arXiv subjects

Uros Seljak

Publications and source records attributed to Uros Seljak.

At least 163 records · Page 9Linked to original sources

arXiv2000

Tentative Detection of Galaxy Spin Correlations in the Tully Catalogue

We report a tentative detection of spin correlations in the Tully catalogue of nearby galaxies. We define a simple but nontrivial spin correlation function, and find an analytic estimate of it in the frame of the linear perturbation theory. Then, we present the observed spin correlation signal from the Tully galaxies with error bars. The three dimensional spin correlation turns out to be significant at the 97% confidence level, detected out to a few $h^{-1}$ Mpc. This observed correlation is consistent with the theoretical prediction based on the gravitational instability picture of galaxy formation. An analysis of systematic errors is also presented. The observed strength of correlation may be sufficient to significantly affect blank field of weak lensing searches.

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arXiv2000

Galaxy-dark matter correlations applied to galaxy-galaxy lensing: predictions from the semi-analytic galaxy formation models

We use semi-analytic models of galaxy formation combined with high resolution N-body simulations to make predictions for galaxy-dark matter correlations and apply them to galaxy-galaxy lensing. We analyze cross-correlation spectra between the dark matter and different galaxy samples selected by luminosity, color or star formation rate. We compare the predictions to the recent detection by SDSS. We show that the correlation amplitude and the mean tangential shear depend strongly on the luminosity of the sample on scales below 1 Mpc, reflecting the correlation between the galaxy luminosity and the halo mass. The cross-correlation cannot however be used to infer the halo profile directly because different halo masses dominate on different scales and because not all galaxies are at the centers of the corresponding halos. We compute the redshift evolution of the cross-correlation amplitude and compare it to those of galaxies and dark matter. We also compute the galaxy-dark matter correlation coefficient and show it is close to unity on scales above r > 1 Mpc for all considered galaxy types. This would allow one to extract the bias and the dark matter power spectrum on large scales from the galaxy and galaxy-dark matter correlations.

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arXiv2000

On Estimating the QSO Transmission Power Spectrum

We present new methods to minimize the systematic and random errors for measuring the transmission power spectrum from the Lyman-alpha forest. Sources of systematic errors explored include metal line contamination and continuum-fitting. We advocate the technique of trend-removal in place of traditional continuum-fitting -- here, a spectrum is normalized by its (smoothly varying) mean rather than its continuum -- this method is easily automated and removes biases introduced by continuum-fitting. Trend- removal can be easily applied to spectra where continuum-fitting is difficult, such as when the resolution or signal-to-noise is low, or for spectra at high redshifts. Furthermore, a measurement of the continuum power spectrum using trend-removal, from either low redshift quasar spectra or the red-side of Lyman-alpha, allows in principle the removal of spurious power introduced by the continuum and thereby expanding scales probed to larger ones. We also derive expressions for the shot-noise bias and variance of the power spectrum estimate, taking into account the non-Poissonian nature of the shot-noise and the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance weighting of the data is given. Finally, we give practical suggestions on observing strategy: the desired resolution and S/N for different purposes, and how to distribute one's finite observing time among quasar targets. Also discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey, which has the potential to measure the power spectrum at z ~ 2-4 accurate to better than 1 % per mode -- the techniques presented here will be useful for tackling the anticipated issues of shot-noise and continuum contamination.

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arXiv2000

Analytic model for galaxy and dark matter clustering

We investigate an analytic model to compute nonlinear power spectrum of dark matter, galaxies and their cross-correlation. The model is based on Press-Schechter halos, which cluster and have realistic dark matter profiles. The total power spectrum is a sum of two contributions, one from correlations betwen the halos and one from correlations within the same halo. We show that such a model can give dark matter power spectra which match well with the results of N-body simulations, provided that concentration parameter decreases with the halo mass. Galaxy power spectrum differs from dark matter power spectrum because pair weighted number of galaxies increases less rapidly than the halo mass, as predicted by theoretical models and observed in clusters. In this case the resulting power spectrum becomes a power law with the slope closed to the observed. Such a model also predicts a later onset of nonlinear clustering compared to the dark matter, which is needed to reconcile the CDM models with the data. Generic prediction of this model is that bias is scale dependent and nonmonotonic. For red or elliptical galaxies bias in power spectrum may be scale dependent even on very large scales. Our predictions for galaxy-dark matter correlations, which can be observed through the galaxy-galaxy lensing, show that these cannot be interpreted simply as an average halo profile of a typical galaxy, because different halo masses dominate at different scales and because larger halos host more than one galaxy. We discuss the prospects of using cross-correlations in combination with galaxy clustering to determine the dark matter power spectrum (ABRIDGED).

