolution of high-redshift galaxies.

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arXiv1997

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.

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arXiv1997

Cosmology with Sunyaev-Zeldovich observations from space

In order to assess the potential of future microwave anisotropy space experiments for detecting clusters by their Sunyaev-Zeldovich (SZ) thermal effect, we have simulated maps of the large scale distribution of their Compton parameter $y$ and of the temperature anisotropy $ΔT/T$ induced by their proper motion. Our model is based on a predicted distribution of clusters per unit redshift and flux density using a Press-Schecter approach. These maps were used to create simulated microwave sky by adding them to the microwave contributions of the emissions of our Galaxy and the primary Cosmic Microwave Background (CMB) anisotropies. In order to simulate measurements representative of what current technology should achieve, ``observations'' were performed according to the instrumental characteristics of the COBRAS/SAMBA space mission. We then analyzed the resulting $y$ and $ΔT/T$ maps which now include both the primary anisotropies and those superimposed due to cluster motions.

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arXiv1996

Extended Perturbation Theory for the Local Density Distribution Function

Perturbation theory makes it possible to calculate the probability distribution function (PDF) of the large scale density field in the small variance limit. For top hat smoothing and scale-free Gaussian initial fluctuations, the result depends only on the linear variance, sigma_linear, and its logarithmic derivative with respect to the filtering scale -(n_linear+3)=dlog sigma_linear^2/dlog L (Bernardeau 1994). In this paper, we measure the PDF and its low-order moments in scale-free simulations evolved well into the nonlinear regime and compare the results with the above predictions, assuming that the spectral index and the variance are adjustable parameters, n_eff and sigma_eff=sigma, where sigma is the true, nonlinear variance. With these additional degrees of freedom, results from perturbation theory provide a good fit of the PDFs, even in the highly nonlinear regime. The value of n_eff is of course equal to n_linear when sigma << 1, and it decreases with increasing sigma. A nearly flat plateau is reached when sigma >> 1. In this regime, the difference between n_eff and n_linear increases when n_linear decreases. For initial power-spectra with n_linear=-2,-1,0,+1, we find n_eff ~ -9,-3,-1,-0.5 when sigma^2 ~ 100.

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arXiv1996

Introductory Overview of Eulerian and Lagrangian Perturbation Theories

These lectures notes give an introduction to the fast developing area of research dealing with perturbative descriptions of the gravitational instability in an expanding universe. I just sketch the outlines of some proofs, and many important contributions (and references) were left out. Many untouched aspects are reviewed in \cite{sah_col} which also contains a useful reference list. In the notes dedicated to the Eulerian approach (§\ref{sec:fluid} - §\ref{sec:further}), I derive the fluid equations starting from a microscopic description, I give their linear solution and some higher order corrections, and describe a number of applications concerning the evolution of the one point probability density function for the density contrast field and for the divergence of the corresponding velocity field. In the notes dedicated to the Lagrangian point of view (§\ref{sec:zeldo} - §\ref{sec:lag_app}), I recall Zeldovich Approximation and its higher orders corrections, and present some applications, like their use in describing the statistical effect of the real space-redshift mapping, or as approximate description of the full non-linear dynamics.

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arXiv1996

Clustering statistics and dynamics

Since the appearance of the classical paper of Lifshitz almost half a century ago, linear stability analysis of cosmological models is textbook knowledge. Until recently, however, little was known about the behavior of higher than linear order terms in the perturbative expansion. These terms become important in the weakly nonlinear regime of gravitational clustering, when the rms mass density contrast is only slightly smaller than unity. In the past, theorists showed little interest in studying this regime, and for a good reason: only a decade ago, it would have been an academic excercise - at scales large enough to probe the weakly nonlinear regime, all measures of clustering were dominated by noise. This is no longer the case with present data. The purpose of this talk is to provide a brief summary of recent advances in weakly nonlinear perturbation theory. We present analytical perturbative results together with results of N-body experiments, conducted to test their accuracy. We compare perturbative predictions with measurements from galaxy surveys. Such comparisons can be used to test the gravitational instability theory and to constrain possible deviations from Gaussian statistics in the initial mass distribution; they can be also used to study the nature of physical processes that govern galaxy formation (``biasing''). We also show how future studies of velocity field statistics can provide a new way to determine the density parameter, $Ω$.

