arXiv subjects
Publications and source records attributed to J. Silk.
We study a model of the dark energy which exhibits a rapid change in its equation of state w(z), such as occurs in vacuum metamorphosis. We compare the model predictions with CMB, large scale structure and supernova data and show that a late-time transition is marginally preferred over standard Lambda-CDM.
Forthcoming CMB experiments will allow us to accurately investigate the power spectrum at very small scales ($\ell > 1000$). We predict the level of the primary anisotropies, given the actual CMB measurements. The secondary anisotropies generated after matter-radiation decoupling contribute additional power in the tail of the CMB power spectrum. Together with the primary anisotropies, we compute the predicted power spectra for three dominant secondary effects induced by photon scattering. We predict these secondary contributions in flat cosmological models for parameters in agreement (to $2σ$) with the values allowed by current parameter estimates.
In this paper we pursue a new technique to search for evidence of a finite Universe, making use of a spherical mexican-hat wavelet decomposition of the microwave background fluctuations. Using the information provided by the wavelet coefficients at several scales we test whether compact orientable flat topologies are consistent with the COBE-DMR data. We consider topological sizes ranging from half to twice the horizon size. A scale-scale correlation test indicates that non-trivial topologies with appropriate topological sizes are as consistent with the COBE-DMR data as an infinite universe. Among the finite models the data seems to prefer a Universe which is about the size of the horizon for all but the hypertorus and the triple-twist torus. For the latter the wavelet technique does not seem a good discriminator of scales for the range of topological sizes considered here, while a hypertorus has a preferred size which is 80% of the horizon. This analysis allows us to find a best fit topological size for each model, although cosmic variance might limit our ability to distinguish some of the topologies.
The CMB Planck satellite (ESA) will provide a catalogue of nearly 30000 galaxy clusters (Sunyaev-Zel'dovich effect) between redshift 0 and approx. 2-3 (depending on the cosmological model). This hughe catalogue of clusters will allow detailed studies of the evolution of the cluster population with redshift which will constrain the cosmological model. The identification of the high redshift clusters should be done with a large telescope (like GTC) since Planck data alone will not provide any clue about the redshift of the clusters. In this work we show that an optical follow up of only 300 clusters (randomly selected from the Planck catalogue) is needed in order to distinguish models with or without a cosmological constant.
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.
The nature of dark matter is increasingly constrained by cosmological data. In this paper, we examine the implications of the Cosmic Microwave Background anisotropy limits on the density of cold dark matter under different theoretical assumptions and combinations of datasets. We infer the constraint $Ω_{cdm}h^2=0.12\pm0.04$ (at 95% c.l.). The CDM models are compared with the shape of the linear matter power spectrum inferred from the 2dF galaxy redshift survey and with the rms mass fluctuations from recent local cluster observations. We found that a value of $σ_8 \sim 1$ as suggested by recent cosmic shear data is not favoured by the CMB data alone nor by combined CMB+SN-Ia, CMB+HST or CMB+2dFGRS analyses. We also extrapolate our bounds on the rms linear mass fluctuations to sub-galactic scales and compare them with recent lensing constraints, finding agreement with the standard $Λ$CDM model.
Cold, dense clouds of gas have been proposed as baryonic candidates for the dark matter in Galactic haloes, and have also been invoked in the Galactic disc as an explanation for the excess faint sub-mm sources detected by SCUBA. Even if their dust-to-gas ratio is only a small percentage of that in conventional gas clouds, these dense systems would be opaque to visible radiation. This presents the possibility of detecting them by looking for occultations of background stars. We examine the possibility that the data sets of microlensing experiments searching for massive compact halo objects can also be used to search for occultation signatures by cold clouds. We compute the rate and timescale distribution of stellar transits by clouds in the Galactic disc and halo. We find that, for cloud parameters typically advocated by theoretical models, thousands of transit events should already exist within microlensing survey data sets. We examine the seasonal modulation in the rate caused by the Earth's orbital motion and find it provides an excellent probe of whether detected clouds are of disc or halo origin.