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arXiv2000

Sunyaev-Zeldovich effect from hydrodynamical simulations: maps and low order statistics

We use moving mesh hydrodynamical simulations to make maps of Sunyaev-Zeldovich effect. We present these maps for several cosmological models and explore their lowest moments. We find that the first moment, the mean Compton $y$ parameter, is typically between $1-2\times 10^{-6}$ for cluster abundance normalized models, the lower value corresponding to the high density models, and scales as $σ_8^{3-4}$. Rms fluctuations at 10' scale have an amplitude $ ΔT/T \sim 1-3 \times 10^{-6}$ in the Rayleigh-Jeans regime. The amplitude of the power spectrum strongly depends on the power spectrum normalization and scales as $σ_8^{6-9}$. On smaller scales ($l>1000$) the spectrum is dominated by halos below $10^{13}M_{\sun}$ and so is sensitive to the thermal history of galactic halo gas. On larger scales ($l<1000$) the power spectrum is less sensitive to nongravitational energy injection, but becomes very noisy, with large field to field variations in the power spectrum caused by rare bright sources in the maps, which dominate in the spectrum over the large scale structure correlations. Cross-correlation power spectrum with weak lensing or projected galaxy distribution is significant and the cross-correlation coefficient is around 0.5 over a wide range of scales. Comparison with Press-Schechter predictions gives very good agreement for all the statistics, indicating that there is no significant contribution to SZ from non-virialized structures. The one point distribution function shows clear deviations from gaussianity and can be well-approximated as log-normal on small scales.

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arXiv1999

Lensing effect on polarization in microwave background: extracting convergence power spectrum

Matter inhomogeneities along the line of sight deflect the cosmic microwave background (CMB) photons originating at the last scattering surface at redshift $z \sim 1100$. These distortions modify the pattern of CMB polarization. We identify specific combinations of Stokes $Q$ and $U$ parameters that correspond to spin 0,$\pm 2$ variables and can be used to reconstruct the projected matter density. We compute the expected signal to noise as a function of detector sensitivity and angular resolution. With Planck satellite the detection would be at a few $σ$ level. Several times better detector sensitivity would be needed to measure the projected dark matter power spectrum over a wider range of scales, which could provide an independent confirmation of the projected matter power spectrum as measured from other methods.

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arXiv1999

Ray Tracing Simulations of Weak Lensing by Large-Scale Structure

We investigate weak lensing by large-scale structure using ray tracing through N-body simulations. Photon trajectories are followed through high resolution simulations of structure formation to make simulated maps of shear and convergence on the sky. Tests with varying numerical parameters are used to calibrate the accuracy of computed lensing statistics on angular scales from about 1 arcminute to a few degrees. Various aspects of the weak lensing approximation are also tested. For fields a few degrees on a side the shear power spectrum is almost entirely in the nonlinear regime and agrees well with nonlinear analytical predictions. Sampling fluctuations in power spectrum estimates are investigated by comparing several ray tracing realizations of a given model. For survey areas smaller than a degree on a side the main source of scatter is nonlinear coupling to modes larger than the survey. We develop a method which uses this effect to estimate the mass density parameter Omega from the scatter in power spectrum estimates for subregions of a larger survey. We show that the power spectrum can be measured accurately from realistically noisy data on scales corresponding to 1-10 Mpc/h. Non-Gaussian features in the one point distribution function of the weak lensing convergence (reconstructed from the shear) are also sensitive to Omega. We suggest several techniques for estimating Omega in the presence of noise and compare their statistical power, robustness and simplicity. With realistic noise Omega can be determined to within 0.1-0.2 from a deep survey of several square degrees.

