1805.11629
Covariances for cosmic shear and galaxy-galaxy lensing in the response approach
Takahashi, Nishimichi, Takada, Shirasaki, Shiroyama
Measure the response of matter and halos projected power spectra P^2D_XY(k) (X,Y are matter and/or haloes), to a large-scale density contrast, delta_b, using separate universe simulations. Show that the fractional response functions, i.e., d lnP^2D_XY(k) / d delta_b, are identical to their respective 3D power spectra within simulation measurement errors. Then, using various N-body sim combinations (small-box simulations with periodic boundary conditions and sub-volumes of large-box simulations) to construct hypothetical observations of mock projected fields, study how super-survey modes, in both parallel and perpendicular directions to the projection direction, affect the covariance matrix of P^2D_XY(k), known as super-sample covariance (SSC). The realists indicate that the SSC term provides dominant contributions to the covariances of matter-matter and matter-halo spectra at small scale but does not provide significant contributions in the halo-halo spectrum. Observe that the LS density contrast in each z shell, i.e., the trace of second-derivative tensor of the LS gravitational potential field, causes most of the SSC effect, and do not observe a SSC signature arising from LS tidal field, within the levels of measurement accuracy. Also develop a response approach to calibrate the SSC term for cosmic shear correlation function and gg WL, and validate the method by comparison with the light-cone, ray-tracing simulations. The method provides a reasonably accurate, albeit computationally inexpensive, way to calibrate the covariance matrix for clustering observables available from wide-area galaxy surveys, without the need to run light-cone simulations.
1805.12084
KiDS-450: enhancing cosmic shear with clipping transformations
Giblin, et al
Present the first "clipped" cosmic shear measurement using data from KiDS-450. "Clipping" transformations suppress the signal from the highest density, non-linear regions of cosmological fields. Demonstrate that these transformations improve constraints on S_8-sigma_8(Omega_m/0.3)^0.5 when used in combinations with conventional 2pt statistics. For the KiDS-450 data, find that the combined measurements improve the constraints on S_8 by 17%, compared to shear correlation functions alone. Determine the expectation value of the clipped shear correlation function using a suite of numerical simulations, and develop methodology to mitigate the impact of masking and shot noise. Future improvements in numerical simulations and mass reconstruction methodology will permit the precise calibration of clipped cosmic shear statistics such that clipping can become a standard tool in WL analysis.
1805.12186
The relative impact of baryons and cluster shape on weak lensing mass estimates of galaxy clusters
Lee, et al
WL depends on the integrated mass along the LoS. Baryons contribute to the mass distribution of galaxy clusters and the resulting mass estimates from lensing analysis. Use the cosmo-OWLS suite of hydro sims to investigate the impact of baryonic processes on the bias and scatter of WL mass estimates of clusters. These estimates are obtained by fitting NFW profiles to mock data using MCMC techniques. In particular, examine the difference inetsimates between DM-only runs and those including various prescriptions for baryonic physics. Find no significant difference in the mass bias when baryonic physics is included, though the overall mass estimates are suppressed when feedback from AGN is included. For lowest-mass systems for which a reliable mass can be obtained (M_200 ~2e14 Msun), find a bias of ~ -10%. The magnitude of the bias tends to decrease for higher mass clusters, consistent with no bias for the most massive clusters which have masses comparable to those found in the CLASH and HFF samples. For the lowest mass clusters, the mass bias is particularly sensitive to the fit radii and the limits placed on the concentration prior, rendering reliable mass estimates difficult. The scatter in mass estimates between the DM-only and the various baryonic runs is less than between different projections of individual clusters, highlighting the importance of triaxiality.
1805.12226
Prospects for determining the mass distributions of galaxy clusters on large scales using weak gravitational lensing
Fong, Bower, Whitehead, Lee, King ,Applegate, McCarthy
For more than 2 decades, the NFW model has stood the test of time; it has been used to describe the distribution of mass in galaxy clusters out to their outskirts. Stacked WL measurements of clusters are now revealing the distribution of mass out to and beyond their viral radii, where the NFW model is no longer applicable. In this study, assess how well the parameterized Diemer+Kravstov (DK) density profile describes the characteristic mass distribution of galaxy clusters extracted from cosmological simulations. This is determined from stacked synthetic lensing measurements of the 50 most massive clusters extracted from the Comso-OWLS simulations, using the DM only run and also the run that most closely matches observations. The characteristics of the data reflect the Weighing the Giants survey and data from the future LSST survey. In comparison with the NFW model, the DK model is favored by the stacked data, in particular for the future LSST data, where the number density of background galaxies is higher. The DK profile depends on the accretion history of clusters which is specified in the current study. Eventually however subsamples of galaxy clusters with qualities indicative of disparate accretion histories could be studied.
1805.12574
The many flavors of photometric redshifts
Salvato, Ilbert, Hoyle
For more than 70 yrs, the measurements of fluxes of galaxies at different wavelengths and driven colors have been used to estimate their corresponding cosmological distances. Form the fields of galaxy and AGN evolution to precision cosmology, the number of scientific projects relying on such distance measurements, called photometric redshifts, have exploded. The benefits of photometric redshifts is that all sources detected in photometric images can have some distance estimate relatively cheaply. The major drawback is that these cheap estimates have a low precision when compared with the resource-expensive spectroscopy. The methodology to estimate redshifts has been through several major revolutions throughout the last decades, triggered by increasingly more stringent requirements on the photo-z accuracy. Here, review the various techniques to obtain photo-zs, from template-fitting to machine learning and hybrid systems. Also describe the state-of-the-art results on current extra-galactic samples and explain how survey strategy choices impact redshift accuracy. Close the review with a description of the photo-z efforts planned for upcoming wide field surveys, which will collect data on billions of galaxies, aiming to solve the most exciting cosmological and astrophysical questions of today.
1805.12590
The Gaia 20pc white dwarf sample
Hollands et al
In Gaia DR2 data, identify 139 white dwarfs within 20 pc of the Sun. 11 are new detections, with the closest of these located at ~8.46 pc. Estimated atmospheric parameters for all stellar remnants based on Gaia parallaxes and photometry. The high precision and completeness of the Gaia astrometry allow for searching of wide binary companions. Re-identify all known binaries where both components have accurate DR2 astrometry, and established the binary of 2 of the 11 newly identified WDs. No new companions where found to previously known 20pc WDs. Finally, estimate the local WD space-density to be 4.49±0.38 e-3 pc^{-3}, having given careful consideration to the distance-dependent Gaia completeness, which misses known objects at short distances, but is close to complete for WDs near 20pc.
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