1703.01575
Weak lensing power spectrum reconstruction by counting galaxies. -- I: the ABS method
Yang, et al
Propose and analytical method of blind separation (ABS) of cosmic magnification from the intrinsic fluctuations of galaxy number density in the observed (lensed) galaxy number density distribution. The ABS method utilizes the different dependencies of the signal (cosmic magnification) and contamination (galaxy intrinsic clustering) in galaxy flux, to separate the two. It works directly on the measured cross galaxy angular power spectra between different flux bins. It determines/reconstructs the lensing power spectrum analytically, without assumptions of galaxy intrinsic clustering and cosmology. It is unbiased in the limit of infinite number of galaxies. In reality the lensing reconstruction accuracy depends on survey configurations, galaxy biases, and other complexities, due to finite number of galaxies and the resulting shot noise in the cross galaxy power spectra. Estimate its performance (systematic and statistical errors) in various cases. Find that state IV DE surveys such as SKA and LSST are capable of reconstructing the lensing power spectrum at z~1 and ell<Z5000 accurately. This lensing reconstruction only requires counting galaxies, and is therefore highly complementary to the cosmic shear measurement by the same surveys.
1703.01679
Astrometric calibration and performance of the Dark Energy Camera
Bernstein et al
Characterize the ability of DECam to perform relative astrometry across its 500Mpix, 3 deg2 science FoV, and across 4 years of operation. This is done using internal comparisons of ~4e7 measurements of high-S/N stellar images obtained in repeat visits to fields of moderate stellar density, with the telescope dithered to move the sources around the array. An empirical astrometric model includes terms for: optical distortions; stray electric fields in the CCD detectors; chromatic terms in the instrumental and atmospheric optics; shifts in CCD relative positions of up to ~10 um when the DECam temperature cycles; and lower-order distortions to each exposure from changes in atmospheric refraction and telescope alignment. Errors in this astrometric model are dominated by stochastic variations with typical amplitudes of 10-30mas (in a 30s exposures) and 5-10 arcmin coherence length, plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of these atmospheric distortions is not closely related to the seeing. Given an astrometric reference catalog at density ~0.7 arcmin^-2, e.g. from Gaia, the typical atmospheric distortions can be interpolated to 7 mac RMS accuracy (for 30s exposures) with 1 arcmin coherence length for residual errors. Remaining detectable error contributors are 2-4 has RMS from unmodelled stray E_fields in the devices, and another 2-4 was RMS from focal plane shifts between camera thermal cycles. Thus the astrometric solution for a single DECam exposures is accurate to 3-6 has (0.02 pixels, or 300 nm) on the focal plane, plus the stochastic atmospheric distortion.
1703.01791
Multipolar moments of weak lensing signal around clusters. Weighing filaments in harmonic space
Gouin, et al
Aim: to understand the connectivity of the cosmic web with unbiased mass tracers like WL is of prime importance to prob the underlying cosmology, seek dynamical signatures of DM, and quantify environmental effects on galaxy formation. Methods: Mock catalogues of galaxy clusters are extracted from the N-body PLUS simulation. For each cluster, the aperture multipolar moments of the convergence are calculated in two annuli (inside and outside the virial radius). By stacking their modulus, a statistical estimator is built to characterize the angular mass distribution around clusters. The moments are compared to predictions from perturbation theory and spherical collapse. Results: The main weakly chromatic excess of multipolar power on large scales is undetstood as arising from the contraction of the primordial cosmic web driven by the growing potential well of the cluster. In the inner region, the initial quadrupole prevails, while centering suppresses odd multipoles, especially m=1. Predictions for the signal amplitude as a function of the cluster-centric distance, mass and redshift are presented. The prospects of measuring this signal are estimated for current and future lensing data sets. Inside the virial radius, multipoles up to m=8-12 can be measured at >10 sigma with 10k clusters at z=0.3. In the outer regions, around 4 Rvir, detection is possible up to m=8 but the noise from intervening large-scale structure dominates. Conclusions: the Euclid mission should provide all the necessary information to study the comics evolution of the connectivity of the cosmic web around lensing clusters using multipolar moments.
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