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arXiv1995

Self-similarity and scaling behavior of scale-free gravitational clustering

We measure the scaling properties of the probability distribution of the smoothed density field in $N$-body simulations of expanding universes with scale-free initial power-spectra, with particular attention to the predictions of the stable clustering hypothesis. We concentrate our analysis on the ratios $S_Q(\ell)\equiv {\bar ξ}_Q/{\bar ξ}_2^{Q-1}$, $Q \leq 5$, where ${\bar ξ}_Q$ is the averaged $Q$-body correlation function over a cell of radius $\ell$. The behavior of the higher order correlations is studied through that of the void probability distribution function. As functions of ${\bar ξ}_2$, the quantities $S_Q$, $3 \leq Q \leq 5$, exhibit two plateaus separated by a smooth transition around ${\bar ξ}_2 \sim 1$. In the weakly nonlinear regime, ${\bar ξ}_2 \la 1$, the results are in reasonable agreement with the predictions of perturbation theory. In the nonlinear regime, ${\bar ξ}_2 > 1$, the function $S_Q({\bar ξ}_2)$ is larger than in the weakly nonlinear regime, and increasingly so with $-n$. It is well-fitted by the expression $S_Q= ({\bar ξ}_2/100)^{0.045(Q-2)}\ {\widetilde S}_Q$ for all $n$. This weak dependence on scale proves {\em a small, but significant departure from the stable clustering predictions} at least for $n=0$ and $n=+1$. The analysis of $P_0$ confirms that the expected scale-invariance of the functions $S_Q$ is not exactly attained in the part of the nonlinear regime we probe, except possibly for $n=-2$ and marginally for $n=-1$. In these two cases, our measurements are not accurate enough to be discriminant.

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arXiv1994

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.

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arXiv1994

A Count Probability Cookbook: Spurious Effects and the Scaling Model

We study the errors brought by finite volume effects and dilution effects on the practical determination of the count probability distribution function P_N(n,L), which is the probability of having N objects in a cell of volume L^3 for a set of average number density n. Dilution effects are particularly relevant to the so-called sparse sampling strategy. This work is mainly done in the framework of the scaling model (Balian \& Schaeffer 1989), which assumes that the Q-body correlation functions obey the scaling relation xi_Q(K r_1,..., K r_Q) = K^{-(Q-1) gamma} xi_N(r_1,..., r_Q). We use three synthetic samples as references to perform our analysis: a fractal generated by a Rayleigh-Lévy random walk with 3.10^4 objects, a sample dominated by a spherical power-law cluster with 3.10^4 objects and a cold dark matter (CDM) universe involving 3.10^5 matter particles.

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arXiv1994

Redshift Distortions of Clustering: A Lagrangian Approach

We study the effects of peculiar velocities on statistical measures of galaxy clustering. These effects occur when distances to the galaxies are estimated from their redshifts. It is assumed that the clustering pattern results from the gravitational instability of initially Gaussian, small-amplitude perturbations of a Friedman-Lemaitre cosmological model. Explicit expressions are given for an arbitrary density parameter Ωof the model, both when the cosmological constant, Λ, is zero, and when the model is spatially flat, Ω+ Λ/3H^2 =1. We show how to compute the skewness, or third moment of the density field, in redshift space, and its Fourier space counterpart, the bispectrum. We rely on a perturbative expansion of particle trajectories in Lagrangian coordinates. This formalism extends to higher orders the Zel'dovich first order solution (1970). We show that a physically consistent and quantitatively accurate analysis of the growth skewness in redshift space can be obtained from second-order Lagrangian theory. We also study the effects of spatial smoothing of the evolved density field. We give analytic expressions for the gravitationally induced skewness as a function of the power spectrum and of Ω, for a spherical top-hat and a Gaussian smoothing filter. We compare our analytical predictions with measurements performed in numerical simulations, and find good agreement. These results should then prove useful in analyzing large scale redshift surveys. In particular, our results, in conjunction with the recent suggestion of Fry (1994), may help to break the coupling between the bias and the cosmological