A general method of deprojecting two-dimensional images to reconstruct the three dimensional structure of the projected object --specifically X-ray, Sunyaev-Zel'dovich (SZ) and gravitational lensing maps of rich clusters of galaxies -- assuming axial symmetry (Zaroubi et. al. 98), is considered. Here we test the applicability of the method for realistic, numerically simulated galaxy clusters, viewed from three orthogonal projections at four redshift outputs. We demonstrate that the assumption of axial symmetry is a good approximation for the 3D structure in this ensemble of galaxy clusters. Applying the method, we demonstrate that a unique determination of the cluster inclination angle is possible from comparison between the SZ and X-ray images and, independently, between SZ and surface density maps. Moreover, the results from these comparisons are found to be consistent with each other and with the full 3D structure inclination angle determination. The radial dark matter and gas density profiles as calculated from the actual and reconstructed 3D distributions show a very good agreement. The method is also shown to provide a direct determination of the baryon fraction in clusters, independent of the cluster inclination angle.
Recent estimates of cosmological parameters derived from Cosmic Microwave Background (CMB) anisotropies are based on the assumption that we know the precise amount of energy density in relativistic particles in the universe, $ω_{rel}$, at all times. There are, however, many possible mechanisms that can undermine this assumption. In this paper we investigate the effect that removing this assumption has on the determination of the various cosmological parameters. We obtain fairly general bounds on the redshift of equality, $z_{eq}= ω_{m}/ω_{rel}=3100_{-400}^{+600}$. We show that $ω_{rel}$ is nearly degenerate with the amount of energy in matter, $ω_m$, and that its inclusion in CMB parameter estimation also affects the present constraints on other parameters such as the curvature or the scalar spectral index of primordial fluctuations. This degeneracy has the effect of limiting the precision of parameter estimation from the MAP satellite, but it can be broken by measurements on smaller scales such as those provided by the Planck satellite mission.
We review the constraints given by the linear matter power spectra data on cosmological and bias parameters, comparing the data from the PSCz survey (Hamilton et al., 2000) and from the matter power spectrum infered by the study of Lyman alpha spectra at z=2.72 (Croft et al., 2000). We consider flat--$Λ$ cosmologies, allowing $Λ$, $H_0$ and $n$ to vary, and we also let the two ratio factors $r_{pscz}$ and $r_{lyman}$ ($r^2_i = \frac{P_{i}(k)}{P_{CMB}(k)}$) vary independently. Using a simple $χ^2$ minimisation technique, we find confidence intervals on our parameters for each dataset and for a combined analysis. Letting the 5 parameters vary freely gives almost no constraints on cosmology, but requirement of a universal ratio for both datasets implies unacceptably low values of $H_0$ and $Λ$. Adding some reasonable priors on the cosmological parameters demonstrates that the power derived by the PSCz survey is higher by a factor $\sim 1.75$ compared to the power from the Lyman $α$ forest survey.
We present a simple phenomenological model of star formation in galaxies that describes the process with a set of a few parameters. The star formation efficiency and the fraction of gas and metals ejected in outflows are assumed to be the main drivers of star formation. The Tully-Fisher and the Faber-Jackson relations in different passbands are used as constraints in the analysis of disk and elliptical galaxies, respectively. We find that a steep correlation between efficiency and maximum rotation velocity is needed in disk systems, whereas elliptical galaxies can be explained by a high star formation efficiency, uncorrelated with the central velocity dispersion. Gas outflows are not important in disks, whereas a correlation of outflows with galaxy mass must be invoked in early-type galaxies to account for the observed colors.
We argue for implementing star formation on a viscous timescale in hydrodynamical simulations of disk galaxy formation and evolution. Modelling two-dimensional isolated disk galaxies with the Bhatnagar-Gross-Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic timescale for star formation is equal to the viscous timescale in disks, the resulting stellar profile is exponential on several scale lengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi-analytical disk formation simulations which either (a) commence star formation in an already exponential gaseous disk, (b) begin a disk simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disk formation until the dark matter halos are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt-like law, which for lack of a suitable timescale, resorts to an efficiency parameter. With star formation prescribed on a viscous timescale however, we find gas and star fractions after $\sim$ 12 Gyr that are consistent with observations without having to invoke any ``fudge factor'' for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous timescale is indeed the natural timescale for star formation.
There are three independent techniques for determining the age of the universe: via cosmochronology of long-lived radioactive nuclei, via stellar modelling and population synthesis of the oldest stellar populations, and via the precision cosmology that has become feasible with the mapping of the acoustic peaks in the cosmic microwave background. We demonstrate that all three methods give completely consistent results, and enable us to set rigorous bounds on the maximum and minimum ages that are allowed for the universe. We present new constraints on the age of the universe by performing a multiband colour analysis of bright cluster ellipticals over a large redshift range (0.3<z<0.9), which allows us to infer the ages of their stellar populations over a wide range of possible formation redshifts and metallicities. Applying a conservative prior to Hubble's constant of H0=70+-15 km/s/Mpc, we find the age of the universe to be 13.2(+3.6 -2.0) Gyr (1-sigma), in agreement both with the estimates from type Ia supernovae, as well as with the latest uranium decay estimates. Without the assumption of any priors, universes older than 18 Gyr are ruled out by the data at the 90 % confidence level.
