Showing papers by "Nabila Aghanim published in 2005"

Cosmological non-Gaussian signature detection: comparing performance of different statistical tests

Abstract: Currently, it appears that the best method for non-Gaussianity detection in the cosmic microwave background (CMB) consists in calculating the kurtosis of the wavelet coefficients. We know that wavelet-kurtosis outperforms other methods such as the bispectrum, the genus, ridgelet-kurtosis, and curvelet-kurtosis on an empirical basis, but relatively few studies have compared other transform-based statistics, such as extreme values, or more recent tools such as higher criticism (HC), or proposed "best possible" choices for such statistics. In this paper, we consider two models for transform-domain coefficients: (a) a power-law model, which seems suited to the wavelet coefficients of simulated cosmic strings, and (b) a sparse mixture model, which seems suitable for the curvelet coefficients of filamentary structure. For model (a), if power-law behavior holds with finite 8th moment, excess kurtosis is an asymptotically optimal detector, but if the 8th moment is not finite, a test based on extreme values is asymptotically optimal. For model (b), if the transform coefficients are very sparse, a recent test, higher criticism, is an optimal detector, but if they are dense, kurtosis is an optimal detector. Empirical wavelet coefficients of simulated cosmic strings have power-law character, infinite 8th moment, while curvelet coefficients of the simulated cosmic strings are not very sparse. In all cases, excess kurtosis seems to be an effective test in moderate-resolution imagery.

Magnetic fields from reionisation

Abstract: We present a complementary study to a new model for generating magnetic fields of cosmological interest. The driving mechanism is the photoionisation process by photons provided by the first luminous sources. Investigating the transient regime at the onset of inhomogeneous reionisation, we show that magnetic field amplitudes as high as $2\times 10^{-16}$ Gauss can be obtained within a source lifetime. Photons with energies above the ionisation threshold accelerate electrons, inducing magnetic fields outside the Stromgren spheres which surround the ionising sources. Thanks to their mean free path, photons with higher energies propagate further and lead to magnetic field generation deeper in the neutral medium. We find that soft X -ray photons could contribute to a significant premagnetisation of the intergalactic medium at a redshift of $z=15$.

Primordial quadrupole-induced polarization from filamentary structures and galaxy clusters

Abstract: We present the first computation of the cosmic microwave background (CMB) polarization power spectrum from galaxy clusters and filaments using hydrodynamic simulations of large-scale structure. We give the E- and B-mode power spectra of the CMB quadrupole-induced polarization between l ~ 560 and 20,000. We find that the contribution from warm ionized gas in filamentary structures dominates the polarized signal from galaxy clusters by more than 1 order of magnitude on large scales (below l ~ 1000) and by a factor of about 2 on small scales (l 10,000). We study the dependence of the power spectra with σ8. Assuming the power spectra vary as σ, we find n = 3.2-4.0 for filaments and n = 3.5-4.6 for clusters.

Adapted method for separating kinetic SZ signal from primary CMB fluctuations

Abstract: In this first attempt to extract amap of the kinetic Sunyaev-Zel'dovich (KSZ) temperature fluctuations from the cosmic microwave background (CMB) anisotropies, we use a method which is based on simple and minimal assumptions. We first focus on the intrinsic limitations of the method due to the cosmological signal itself. We demonstrate using simulated maps that the KSZ reconstructed maps are in quite good agreement with the original input signal with a correlation coefficient between original and reconstructed maps of 0.78 on average, and an error on the standard deviation of the reconstructed KSZ map of only 5% on average. To achieve these results, our method is based on the fact that some first-step component separation provides us with (i) a map of Compton parameters for the thermal Sunyaev-Zel'dovich (TSZ) effect of galaxy clusters, and (ii) a map of temperature fluctuations which is the sum of primary CMB and KSZ signals. Our method takes benefit from the spatial correlation between KSZ and TSZ effects which are both due to the same galaxy clusters. This correlation allows us to use the TSZ map as a spatial template in order to mask, in the CMB + KSZ map, the pixels where the clusters must have imprinted an SZ fluctuation. In practice, a series of TSZ thresholds is defined and for each threshold, we estimate the corresponding KSZ signal by interpolating the CMB fluctuations on the masked pixels. The series of estimated KSZ maps is finally used to reconstruct the KSZ map through the minimisation of a criterion taking into account two statistical properties of the KSZ signal (KSZ dominates over primary anisotropies at small scales, KSZ fluctuations are non-Gaussian distributed). We show that the results are quite sensitive to the effect of beam convolution, especially for large beams, and to the corruption by instrumental noise.

Magnetic fields from reionisation

Abstract: We present a complementary study to a new model for generating magnetic fields of cosmological interest. The driving mechanism is the photoionisation process by photons provided by the first luminous sources. Investigating the transient regime at the onset of inhomogeneous reionisation, we show that magnetic field amplitudes as high as $2 \times 10^{-16}$ Gauss can be obtained within a source lifetime. Photons with energies above the ionisation threshold accelerate electrons, inducing magnetic fields outside the Stroemgren spheres which surround the ionising sources. Thanks to their mean free path, photons with higher energies propagate further and lead to magnetic field generation deeper in the neutral medium. We find that soft X-ray photons could contribute to a significant premagnetisation of the intergalactic medium at a redshift of z=15.

Cosmological non-Gaussian Signature Detection: Comparing Performance of Different Statistical Tests

Abstract: Currently, it appears that the best method for non-Gaussianity detection in the Cosmic Microwave Background (CMB) consists in calculating the kurtosis of the wavelet coefficients. We know that wavelet-kurtosis outperforms other methods such as the bispectrum, the genus, ridgelet-kurtosis and curvelet-kurtosis on an empirical basis, but relatively few studies have compared other transform-based statistics, such as extreme values, or more recent tools such as Higher Criticism (HC), or proposed `best possible' choices for such statistics. In this paper we consider two models for transform-domain coefficients: (a) a power-law model, which seems suited to the wavelet coefficients of simulated cosmic strings; and (b) a sparse mixture model, which seems suitable for the curvelet coefficients of filamentary structure. For model (a), if power-law behavior holds with finite 8-th moment, excess kurtosis is an asymptotically optimal detector, but if the 8-th moment is not finite, a test based on extreme values is asymptotically optimal. For model (b), if the transform coefficients are very sparse, a recent test, Higher Criticism, is an optimal detector, but if they are dense, kurtosis is an optimal detector. Empirical wavelet coefficients of simulated cosmic strings have power-law character, infinite 8-th moment, while curvelet coefficients of the simulated cosmic strings are not very sparse. In all cases, excess kurtosis seems to be an effective test in moderate-resolution imagery.