arXiv subjects

J. Silk

Publications and source records attributed to J. Silk.

161 records · Page 9Linked to original sources

arXiv1999

Analysis of CMB maps with 2D wavelets

We consider the 2D wavelet transform with two scales to study sky maps of temperature anisotropies in the cosmic microwave background radiation (CMB). We apply this technique to simulated maps of small sky patches of size 12.8 \times 12.8 square degrees and 1.5' \times 1.5' pixels. The relation to the standard approach, based on the cl's is established through the introduction of the scalogram. We consider temperature fluctuations derived from standard, open and flat-Lambda CDM models. We analyze CMB anisotropies maps plus uncorrelated Gaussian noise (uniform and non-uniform) at idfferent S/N levels. We explore in detail the denoising of such maps and compare the results with other techniques already proposed in the literature. Wavelet methods provide a good reconstruction of the image and power spectrum. Moreover, they are faster than previously proposed methods.

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arXiv1999

Detection of Neutralino Annihilation Photons from External Galaxies

We consider neutralino annihilation in dense extragalactic systems known to be dominated by dark matter, in particular M87 and several local dwarf spheroidal galaxies. These annihilations can produce energetic gamma rays which may be visible to atmospheric Cerenkov telescopes. We explore the supersymmetric parameter space, and compute the expected flux of gamma--rays coming from these objects. It is shown that some parts of the parameter space lead to a signal observable with the next generation of Cerenkov telescopes, provided the supersymmetric dark matter has a clumpy structure, as may be expected in a hierarchical scenario for structure formation.

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arXiv1999

The reheating and reionization history of the universe

We incorporate quasars into an analytic model to describe the reheating and reionization of the universe. In combination with a previous study of galaxies and Lyman-$α$ clouds, we are able to provide a unified description of structure formation, verified against a large variety of observations. We also take into account the clumping of the baryonic gas in addition to the presence of collapsed objects. We consider two cosmologies: a critical universe with a CDM power-spectrum and an open universe with $Ω_0=0.3$, $Λ=0$. The derived quasar luminosity function agrees reasonably well with observations at $z<4.5$ and with constraints over larger redshifts from the HDF. The radiation produced by these objects at $z \sim 16$ slowly reheats the universe which gets suddenly reionized at $z_{ri}=6.8$ for the open universe ($z_{ri}=5.6$ for the critical density universe). The UV background radiation simultaneously increases sharply to reach a maximum of $J_{21} = 0.18$ at $z=2.6$, but shows strong ionization edges until $z \leq 1$. The metallicity of the gas increases quickly at high $z$ and is already larger than $0.01 Z_{\odot}$ at $z=10$. The QSO number counts and the helium opacity constrain the reionization redshift to be $z_{ri} \sim 6$. We confirm that a population of faint quasars is needed in order to satisfy the observations. Due to the low reionization redshift, the damping of CMB fluctuations is quite small, but future observations (e.g. with the NGST) of the multiplicity functions of radiation sources and of the HI and HeII opacities will strongly constrain scenarios in which reionization is due to QSOs.

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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

Simulated NGST Observations

This paper presents the results of simulated NGST observations. The goal of this work is to study the detectability of very high redshift galaxies and to try to characterize Wide Deep-Field observations. The main points made in this paper are 1) high-z galaxies observable by NGST would be similar to local starburst galaxies 2) even if they exist, large spiral galaxies similar to local ones would not be observed by a NGST-like telescope and 3) the density of high-z galaxies detected so far is apparently moderate.

