1509.00010
The view from the boundary: a new void stacking method
Cautun, Cai, Frenk
Introduce a new method for stacking voids and deriving their profile that greatly increases the potential of voids as a tool for precision cosmology. Given that voids are highly non-spherical and have most of their mass at their edge, voids are better described relative to their boundary rather than relative to their centre, as in the conventional spherical stacking approach. The boundary profile is obtained by computing the distance of each volume element from the void boundary. Voids can then be stacked and their profiles computed as a function of this boundary distance. This approach enhances the WL signal of voids, both shear and convergence, by a factor of two when compared to the spherical stacking method. It also results in steeper void density profiles that are characterized by a very slow rise inside the void and a pronounced density ridge at the void boundary, in qualitative agreement with theoretical models of expanding spherical under densities. The resulting boundary density profile is self-similar when rescaled by the thickness of the density ridge, implying that the average rescaled profile is independent of void size. The boundary velocity profile is characterized by outflows in the inner regions whose amplitude scales with void size, and by a strong inflow into the filaments and walls delimiting the void. This new picture enables a straightforward discrimination between collapsing and expanding voids both for individual objects as well as for stacked samples.
1509.00020
The impact of correlated noise on galaxy shape estimation for weak lensing
Gurvich, Mandelbaum
The robust estimation of the tiny distortion (shears) of galaxy shapes caused by WL in the presence of much larger shape distortions due to the PSF has been widely investigated. One major problem is that most galaxy shape measurement methods are subject to bias due to pixel noise in the images ("noise bias"). Noise bias is usually characterized using uncorrelated noise fields; however, real images typically have low-level noise correlation due to galaxies below the detection threshold, and some types of image processing can induce further noise correlations. Investigate the effective detection significance and its impact on noise bias in the presence of correlated noise for one method of galaxy shape estimation. For a fixed noise variance, the biases in galaxy shape estimates can differ substantially for uncorrelated versus correlated noise. However, use of an estimate of detection significance that accounts for the noise correlations can almost entirely remove these differences, leading to consistent values of noise bias as a function of detection significance for correlated and uncorrelated noise. Confirm the robustness of this finding to properties of the galaxy, the PSF, and the noise field, and quantify the impact of anisotropy in the noise correlations. Results highlight the importance of understanding the pixel noise model and its impact on detection significances when correcting for noise bias on WL.
1509.00064
Estimating the power spectrum covariance matrix with fewer mock samples
Pearson, Samushia
The covariance matrices of P(k) measurements from galaxy surveys are difficult to compute theoretically. The current best practice is to estimate covariance matrices by computing a sample covariance of a large number of mock catalogues. The next generation of galaxy surveys will require thousands of large volume mocks to determine the covariance matrices to desired accuracy. The errors in the inverse covraince matrix are larger and scale with the number of P(k) bins, making the problem even more acute. Develop a method of estimating covariance matrices using a theoretically justified, few-parameter model, calibrated with mock catalogues. Using a set of 600 BOSS DR11 mock catalogues, show that a seven parameter model is sufficient to fit the covariance matrix of DR11 P(k) measurements. The covariance computed with this method is better than the sample covariance at any number of mocks and only ~100 mocks are required for it to fully converge and the inverse covariance matrix converges at the same rate. This method should work equally well for the next generation of galaxy surveys, although a demand for higher accuracy may require adding extra parameters to the fitting function.
1509.00150
Cosmic transients test Einstein's Equivalence Principle out to GeV Energies
Gao, Wu, Mészáros
The EEP can be proved with astrophysical sources emitting simultaneously different types of neutral particles, or particles with varying energies, by testing their time of flight through the same gravitational field. Use the time delays between correlated photons from cosmo transients to constrain the accuracy of the EEP. Take data from two gamma-ray bursts as an example, and use, as a lower limit to the theoretical time delays between different energies, delays arising from only the gravitational field of our own galaxy. Then show that the parameterized post-Newtonian parameter gamma is the same for photons over energy ranges between eV and MeV and GeV to a part in 1e-7, which is at least one order of magnitude better than previous limits. Combining this bound on the wavelength dependence of gamma with the absolute bound |gamma-1|<0.3% from light-deflection measurements at optical (eV) wavelengths, thus extend this absolute bound on gamma to GeV energies.
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