Thursday. Friday.
1707.06233
Cosmic shear calibration with forward modeling
Bruderer, Nicola, Amara, Refregier, Herbel, Kacprzak
WL is a powerful probe of the dark sector of the Universe. One of the main challenges for this technique is the treatment of systematics in the measurement of cosmic shear from galaxy shapes. In an earlier work, Refregier+Amara (2014) have proposed the Monte Carlo control loops (MCCL) to overcome these effects using a forward modeling approach. Focus here on one of the control loops in this method, the task of which is the calibration of the shear measurement. For this purpose, first consider the requirements on the shear systematics for a given survey and propagate them to different systematic terms. Use two 1-pt statistics to calibrate the shear measurement and six further 1-pt statistics as diagnostics. Also propagate the systematics levels that is estimated form the 1pt functions to the 2pt functions for the different systematic error sources. This allows assessment of the consistency between the systematics levels measured in different ways. To test the method, construct synthetic sky surveys with an area of 1700 deg^2. With some simplifying assumptions, able to meet the requirements on the shear calibration for this survey configurations. Furthermore, account for the total residual shear systematics in terms of the contributing sources. Discuss how this MCCL framework can be applied to current and future WL surveys.
1707.06312
ICE-COLA: fast simulations for weak lensing observables
Izard, Fosalba, Crocce
Approximate methods to full N-body sims provide a fast and accurate solution to the development of mock catalogues for the modeling of galaxy clustering observables. In this paper, extend ICE-COLA (Izard+2016), based on an optimized implementation of the approximate COLA method, to produce WL maps and halo catalogues in the light cone using an integrated and self consistent approach. Show that despite the approximate dynamics, the catalogues thus produced enable an accurate modeling of WL observables one decade beyond the characteristic scale where the growth becomes non-linear. In particular, compare ICE-COLA to the MICE-GC N-body simulation for some fiducial cases representative of upcoming surveys and find that, for sources at z=1, their convergence power spectra agree to within one percent up to high multipoles (i.e., of order 1000). The corresponding shear two point functions, xi+ and xi-, yield similar accuracy down to 2 and 20 arcmin respectively, while tangential shear around a z=0.5 lens sample is accurate down to 4 arcmin. Show that such accuracy is stable against an increased angular resolution of the weak lensing maps. Hence, this opens the possibility of using approximate methods for the joint modeling of galaxy clustering and weak lensing observables and their covariance in ongoing and future galaxy surveys.
1707.06529
Massive data compression for parameter-dependent covariance matrices
Heavens, et al
Show how the massive data compression algorithm MPED can be used to reduce, by orders of magnitude, the number of simulated datasets that are required to estimate the covariance matrix required for the analysis of gaussian-distributed data. This is relevant when the covariance matrix cannot be calculated directly. The compression is especially valuable when the covariance matrix varies with the model parameters. In this case, it may be prohibitively expensive to run enough simulations to estimate the full covariance matrix throughout the parameter space. This compression may be particularly valuable for the next-generation of weak lensing surveys, such as proposed for Euclid and LSST, for which the number of summary data (such as band power or shear correlation estimates) is very large, ~1e4, due to the large number of tomographic redshift bins that the data will be divided into. In the pessimistic case where the covariance matrix is estimated separately for all points in an MCMC analysis, This may require an unfeasible 1e9 simulations. Show here that MOPED can reduce this number by a factor of 1000, or a factor of ~1e6 if some regularity in the covariance matrix is assumed, reducing the number of simulations required to a manageable 1e3, making an otherwise intractable analysis feasible.
1707.06559
The mass dependence of dark matter halo alignments with large-scale structure
Piras, Joachimi, et al
Tidal gravitational forces can modify the shape of galaxies and clusters of galaxies, thus correlating their orientation with the surrounding matter density field. Study the dependence of this phenomenon, known as IA, on the mass of the DM haloes that host these bright structures, analyzing the Millennium and MIllennium-XXL N-body sims. Closely follow the observational approach, measuring the halo position-halo shape alignment and subsequently dividing out the dependence on halo bias. Derive a theoretical scaling of the IA amplitude with mass in a DM universe, and predict a power-law with slope beta_M in the range 1/3 to 1/2, depending on mass scale. Find that the sim data agree with each other and with the theoretical prediction remarkably well over three orders of magnitude in mass, with the joint analysis yielding an estimate of beta_M=0.36±0.01. This result does not depend on z or on the details of the halo shape measurements. The analysis is repeated on observational data, obtaining a significantly higher value, eta_M=0.56±0.05. There are also small but significant deviations from the simple model in the sim signals at both the high- and low-mass end. Discuss possible reasons for these discrepancies, and argue that they can be attributed to physical processes not captured in the model or in the DM-only simulations.
1707.06627
KiDS-450 + 2dFLenS: Cosmological parameter constraints from weak gravitational lensing tomography and overlapping redshift-space galaxy clustering
Joudaki, Blake, et al
Perform a combined analysis of cosmic shear tomography, gg lensing tomography, and redshift-space multiple power spectra (monopole and quadrupole) using 450 deg^2 of imaging data by KiDS overlapping with 2 spectroscopic surveys: 2dFLenS and BOSS. Restrict the gg lensing and multipole power spectrum measurements to the overlapping regions with KiDS, and self-consistently compute the full covariance between the different observables using a large suite of N-body sims. Methodically analyze different combinations of the observables, finding that gg lensing measurements are particularly useful in improving the constraint on the intrinsic alignment amplitude (by 30%, positive at 3.5 sigma in the fiducial data analysis), while the multipole power spectra are useful in tightening the constraints along the lensing degeneracy direction (e.g. factor of 2 stronger matter density constraint in the fiducial analysis). The fully combined constraint on S8=sigma8 sqrt(Omega_m/0.3) = 0.742±0.035, which is an improvement by 20% compared to KiDS alone, corresponds to a 2.6 sigma discordance with Planck, and is not significantly affected by fitting to a more conservative set of scales. The tightening of the parameter space disables resolving the discordance with an extended cosmology that is simultaneously favored in a model selection sense, including the sum of neutrino masses, curvature, evolving DE, and modified gravity. The complementarity of the observables allow for constraints on modified gravity degrees of freedom that are not simultaneously bounded with either probe alone, and up to a factor of 3 improvement in the S8 constraint in the extended cosmology compared to KiDS alone.
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