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arXiv1994

Perturbative Lagrangian Approach to Gravitational Instability

This paper deals with the time evolution in the matter era of perturbations in Friedman-Lemaitre models with arbitrary density parameter $Ω$, with either a zero cosmological constant, $Λ= 0$, or with a non-zero cosmological constant in a spatially flat Universe. Unlike the classical Eulerian approach where the density contrast is expanded in a perturbative series, this analysis relies instead on a perturbative expansion of particles trajectories in Lagrangian coordinates. This brings a number of advantages over the classical analysis. In particular, it enables the description of stronger density contrasts. Indeed the linear term is the famous Zel'dovich approximate solution (1970). We present here a systematic and detailed account of this approach. We give analytical results (or fits to numerical results) up to the third order. We then proceed to explore the link between the lagrangian description and statistical measures. We show in particular that Lagrangian perturbation theory provides a natural framework to compute the effect of redshift distortions, using the skewness of the density distribution function as an example. Finally, we show how well the second order theory does as compared to other approximat- ions in the case of spherically symmetric perturbations. We also compare this second order approximation and Zel'dovich solution to N-body simulations in the description of large-scale structure formation starting from a power law (n=-2) power spectrum of Gaussian perturbation. We find that second order theory is both simple and powerful.

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arXiv1993

Perturbation Theory Confronts Observations~: Implications for the ``Initial'' Conditions and Omega

This paper covers the material of our two talks. We describe a series of projects based upon perturbative expansions to follow the gravitational evolution of the one point probability distribution functions (PDFs) for the density contrast field and for the divergence of the corresponding velocity field. The Edgeworth expansion greatly simplifies the problem. Indeed, the PDF can be described through several low order moments, with the first non-trivial contribution coming from the skewness, or third moment. Gaussian initial conditions imply that the skewness of the smoothed density field is proportional to the square of the variance. The constant of proportionality is then a function of the logarithmic slope of the variance versus scale, and is otherwise independent of scale. This constant is also practically independent of the density parameter $Ω$, and has very nearly the same value when distances are estimated by redshifts. To show the latter property, we also briefly discuss our Lagrangian approach to perturbation theory. Finally, the observed skewness does depend, of course, on the relation between the distribution of light and mass; we describe the case of local biasing. For the divergence of the velocity field, the skewness of its PDF is also proportional to the square of its variance, but the ratio now strongly depends on the value of $Ω$. This offers a new method to determine the density parameter which does not involve comparisons

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arXiv1993

Skewness Induced by Gravity

Large-scale structures, observed today, are generally believed to have grown from random, small-amplitude inhomogeneities, present in the early Universe. We investigate how gravitational instability drives the distribution of these fluctuations away from the initial state, assumed to be Gaussian. Using second order perturbation theory, we calculate the skewness factor, $S_3 \equiv \langle δ^3 \rangle \, /\, \langle δ^2 \rangle^2$. Here the brackets, $\langle \ldots \rangle$, denote an ensemble average, and $δ$ is the density contrast field, smoothed with a low pass spatial filter. We show that $S_3$ decreases with the slope of the fluctuation power spectrum; it depends only weakly on $Ω$, the cosmological density parameter. We compare perturbative calculations with N-body experiments and find excellent agreement over a wide dynamic range. If galaxies trace the mass, measurements of $S_3$ can be used to distinguish models with Gaussian initial conditions from their non-Gaussian alternatives.

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