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arXiv1999

CMBFAST for spatially closed universes

We extend the cosmological linear perturbation theory code CMBFAST to closed geometries. This completes the implementation of CMBFAST to all types of geometries and allows the user to perform an unlimited search in the parameter space of models. This will be specially useful for placing confidence limits on cosmological parameters from existing and future data. We discuss some of the technical issues regarding the implementation.

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arXiv1999

Limits on the Density of Compact Objects from High Redshift Supernovae

Due to the effects of gravitational lensing, the magnification distribution of high redshift supenovae can be a powerful discriminator between smooth dark matter and dark matter consisting of compact objects. We use high resolution N-body simulations in combination with the results of simulations with compact objects to determine the magnification distribution for a Universe with an arbitrary fraction of the dark matter in compact objects. Using these distributions we determine the number of type Ia SNe required to measure the fraction of matter in compact objects. It is possible to determine a 20% fraction of matter in compact objects with 100-400 well measured SNe at $z \sim 1$, assuming the background cosmological model is well determined.

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arXiv1999

Lensing Induced Cluster Signatures in Cosmic Microwave Background

We show that clusters of galaxies induce step-like wiggles on top of the cosmic microwave background (CMB). The direction of the wiggle is parallel to the large scale gradient of CMB allowing one to isolate the effect from other small scale fluctuations. The effect is sensitive to the deflection angle rather than its derivative (shear or magnification) and is thus tracing outer parts of the cluster with higher sensitivity than some other methods. A typical amplitude of the effect is $10μK (σ_v/1400 kms^{-1})^2$ where $σ_v$ is the velocity dispersion of the cluster and several $μK$ signals extend out to a fraction of a degree. We derive the expressions for the temperature profile for several simple parameterized cluster models and identify some degeneracies between parameters. Finally, we discuss how to separate this signal from other imprints on CMB using custom designed filters. Detection of this effect is within reach of the next generation of small scale CMB telescopes and could provide information about the cluster density profile beyond the virial radius.

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arXiv1999

Simulating Weak Lensing by Clusters and Large-Scale Structure

Selected results on estimating cosmological parameters from simulated weak lensing data with noise are presented. Numerical simulations of ray tracing through N-body simulations have been used to generate shear and convergence maps due to lensing by large-scale structure. Noise due to the intrinsic ellipticities of a finite number of galaxies is added. In this contribution we present our main results on estimation of the power spectrum and density parameter Omega from weak lensing data on several degree sized fields. We also show that there are striking morphological differences in the weak lensing maps of clusters of galaxies formed in models with different values of Omega.

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arXiv1998

Direct Signature of Evolving Gravitational Potential from Cosmic Microwave Background

We show that time dependent gravitational potential can be directly detected from the cosmic microwave background (CMB) anisotropies. The signature can be measured by cross-correlating the CMB with the projected density field reconstructed from the weak lensing distortions of the CMB itself. The cross-correlation gives a signal whenever there is a time dependent gravitational potential. This method traces dark matter directly and has a well defined redshift distribution of the window projecting over the density perturbations, thereby avoiding the problems plaguing other proposed cross-correlations. We show that both MAP and Planck will be able to probe this effect for observationally relevant curvature and cosmological constant models, which will provide additional constraints on the cosmological parameters.

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arXiv1998

Reconstructing Projected Matter Density from Cosmic Microwave Background

Gravitational lensing distorts the cosmic microwave background (CMB) anisotropies and imprints a characteristic pattern onto it. The distortions depend on the projected matter density between today and redshift $z \sim 1100$. In this paper we develop a method for a direct reconstruction of the projected matter density from the CMB anisotropies. This reconstruction is obtained by averaging over quadratic combinations of the derivatives of CMB field. We test the method using simulations and show that it can successfully recover projected density profile of a cluster of galaxies if there are measurable anisotropies on scales smaller than the characteristic cluster size. In the absence of sufficient small scale power the reconstructed maps have low signal to noise on individual structures, but can give a positive detection of the power spectrum or when cross correlated with other maps of large scale structure. We develop an analytic method to reconstruct the power spectrum including the effects of noise and beam smoothing. Tests with Monte Carlo simulations show that we can recover the input power spectrum both on large and small scales, provided that we use maps with sufficiently low noise and high angular resolution.