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arXiv1998

The Age and Metallicity Range of Early-Type Galaxies in Clusters

We present an unbiased method for evaluating the ranges of ages and metallicities which are allowed by the photometric properties of the stellar populations that dominate the light of early-type galaxies in clusters. The method is based on the analysis of morphologically-classified early-type galaxies in 17 clusters at redshifts 0.3<z<0.9 and in the nearby Coma cluster using recent stellar population synthesis models that span a wide range of metallicities. We confirm that metallicity effects must play a role in the origin of the slope of the color-magnitude relation for cluster early-type galaxies. We show, however, that the small scatter of the color-magnitude relation out to redshifts z=1 does not formally imply a common epoch of major star formation for all early-type galaxies. Instead, it requires that galaxies assembling more recently be on average more metal-rich than older galaxies of similar luminosity. Regardless of the true ages and metallicities of early-type galaxies within the allowed range, their photometric properties and the implied strengths of several commonly used spectral indices are found to be consistent with apparently passive evolution of the stellar populations. Also, the implied dependence of the mass-to-light ratio on galaxy luminosity is consistent with the observed trend. The results of our unbiased analysis define the boundaries in age and metallicity that must be satisfied by theoretical studies aimed at explaining the formation and evolution of early-type galaxies in clusters.

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arXiv1998

Deprojection of Rich Cluster Images

We consider a general method of deprojecting 2D images to reconstruct the 3D structure of the projected object, assuming axial symmetry. The method consists of the application of the Fourier Slice Theorem to the general case where the axis of symmetry is not necessarily perpendicular to the line of sight, and is based on an extrapolation of the image Fourier transform into the so-called cone of ignorance. The method is specifically designed for the deprojection of X-ray, Sunyaev-Zeldovich (SZ) and gravitational lensing maps of rich clusters of galaxies. For known values of the Hubble constant, H0, and inclination angle, the quality of the projection depends on how exact is the extrapolation in the cone of ignorance. In the case where the axis of symmetry is perpendicular to the line of sight and the image is noise-free, the deprojection is exact. Given an assumed value of H0, the inclination angle can be found by matching the deprojected structure out of two different images of a given cluster, e.g., SZ and X-ray maps. However, this solution is degenerate with respect to its dependence on the assumed H0, and a third independent image of the given cluster is needed to determine H0 as well. The application of the deprojection algorithm to upcoming SZ, X-ray and weak lensing projected mass images of clusters will serve to determine the structure of rich clusters, the value of H0, and place constraints on the physics of the intra-cluster gas and its relation to the total mass distribution.

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arXiv1998

Cosmic Star Formation and Type Ia/II Supernova Rates at high z

We study how Type Ia/II supernova rates at various redshifts can be used to constrain the cosmic star formation rate (CSFR) history. We use a spectrophotometric model of galaxy evolution which provides a self-consistent description of global properties of local galaxies. We have chosen two CSFR histories, with and without dust extinction, both normalized in order to reproduce the observed comoving luminosity densities at different wavelengths. We show that as expected, the SNII rates provide a direct measurement of the instantaneous CSFR, whereas the SNIa rate depends on the CSFR history during the past few Gyrs. Therefore the SNIa rate detected up to $z {\sim}$ 1 is able to provide new insight on the CSFR at higher redshift. The comparison with the data are in good agreement with both local and high-$z$ measurement and provides an additional evidence of the validity of the CSFR up to $z$ = 1, but suggests values at $z >$ 1 higher than those of Madau et al. (1996) and consistent with the presence of extinction. We also show that Type Ia supernova observations at $z{\sim}$ 1 can already put constraints on the still puzzling nature of Type Ia progenitors.

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arXiv1997

Correlation of Excursion Sets for Non-Gaussian CMB Temperature Distributions

We present a method, based on the correlation function of excursion sets above a given threshold, to test the Gaussianity of the CMB temperature fluctuations in the sky. In particular, this method can be applied to discriminate between standard inflationary scenarios and those producing non-Gaussianity such as topological defects. We have obtained the normalized correlation of excursion sets, including different levels of noise, for 2-point probability density functions constructed from the Gaussian, χ_n^2 and Laplace 1-point probability density functions in two different ways. Considering subdegree angular scales, we find that this method can distinguish between different distributions even if the corresponding marginal probability density functions and/or the radiation power spectra are the same.

