arXiv subjects

R. Schaeffer

Publications and source records attributed to R. Schaeffer.

18 recordsLinked to original sources

arXiv2003

The phase-diagram of the IGM and the entropy floor of groups and clusters: are clusters born warm?

We point out that two problems of observational cosmology, the facts i) that > 60% of the baryonic content of the universe is not observed at z=0 and ii) that the properties of small clusters do not agree with simple expectations, could be closely related. As shown by recent studies, the shock-heating associated with the formation of large-scale structures heats the intergalactic medium (IGM) and leads to a ``warm IGM'' component for the gas. In the same spirit, we suggest the intracluster medium (ICM) to be a mixture of galaxy-recycled, metal enriched gas and intergalactic gas, shock-heated by the collapsing much larger scales. This could be obtained through two processes: 1) the late infalling gas from the external warm IGM is efficiently mixed within the halo and brings some additional entropy, or 2) the shocks generated by larger non-linear scales are also present within clusters and can heat the ICM. We show that if assumption (1) holds, the entropy brought by the warm IGM is sufficient to explain the observed properties of clusters, in particular the entropy floor and the LX-T relation. On the other hand, we briefly note that the scenario (2) would require a stronger shock-heating because of the larger density of the ICM as compared with filaments. Our scenario of clusters being "born warm" can be checked through the predicted redshift evolution of the entropy floor.

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arXiv2002

The phase-diagram of cosmological baryons

We investigate the behaviour of cosmological baryons at low redshifts $z<5$ after reionization through analytic means. In particular, we study the density-temperature phase-diagram which describes the history of the gas. We show how the location of the matter in this $(ρ,T)$ diagram expresses the various constraints implied by usual hierarchical scenarios. This yields robust model-independent results which agree with numerical simulations. The IGM is seen to be formed via two phases: a ``cool'' photo-ionized component and a ``warm'' component governed by shock-heating. We also briefly describe how the remainder of the matter is distributed over galaxies, groups and clusters. We recover the fraction of matter and the spatial clustering computed by numerical simulations. We also check that the soft X-ray background due to the ``warm'' IGM component is consistent with observations. We find in the present universe a baryon fraction of 7% in hot gas, 24% in the warm IGM, 38% in the cool IGM, 9% within star-like objects and, as a still un-observed component, 22% of dark baryons associated with collapsed structures, with a relative uncertainty no larger than 30% on these numbers.

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arXiv2001

Constraining Dark Matter candidates from structure formation

We show that collisional damping of adiabatic primordial fluctuations yields constraints on the possible range of mass and interaction rates of Dark Matter particles. Our analysis relies on a general classification of Dark Matter candidates, that we establish independently of any specific particle theory or model. From a relation between the collisional damping scale and the Dark Matter interaction rate, we find that Dark Matter candidates must have cross-sections at decoupling smaller than $ 10^{-33} \frac{m_{dm}}{1 MeV} cm^2$ with photons and $10^{-37} \frac{m_{dm}}{1 MeV} cm^2$ with neutrinos, to explain the observed primordial structures of $10^9$ Solar mass. These damping constraints are particularly relevant for Warm Dark Matter candidates. They also leave open less known regions of parameter space corresponding to particles having rather high interaction rates with other species than neutrinos and photons.

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arXiv2001

The redshift evolution of bias and baryonic matter distribution

We study the distribution of baryonic and luminous matter within the framework of a hierarchical scenario. Using an analytical model for structure formation which has already been checked against observations for galaxies, Lyman-$α$ clouds, clusters and reionization processes, we present its predictions for the bias of these objects. We describe its dependence on the luminosity (for galaxies or quasars) or the column density (for Lyman-$α$ absorbers) of the considered objects. We also study its redshift evolution, which can exhibit an intricate behaviour. These astrophysical objects do not trace the dark matter density field, the Lyman-$α$ forest clouds being undercorrelated and the bright galaxies overcorrelated, while the intermediate class of Lyman-limit systems is seen to sample the matter field quite well. We also present the distribution of baryonic matter over these various objects. We show that light does not trace baryonic mass, since bright galaxies which contain most of the stars only form a small fraction of the mass associated with virialized and cooled halos. We consider two cosmologies: a critical density universe and an open universe. In both cases, our results agree with observations and show that hierarchical scenarios provide a good model for structure formation and can describe a wide range of objects which spans at least the seven orders of magnitude in mass for which data exist. More detailed observations, in particular of the clustering evolution of galaxies, will constrain the astrophysical models involved.