A simple chemical enrichment code is described where the two basic mechanisms driving the evolution of the ages and metallicities of the stellar populations are the star formation efficiency and the fraction of gas ejected from the galaxy. Using the observed Tully-Fisher relation in different passbands as a constraint, it is found that a steep correlation between the maximum disk rotational velocity and star formation efficiency must exist either for a linear or a quadratic Schmidt law. Outflows do not play a major role. The redshift evolution of disk galaxies is explored, showing that a significant change in the slope of the Tully-Fisher relation is expected because of the different age distributions of the stellar components in high and low-mass disk galaxies. The slope measured in the rest frame B,K bands is found to change from 3(B); 4(K) at z=0 up to 4.5(B); 5(K) at z~1, with a slight dependence on formation redshift.
In this work we show how well can the cosmological parameters be constrained using galaxy cluster data. We also show how in the process of fitting the model which attempts to describe the data it is possible to get some information about the cluster scaling relations T-M and L_x-T. Among other conclusions we found that only low density universes (Omega approx. 0.3 with or without cosmological constant) are compatible with recent data sets. These constraints will be improved with future SZ data.
We have used recent X-ray and optical data in order to impose some constraints on the cosmology and cluster scaling relations. Generically two kind of hypotheses define our model. First we consider that the cluster population is well described by the standard Press-Schechter (PS) formalism, and second, these clusters are supposed to follow scaling relations with mass: Temperature-Mass (T-M) and X-ray Luminosity-Mass (L_x - M). As a difference with many other authors we do not assume specific scaling relations to model cluster properties such as the usual $T-M$ virial relation or one observational determination of the $L_x-T$ relation. Instead we consider general free parameter scaling relations. With the previous model (PS plus scalings) we fit our free parameters to several X-ray and optical data with the advantage over many other works that we consider all the data sets at the same time. This prevents us from being inconsistent with some of the available observations. Among other interesting conclusions, we find that only low-density universes are compatible with all the data considered and that the degeneracy between $Ω_m$ and $σ_8$ is broken. Also we obtain interesting limits on the parameters characterizing the scaling relations.
We show how future measurements of the SZ effect (SZE) can be used to constrain the cosmological parameters. We combine the SZ information expected from the Planck full-sky survey, N(S), where no redshift information is included, with the N(z) obtained from an optically-identified SZ-selected survey covering less than 1% of the sky. We demonstrate how with a small subsample (approx. 300 clusters) of the whole SZ catalogue observed optically it is possible to drastically reduce the degeneracy among the cosmological parameters. We have studied the requirements for performing the optical follow-up and we show the feasibility of such a project. Finally we have compared the cluster expectations for Planck with those expected for Newton-XMM during their lifetimes. It is shown that, due to its larger sky coverage, Planck will detect a factor approx. 5 times more clusters than Newton-XMM and also with a larger redshift coverage.
We present an analytic model, based on previous works which tackled the reionization history of the universe, which allows us to describe the generation of the secondary CMB anisotropies due to the kinetic SZ effect. We take into account the "patchy pattern" of reionization (HII bubbles), the cross-correlations of these ionized regions, the small-scale fluctuations of the matter density field and the contribution from collapsed objects. For an open universe, we find that the power-spectrum $l(l+1)C_l/(2π)$ exhibits a plateau of height $10^{-13}$ in the range $10^3 < l < 10^6$. We find that for large wavenumbers $l > 10^4$ the signal is dominated by the contribution from collapsed halos while for $l < 10^4$ it is governed by the large-scale correlations of HII bubbles. This implies that one cannot discriminate reionization by stars from a quasar-driven scenario since the size of ionized regions never dominates the behaviour of the anisotropies. Moreover, the secondary CMB anisotropies arise from a broad range of redshifts (7.5<z<10 for the IGM and 0<z<7 for galactic halos). The signal expected in our model might bias the cosmological parameter estimation from CMB experiments such as Planck and could be detected by future mm-wavelength interferometers (e.g., ALMA).