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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

Dark baryons and rotation curves

The best measured rotation curve for any galaxy is that of the dwarf spiralXXXX DDO 154, which extends out to about 20 disk scale lengths. It provides an ideal laboratory for testing the universal density profile prediction from high resolution numerical simulations of hierarchical clustering in cold dark matter dominated cosmological models. We find that the observed rotation curve cannot be fit either at small radii, as previously noted, or at large radii. We advocate a resolution of this dilemma by postulating the existence of a dark spheroid of baryons amounting to several times the mass of the observed disk component and comparable to that of the cold dark matter halo component. Such an additional mass component provides an excellent fit to the rotation curve provided that the outer halo is still cold dark matter-dominated with a density profile and mass-radius scaling relation as predicted by standard CDM-dominated models. The universal existence of such dark baryonic spheroidal components provides a natural explanation of the universal rotation curves observed in spiral galaxies, may have a similar origin and composition to the local counterpart that has been detected as MACHOs in our own galactic halo via gravitational microlensing, and is consistent with, and even motivated by, primordial nucleosynthesis estimates of the baryon fraction.

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arXiv1996

From Local Velocities to Microwave Background

The mass density field as extracted from peculiar velocities in our cosmological neighborhood is mapped back in time to the CMB in two ways. First, the density power spectrum ($P_k$) is translated into a temperature angular power spectrum of sub-degree resolution ($C_l$) and compared to observations. Second, the local density field is translated into a temperature map in a patch on the last-scattering surface of a distant observer. A likelihood analysis of the Mark III peculiar velocity data have constrained the range of parameters for $P_k$ within the family of COBE-normalized CDM models (Zaroubi et al 1996), favoring a slight tilt in the initial spectrum, $n<1$. The corresponding range of $C_l$'s is plotted against current observations, indicating that the CMB data can tighten the constraints further: only models with ``small'' tilt ($n\sim 0.9$) and ``high'' baryonic content ($Ω_b \sim 0.1$) could survive the two data sets simultaneously. The local mass density field that has been recovered from the velocities via a Wiener method is convolved with a Boltzmann calculation to recover $10'$ resolution temperature maps as viewed from different directions. The extent of the CMB patch and the amplitude of fluctuations depend on the choice of cosmological parameters, e.g., the local $100\hmpc$ sphere corresponds to $90'$ to $30'$ at the CMB for $Ω$ between 1 and 0 respectively. The phases of the temperature map are correlated with those of the density field, contrary to the contribution of the Sachs-Wolfe effect alone. This correlation suggests the possibility of an inverse reconstruction of the underlying density field from CMB data with interesting theoretical implications.

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arXiv1994

Structure Formation with Decaying Neutrinos

We consider the effects of a massive, unstable neutrino on the evolution of large--scale structure and anisotropies in the cosmic microwave background. Comparison with large--scale structure data allows us to rule out a wide range of masses and lifetimes for such neutrinos. We also define a range of masses and lifetimes which delay matter--radiation equality and improve the agreement with the data of Cold Dark Matter models with critical density.

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arXiv1994

Baryonic Dark Matter and the Diffuse Gamma Ray Background

We show that the baryonic gas content of the halo of our galaxy can be probed by performing a multipole expansion on the distribution of diffuse background gamma-ray emission. While the monopole moment (isotropic background) can be used to constrain the baryonic fraction of the halo gas, the quadrupole to monopole ratio is a sensitive probe of the distribution of gas in the halo, {\it i.e.} the degree of flattening of the gas distribution. The predicted diffuse gamma ray flux is found to be very sensitive to the adopted cosmic ray density distribution throughout the halo. If the cosmic rays are uniform, then the upper bound on the gas fraction is $16.6\%$ regardless of the flattening of the halo gas distribution. However, this bound can be weakened by taking into account the removal of flux in and close to the galactic plane, especially for a oblate ($e <1$) gas distribution. On the other hand, in the more realistic situation that the cosmic rays linearly trace the smoothed halo mass (and halo gas) distribution, then a stringent bound on the baryonic gas fraction in the halo $η$, $η\ltwid 3\%$, can be placed with existing data, regardless of halo flattening.