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arXiv2000

Multiplicity Functions and X-ray emission of Clusters and Groups versus Galaxies and Quasars

We use a unified analytical formulation for the multiplicity functions of clusters and galaxies which is free from the cloud-in-cloud problem encountered in earlier approaches and well adapted to the description of the non-linear clustering features. It is especially suited to simultaneously describe rich clusters, groups and galaxies, consistently with the hierarchical picture of gravitational clustering and their evolution in time. Using a simple model for the X-ray luminosity (taking into account entropy considerations), we obtain the X-ray luminosity distribution of groups and clusters. Then, using the same formalism we derive the galaxy and quasar multiplicity functions. In particular, we show that the use of the standard Press-Schechter prescription leads to erroneous conclusions at low redshifts while our approach provides a reasonable agreement with observations in a natural fashion because it is able to distinguish galactic halos from groups or clusters. Thus, we obtain a global and consistent picture of the X-ray emissions from all structures. In particular, we show that future observations (e.g., from AXAF) could provide interesting information on galaxy evolution. Indeed, they will constrain the importance of a possible hot diffuse gaseous phase in galactic halos and they could reveal massive galaxies which are just being formed, through the X-ray emission of their cooling gas.

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arXiv2000

Decomposing Quantum Fields on Branes

We provide a method to decompose the two-point function of a quantum field on a warped manifold in terms of fields living on a lower-dimensional manifold. We discuss explicit applications to Minkowski, de Sitter and anti-de Sitter quantum field theories. This decomposition presents a remarkable analogy with the holography principle, in the sense that physics in d+1 dimensions may be encoded into the physics in one dimension less. Moreover in a context a la Randall--Sundrum, the method outlined here allows a mechanism of generation of mass-spectra in the 3-brane (or more generally a d-1-brane).

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arXiv2000

AdS/CFT correspondence for n-point functions

We provide a new general setting for scalar interacting fields on the covering of a d+1-dimensional AdS spacetime. The formalism is used at first to construct a one-paramater family of field theories, each living on a corresponding spacetime submanifold of AdS, which is a cylinder $R\times S_{d-1}$. We then introduce a limiting procedure which directly produces Luescher-Mack CFT's on the covering of the AdS asymptotic cone. Our AdS/CFT correspondence is generally valid for interacting fields, and is illustrated by a complete treatment of two-point functions, the case of Klein-Gordon fields appearing as particularly simple in our context. We also show how the Minkowskian representation of these boundary CFT's can be directly generated by an alternative limiting procedure involving Minkowskian theories in horocyclic sections (nowadays called (d-1)-branes, 3-branes for AdS_5). These theories are restrictions to the brane of the ambient AdS field theory considered. This provides a more general correspondence between the AdS field theory and a Poincare' invariant QFT on the brane, satisfying all the Wightman axioms. The case of two-point functions is again studied in detail from this viewpoint as well as the CFT limit on the boundary.

hep-th↗

arXiv2000

Scaling laws in gravitational clustering for counts-in-cells and mass functions

We present in this article an analysis of some of the properties of the density field realized in numerical simulations for power-law initial power-spectra in the case of a critical density universe. We compare our numerical results in the non-linear regime with the predictions of a specific scaling model, focusing on its much wider range of applicability, which is one of its main advantages over the standard Press-Schechter approximation. We first check that the two-point correlation functions agree with the stable-clustering ansatz. Next we show that the statistics of the counts-in-cells obey the scaling law predicted by our scaling model. Then, we turn to mass functions of overdense and underdense regions. We first consider the mass function of "just collapsed" objects defined by a density threshold $Δ~177$. We note that the usual Press-Schechter prescription agrees reasonably well with the simulations (although there are some discrepancies) while the numerical results are also consistent with the scaling model. Then, we consider more general mass functions defined by different density thresholds which can even be negative. This is out of reach of the Press-Schechter approach while our scaling model can handle these mass functions and it shows a reasonably good agreement with numerical results. Finally, we consider objects defined by a constant radius condition. Thus, we find that the scaling model allows one to study many different classes of objects and it clarifies the links between various statistical tools.