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arXiv1998

Measuring Dark Matter Power Spectrum from Cosmic Microwave Background

We propose a method to extract the projected power spectrum of density perturbations from the distortions in the cosmic microwave background (CMB). The distortions are imprinted onto the CMB by the gravitational lensing effect and can be extracted using a combination of products of CMB derivatives. We show that future CMB experiments such as Planck will be able to extract the power spectrum with high statistical significance over two orders of magnitude in angle. The method proposed here traces dark matter directly to higher redshift (up to $z \sim 1100$) and larger scale (few Gpc) than any other currently known method. It also traces large scale structure in the linear regime, allowing simple interpretation in terms of cosmological models. By providing additional and complementary information to the one from the primary CMB analysis it will strengthen further the scientific return of future CMB experiments.

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arXiv1998

A Complete Treatment of CMB Anisotropies in a FRW Universe

We generalize the total angular momentum method for computing Cosmic Microwave Background anisotropies to Friedman-Robertson-Walker (FRW) spaces with arbitrary geometries. This unifies the treatment of temperature and polarization anisotropies generated by scalar, vector and tensor perturbations of the fluid, seed, or a scalar field, in a universe with constant comoving curvature. The resulting formalism generalizes and simplifies the calculation of anisotropies and, in its integral form, allows for a fast calculation of model predictions in linear theory for any FRW metric.

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arXiv1998

Polarization of Microwave Background: Statistical and Physical Properties

We discuss statistical and physical properties of cosmic microwave background polarization, both in Fourier and in real space. The latter allows for a more intuitive understanding of some of the geometric signatures. We present expressions that relate electric and magnetic type of polarization to the measured Stokes parameters in real space and discuss how can be constructed locally. We discuss necessary conditions to create polarization and present simulated maps and correlation functions for some typical models.

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arXiv1998

Numerical Investigations of Weak Lensing by Large-Scale Structure

We use numerical simulations of ray tracing through N-body simulations to investigate weak lensing by large-scale structure. These are needed for testing the analytic predictions of two-point correlators, to set error estimates on them and to investigate nonlinear gravitational effects in the weak lensing maps. On scales larger than 1 degree gaussian statistics suffice and can be used to estimate the sampling, noise and aliasing errors on the measured power spectrum. For this case we describe a minimum variance inversion procedure from the 2-d to 3-d power spectrum and discuss a sparse sampling strategy which optimizes the signal to noise on the power spectrum. On degree scales and smaller the shear and convergence statistics lie in the nonlinear regime and have a non-gaussian distribution. For this regime ray tracing simulations are useful to provide reliable error estimates and calibration of the measurements. We show how the skewness and kurtosis can in principle be used to probe the mean density in the universe, but are sensitive to sampling errors and require large observed areas. The probability distribution function is likely to be more useful as a tool to investigate nonlinear effects. In particular, it shows striking differences between models with different values of the mean density $Ω_m$.

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arXiv1998

Gravitational Lensing Effect on Cosmic Microwave Background Polarization

We investigate the effect of gravitational lensing by matter distribution in the universe on the cosmic microwave background (CMB) polarization power spectra and temperature-polarization cross-correlation spectrum. As in the case of temperature spectrum gravitational lensing leads to smoothing of narrow features and enhancement of power on the damping tail of the power spectrum. Because acoustic peaks in polarization spectra are narrower than in the temperature spectrum the smoothing effect is significantly larger and can reach up to 10\% for $l<1000$ and even more above that. A qualitatively new feature is the generation of $B$ type polarization even when only $E$ is intrinsically present, such as in the case of pure scalar perturbations. This may be directly observed with Planck and other future small scale polarization experiments. The gravitational lensing effect is incorporated in the new version (2.4) of CMBFAST code.

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