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arXiv1994

Point Sources in the Cosmic Microwave Background?

Non-Gaussian imprints on the cosmic microwave background radiation (CBR) sky are within the grasp of current experiments. A clear non-Gaussian signature would be point-like sources. We have examined the nature of possible point sources that were tentatively identified in a recent high frequency CBR experiment with half-degree resolution (Cheng et al. 1994: MSAM). The effects of local foreground sources, including cold dust clouds, radio sources and the Sunyaev-Zeldovich effect due to foreground rich clusters are considered, and the effective spectral slopes of these various foreground contaminations are calculated. Radio source emission and the Sunyaev-Zeldovich effect are ruled out as the explanation of the possible MSAM sources. Modeles are examined of extremely cold dust clouds which are located in the solar neighborhood, the interstellar medium, the galactic halo or at cosmological distances. We estimate the cloud mass and dust grain parameters, and in particular the grain size distribution, that are required in order to produce a detectable signal in an MSAM-type experiment. It is shown that cold dust clouds can have an important effect on CBR experiments only if the clouds are nearby, and located within a disc scale height of the solar neighborhood. Cold dust emission remains a possible source of far-infrared signal at the level of the detected CBR fluctuations on degree scales, but may be ruled out if the dust emissivity index satisfies $α= 1.5 \pm 0.5$ .

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arXiv1994

The Case for a Hubble Constant of 30 km/s/Mpc

Although cosmologists have been trying to determine the value of the Hubble constant for nearly 65 years, they have only succeeded in limiting the range of possibilities: most of the current observational determinations place the Hubble constant between 50 km/s/Mpc and 90 km/s/Mpc. The uncertainty is unfortunate because this fundamental parameter of cosmology determines both the distance scale and the time scale, and thereby affects almost all aspects of cosmology. Here we make the case for a Hubble constant that is even smaller than the lower bound of the accepted range, arguing on the basis of the great advantages, all theoretical in nature, of a Hubble constant of around 30 km/s/Mpc. Those advantages are: (1) a comfortable expansion age that avoids the current age crisis; (2) a cold dark matter power spectrum whose shape is in good agreement with the observational data and (3) which predicts an abundance of clusters in close agreement with that of x-ray selected galaxy clusters; (4) a nonbaryonic to baryonic mass ratio that is in better agreement with recent determinations based upon cluster x-ray studies. In short, such a value for the Hubble constant cures almost all the ills of the current theoretical orthodoxy, a flat Universe comprised predominantly of cold dark matter.

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arXiv1994

Evolution of Galaxies in Clusters

We use new models of stellar population synthesis to estimate the fraction of stars formed during the last major bursts of star formation in E/S0 galaxies in low-redshift clusters ($z\simlt0.4$) from the spectral signatures of intermediate-age stars. We find that the mass fraction of stars formed in late bursts in early-type galaxies in clusters must have decreased smoothly with redshift, from about 8\% at $z\approx1$ to less than 1\% at $z\approx0$. This result, which we interpret as a constraint on stellar mass added in mergers, is nearly independent of the assumed ages and morphological types of the progenitor galaxies prior to the last major bursts of star formation. We then compute the implied color and 4000 Å break evolution for progenitors of E/S0 galaxies in clusters at redshifts $0\simlt z\simlt1$. We investigate a conservative model in which all present-day E/S0 galaxies are assumed to initially be elliptical galaxies and to then undergo bursts of star formation at the rate estimated from the signatures of intermediate-age stars at low redshifts. This model reproduces well the observed spread of colors and 4000 Å breaks of galaxies in high-redshift clusters, but underestimates the fraction of galaxies in the blue tail of the distribution. Such a discrepancy may be interpreted as an increasing fraction of spiral galaxies in clusters at high redshift, as suggested independently by recent {\it HST} observations of intermediate-redshift clusters. The current data do not seem to require morphological evolution of cluster galaxies out to $z\sim0.4$, but suggest that either morphological or strong luminosity evolution might have played a major role at $z\simgt0.7$.

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