astro-ph↗

arXiv1999

Halo correlations in nonlinear cosmic density fields

The question we address in this paper is the determination of the correlation properties of the dark matter halos appearing in cosmic density fields once they underwent a strongly nonlinear evolution induced by gravitational dynamics. Assuming that the high-order correlation functions of the matter field behave as products of two-body correlation functions, we derive the correlation properties of the halos, that are assumed to represent the correlation properties of galaxies or clusters. The hierarchical pattern originally induced by gravity is shown to be conserved for the halos. The strength of their correlations at any order varies, however, but is found to depend only on their internal properties, namely on the parameter x=m/r^(3-gamma) where m is the mass of the halo, r its size and gammma is the power law index of the two-body correlation function. We were able to derive the explicit form of the generating function of the moments of the halo count probability distribution function. In particular we show explicitely that, generically, S_p(x) -> p^(p-2) in the rare halo limit. Various illustrations of our general results are presented. As a function of the properties of the underlying matter field, we construct the count probabilities for halos and in particular discuss the halo void probability. We also evaluate the dependence of the halo mass function on the environment. We found that within clusters, hierarchical clustering implies the higher masses are favored. We stress that this bias is naturally induced by gravity.

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arXiv1999

The redshift evolution of Lyman-$α$ absorbers

We present a model for the Lyman-alpha absorbers that treats all objects (from the low-density forest clouds to the dense damped systems) in a unified description. This approach is consistent with an earlier model of galaxies (luminosity function, metallicity) but also with the known description of the density field in the small-scale non-linear regime. We consider two cosmological models: a critical universe $Ω=1$ with a CDM power-spectrum, and an open CDM universe with $Ω_0=0.3$, $Λ=0$. We reproduce the available data on column density distribution as a function of redshift, the value of the main new parameter, the background ionizing UV flux, being consistent with the observed limits. This allows a quantitatively trustable analytical description of the opacity, mass, size, velocity dispersion and metallicity of these absorbers, over a range of column densities spanning 10 orders of magnitude. Moreover, together with an earlier model of galaxy formation this draws a unified picture of the redshift evolution of structures in the universe, from underdense clouds to massive high density galaxies, from weak to very deep potential wells.

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arXiv1998

Biasing and high-order statistics from the SSRS2

We analyze different volume-limited samples extracted from the Southern Sky Redshift Survey (SSRS2), using counts-in-cells to compute the Count Probability Distribution Function (CPDF).From the CPDF we derive volume-averaged correlation functions to fourth order and the normalized skewness and kurtosis $S_3 = \bar{ξ_3}/\bar{ξ_2}^2$ and $S_4=\bar{ξ_4}/\bar{ξ_2}^3$. We find that the data satisfies the hierarchical relations in the range $0.3 \lsim \bar{ξ_2} \lsim 10$. In this range, we find $S_3$ to be scale-independent with a value of $\sim 1.8$, in good agreement with the values measured from other optical redshift surveys probing different volumes, but significantly smaller than that inferred from the APM angular catalog. In addition, the measured values of $S_3$ do not show a significant dependence on the luminosity of the galaxies considered. This result is supported by several tests of systematic errors that could affect our measures and estimates of the cosmic variance determined from mock catalogs extracted from N-body simulations. This result is in marked contrast to what would be expected from the strong dependence of the two-point correlation function on luminosity in the framework of a linear biasing model. We discuss the implications of our results and compare them to some recent models of the galaxy distribution which address the problem of bias.

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arXiv1998

Towards a consistent model of the Galaxy: II. Derivation of the model

We use the calculations derived in a previous paper (Méra, Chabrier and Schaeffer, 1997), based on observational constraints arising from star counts, microlensing experiments and kinematic properties, to determine the amount of dark matter under the form of stellar and sub-stellar objects in the different parts of the Galaxy. This yields the derivation of different mass-models for the Galaxy. In the light of all the afore-mentioned constraints, we discuss two models that correspond to different conclusions about the nature and the location of the Galactic dark matter. In the first model there is a small amount of dark matter in the disk, and a large fraction of the dark matter in the halo is still undetected and likely to be non-baryonic. The second, less conventional model is consistent with entirely, or at least predominantly baryonic dark matter, under the form of brown dwarfs in the disk and white dwarfs in the dark halo. We derive observational predictions for these two models which should be verifiable by near future infrared and microlensing observations.

