59,674 research outputs found
Covariance of cross-correlations: towards efficient measures for large-scale structure
We study the covariance of the cross-power spectrum of different tracers for
the large-scale structure. We develop the counts-in-cells framework for the
multi-tracer approach, and use this to derive expressions for the full
non-Gaussian covariance matrix. We show, that for the usual auto-power
statistic, besides the off-diagonal covariance generated through gravitational
mode-coupling, the discreteness of the tracers and their associated sampling
distribution can generate strong off-diagonal covariance, and that this becomes
the dominant source of covariance as k>>k_f=2 pi/L. On comparison with the
derived expressions for the cross-power covariance, we show that the
off-diagonal terms can be suppressed, if one cross-correlates a high
tracer-density sample with a low one. Taking the effective estimator efficiency
to be proportional to the signal-to-noise ratio (SN), we show that, to probe
clustering as a function of physical properties of the sample, i.e. cluster
mass or galaxy luminosity, then the cross-power approach can out perform the
auto-power one by factors of a few. We confront the theory with measurements of
the mass-mass, halo-mass, and halo-halo power spectra from a large ensemble of
N-body simulations. We show that there is a significant SN advantage to be
gained from using the cross-power approach when studying the bias of rare
haloes. The analysis is repeated in configuration space and again SN
improvement is found. We estimate the covariance matrix for these samples, and
find strong off-diagonal contributions. The covariance depends on halo mass,
with higher mass samples having stronger covariance. In agreement with theory,
we show that the covariance is suppressed for the cross-power. This work points
the way towards improved estimators for clustering studies.Comment: Several significant improvements to the earlier version: for instance
it is shown more clearly how shot noise corrections may generate off-diagonal
covariance in the power spectrum. Original version submitted to MNRAS on 18th
September 2008. This version 18 pages, 7 figure
SKA Weak Lensing III: Added Value of Multi-Wavelength Synergies for the Mitigation of Systematics
In this third paper of a series on radio weak lensing for cosmology with the
Square Kilometre Array, we scrutinise synergies between cosmic shear
measurements in the radio and optical/near-IR bands for mitigating systematic
effects. We focus on three main classes of systematics: (i) experimental
systematic errors in the observed shear; (ii) signal contamination by intrinsic
alignments; and (iii) systematic effects due to an incorrect modelling of
non-linear scales. First, we show that a comprehensive, multi-wavelength
analysis provides a self-calibration method for experimental systematic
effects, only implying <50% increment on the errors on cosmological parameters.
We also illustrate how the cross-correlation between radio and optical/near-IR
surveys alone is able to remove residual systematics with variance as large as
0.00001, i.e. the same order of magnitude of the cosmological signal. This also
opens the possibility of using such a cross-correlation as a means to detect
unknown experimental systematics. Secondly, we demonstrate that, thanks to
polarisation information, radio weak lensing surveys will be able to mitigate
contamination by intrinsic alignments, in a way similar but fully complementary
to available self-calibration methods based on position-shear correlations.
Lastly, we illustrate how radio weak lensing experiments, reaching higher
redshifts than those accessible to optical surveys, will probe dark energy and
the growth of cosmic structures in regimes less contaminated by non-linearities
in the matter perturbations. For instance, the higher-redshift bins of radio
catalogues peak at z~0.8-1, whereas their optical/near-IR counterparts are
limited to z<0.5-0.7. This translates into having a cosmological signal 2 to 5
times less contaminated by non-linear perturbations.Comment: 16 pages, 10 figures, 2 tables; improved discussion of experimental
systematics in Sec. 2; updated to match published versio
Evolution of the Clustering of Photometrically Selected SDSS Galaxies
We measure the angular auto-correlation functions (w) of SDSS galaxies
selected to have photometric redshifts 0.1 < z < 0.4 and absolute r-band
magnitudes Mr < -21.2. We split these galaxies into five overlapping redshift
shells of width 0.1 and measure w in each subsample in order to investigate the
evolution of SDSS galaxies. We find that the bias increases substantially with
redshift - much more so than one would expect for a passively evolving sample.
We use halo-model analysis to determine the best-fit
halo-occupation-distribution (HOD) for each subsample, and the best-fit models
allow us to interpret the change in bias physically. In order to properly
interpret our best-fit HODs, we convert each halo mass to its z = 0 passively
evolved bias (bo), enabling a direct comparison of the best-fit HODs at
different redshifts. We find that the minimum halo bo required to host a galaxy
decreases as the redshift decreases, suggesting that galaxies with Mr < -21.2
are forming in halos at the low-mass end of the HODs over our redshift range.
We use the best-fit HODs to determine the change in occupation number divided
by the change in mass of halos with constant bo and we find a sharp peak at bo
~ 0.9 - corresponding to an average halo mass of ~ 10^12Msol/h. We thus present
the following scenario: the bias of galaxies with Mr < -21.2 decreases as the
Universe evolves because these galaxies form in halos of mass ~ 10^12Msol/h
(independent of redshift), and the bias of these halos naturally decreases as
the Universe evolves.Comment: 17 pages, 14 figures, matches version accepted for publication in
MNRA
A new statistical test based on the wavelet cross-spectrum to detect time–frequency dependence between non-stationary signals: Application to the analysis of cortico-muscular interactions
The study of the correlations that may exist between neurophysiological signals is at the heart of modern techniques for data analysis in neuroscience. Wavelet coherence is a popular method to construct a time-frequency map that can be used to analyze the time-frequency correlations be- tween two time series. Coherence is a normalized measure of dependence, for which it is possible to construct confidence intervals, and that is commonly considered as being more interpretable than the wavelet cross-spectrum (WCS). In this paper, we provide empirical and theoretical arguments to show that a significant level of wavelet coherence does not necessarily correspond to a significant level of dependence between random signals, especially when the number of trials is small. In such cases, we demonstrate that the WCS is a much better measure of statistical dependence, and a new statistical test to detect significant values of the cross-spectrum is proposed. This test clearly outperforms the limitations of coherence analysis while still allowing a consistent estimation of the time-frequency correlations between two non-stationary stochastic processes. Simulated data are used to investigate the advantages of this new approach over coherence analysis. The method is also applied to experimental data sets to analyze the time-frequency correlations that may exist between electroencephalogram (EEG) and surface electromyogram (EMG)
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