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arXiv1998

Towards a consistent model of the Galaxy: I. kinematic properties, star counts and microlensing observations

We examine the most recent observational constraints arising from i) small-scale and large-scale Galactic dynamical properties, ii) star counts at faint magnitude and iii) microlensing experiments. From these constraints, we determine the halo and disk stellar mass functions and stellar content down to the bottom of the main sequence, which yields the normalization of the halo/disk total stellar population, and we infer the contributions of sub-stellar objects to the mass budget of the various Galactic regions. The consistent analysis of star counts and of the overall microlensing observations in the Bulge are compatible with a small contribution of brown dwarfs to the Galactic mass budget $ρ_{BD}/ρ_* \leq 0.2 $. However the separate bulge/disk analysis based on the bulge clump giants is compatible with a substantial population of disk brown dwarfs, $Σ_{BD}/Σ_*\leq 1 $. More statistics of microlensing events towards the Galactic center and a better determination of the velocity dispersions in the bulge should break this degeneracy of solutions. For the halo, we show that a steep mass-function in the dark halo is excluded and that low-mass stars and brown dwarfs represent a negligible fraction of the halo dark matter, and thus of the observed events towards the LMC. The nature of these events remains a puzzle and halo white dwarfs remain the least unlikely candidates.

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arXiv1997

Collision Induced Galaxy Formation

We present a semi-analytical model in which galaxy collisions and strong tidal interactions, both in the field and during the collapse phase of groups and clusters help determine galaxy morphology. From a semi-analytical analysis based on simulation results of tidal collisions (Aguilar & White 1985), we propose simple rules for energy exchanges during collisions that allow to discriminate between different Hubble types: efficient collisions result in the disruption of disks and substantial star formation, leading to the formation of elliptical galaxies; inefficient collisions allow a large gas reservoir to survive and form disks. Assuming that galaxy formation proceeds in a Omega_0=1 Cold Dark Matter universe, the model both reproduces a number of observations and makes predictions, among which are the redshifts of formation of the different Hubble types in the field. When the model is normalized to the present day abundance of X-ray clusters, the amount of energy exchange needed to produce elliptical galaxies in the field implies that they formed at z>2.5 while spiral galaxies formed at z<1.5. The model also offers a natural explanation for biasing between the various morphological types. We find that the present day morphology-density relation in the field is well reproduced under the collision hypothesis. Finally, predictions of the evolution of the various galaxy populations with redshift are made, in the field as well as in clusters.

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arXiv1997

Scaling in Gravitational Clustering, 2D and 3D Dynamics

Perturbation Theory (PT) applied to a cosmological density field with Gaussian initial fluctuations suggests a specific hierarchy for the correlation functions when the variance is small. In particular quantitative predictions have been made for the moments and the shape of the one-point probability distribution function (PDF) of the top-hat smoothed density. In this paper we perform a series of systematic checks of these predictions against N-body computations both in 2D and 3D with a wide range of featureless power spectra. In agreement with previous studies, we found that the reconstructed PDF-s work remarkably well down to very low probabilities, even when the variance approaches unity. Our results for 2D reproduce the features for the 3D dynamics. In particular we found that the PT predictions are more accurate for spectra with less power on small scales. The nonlinear regime has been explored with various tools, PDF-s, moments and Void Probability Function (VPF). These studies have been done with unprecedented dynamical range, especially for the 2D case, allowing in particular more robust determinations of the asymptotic behaviour of the VPF. We have also introduced a new method to determine the moments based on the factorial moments. Results using this method and taking into account the finite volume effects are presented.

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arXiv1996

Biasing in the Galaxy Distribution

We investigate the variation of galaxy clustering with luminosity using the recently completed SSRS2 sample. Clustering measurements based on the two-point correlation function and the variance of counts in cells reveal the existence of a strong dependence of the clustering amplitude on luminosity for galaxies brighter than L*, while no significant variation is detected for fainter galaxies. We derive a relative bias versus magnitude relation which can be compared with theoretical predictions. Existing models of galaxy formation cannot adequately reproduce the simultaneous steep rise of biasing at high luminosities and the plateau at the low-luminosity end. Improved modeling of the halo-galaxy relation and larger samples including low luminosity galaxies are required to draw more definitive conclusions.

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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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arXiv1993

The Fundamental Plane of Galaxy Clusters

Velocity dispersion $σ$, radius $R$ and luminosity $L$ of elliptical galaxies are known to be related, leaving only two degrees of freedom and defining the so-called ``fundamental plane". In this {\em Letter} we present observational evidence that rich galaxy clusters exhibit a similar behaviour. Assuming a relation $L \propto R^ασ^{2 β}$, the best-fit values of $α$ and $β$ are very close to those defined by galaxies. The dispersion of this relation is lower than 10 percent, i.e. significantly smaller than the dispersion observed in the $L-σ$ and $L-R$ relations. We briefly suggest some possible implications on the spread of formation times of objects and on peculiar velocities of galaxy clusters.

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