1507.05090
Weak lensing by galaxy troughs in DES science verification data
Gruen et al
Measure the radial alignment of BG galaxies around galaxy "troughs", under densities in projections of the foreground galaxy field over a wide range of redshift in SV data from DES. Detection of the shear signal is highly significant (10 to 15 sigma for the smallest angular scales) for troughs with 0.2<z<0.5 of the projected galaxy field and angular diameters of 10' to 1 deg. These measurements probe the connection between the galaxy, matter density, and convergence fields. By assuming galaxies are biased tracers of the matter density with Poissonian noise, find agreement of the measurements with predictions in a fiducial LCDM model. The prediction for the lensing signal on large trough scales is virtually independent of the details of the underling model for the connection of galaxies and matter. The comparison of the shear around toughs with that around cylinders with large galaxy counts is consistent with a symmetry between galaxy and matter over- and under densities. In addition, measure the 2 point angular correlation of troughs with galaxies which, in contrast to the lensing signal, is sensitive to galaxy bias on all scales. The lensing signal of troughs and their clustering with galaxies is therefore a promising probe of the statistical properties of matter under densities and their connection to the galaxy field.
1507.05092
Interloper bias in future large-scale structure surveys
Pullen, Hirata, Dore, Raccanelli
LSS of observable universe with emission line galaxies: next generation spectroscopic surveys. Interloping emission lines can masquerade as the survey's intended emission line at different redshifts. Interloping lines from galaxies that are not removed can contaminate the power spectrum measurement, mixing correlations from various redshifts and diluting the true signal. Assess the potential for power spectrum contamination, finding that an interloper fraction worse than 0.2% could bias PS measurements for future surveys by more than 10% of statistical errors, while also biasing inferences biased on the PS. Also construct a formalism for predicting biases for cosmo parameter measurements, and demonstrate that a 0.3% interloper fraction could bias measurements of the growth rate by more than 10% of the error, which can affect constraints from upcoming surveys on gravity. Use COSMOS mock catalog (CMC), with the emission lines re-scalesd to better reproduce recent data, to predict potential interloper fractions for the PFS and WFIRST. Find that secondary line identification, or confirming galaxy redshifts by finding correlated emission liens, is able to remove interloping emission lines in PFS. For WFIRST, use the CMC to predict that the 0.2% target can be reached for the WFIRST Ha survey, but sensitive optical and NIR photometry will be required. For the WFIRST [OIII] survey, the predicted interloper fractions reach several percent and their effects will have to be estimated and removed statistically (e.g. with deep training samples).
1507.05137
The difference imaging pipeline for the transient search in the Dark Energy Survey
Kessler, et al
DiffImg pipeline used to detect transients in deep images from DES-SN program in its first observing season from Aug 2013 to Feb 2014. Search for transients in 10 x 3-deg^2 fields are repeatedly observed in the griz passbands with a cadence of about 1 week. The observing strategy has been optimized to measure high-quality light curves and redshifts for thousands of SN Ia with the goal of measuring DE parameters. The essential DiffImg functions are to align each search image to a deep reference image, do a pixel-by-pixel subtraction and then examine the subtracted image for significant positive detections of point-source objects. The vast majority of detections are subtraction artifacts, but after selection requirements and image filtering with an automated scanning program, there are 130 detections per deg^2 per observation in each band, of which only 25% are artifacts. Of the 7500 transients discovered by DES-SN in its first observing season, each requiring a detection on atlas 2 separate nights, MC simulations predict that 27% are expected to be SN. Another 30% of the transients are artifacts, and most of the remaining transients are AGN and variable stars. Fake SNe Ia are overlaid onto the images to rigorously evaluate detection efficiencies, and to understand the DiffImg performance. The DiffImg efficiency measured with fake SNe agrees well with expectations from MC sims that uses analytical calculates of the fluxes and their uncertainties. In the 8 "shallow" fields with single-epoch 50% completeness depth 23.5, the SN Ia efficiency falls to 1/2 at z~0.7, in the 2 "deep" fields with mag-depth 24.5, the efficiency falls to 1/2 at z~1.1.
1507.05177
Observation of two new L4 Neptune Trojans in the Dark Energy Survey Supernova fields
Gerdes, et al
Report the discovery of the 8th and 9th known Trojans in stable orbits around Neptune's leading Lagrange point, L4. Both are in high-inclination orbits (18.8 and 19.4 deg, respectively). With an eccentricity of 0.104, 2014 QOK_441 has the most eccentric orbit of the 11 known stable Neptune Trojans. Describe the search procedure and investigate the object's long-term dynamical stability and physical properties.
1507.05334
On scale-dependent cosmic shear systematic effects
Kitching, Taylor, Cropper, Hoekstra, Hood, Massey, Niemi
Investigate the impact that realistic scale-dependence systematic effects may have on cosmic shear tomography. Model spatially varying residual ellipticity and size variations in WL measurements and propagate these thorough to predicted changes in the uncertainty and bias of cosmo parameters. Show that the survey strategy - whether it is regular or randomized - is an important factor in determining the impact of a systematic effect: a purely randomized survey strategy produces the smallest biases, at the expense of larger parameter uncertainties, and a very regularized survey strategy produces large biases, but unaffected uncertainties. However, by removing, or modeling, the affected scales (l-modes) in the regular cases the biases are reduced to negligible levels. Find that the integral of the systematic PS is not a good metric for dark energy performance, and advocate that systematic effects should be modeled accurately in real space, where they enter the measurement process, and their effect subsequently propagated into power spectrum contributions.
1507.05339
Path-integral evidence
Kitching, Taylor
Present a Bayesian formalism for the goodness-of-fit that is the evidence for a fixed functional form over the evidence for all functions that are a general perturbation about this form. This is done under the assumption that the statistical properties of the data can be modeled by a multivariate Gaussian distribution. Use this to show how one can optimize an experiment to find evidence for a fixed function over perturbations about this function. Apply this formalism to an illustrative problem of measuring perturbations in the dark energy EoS about a cosmological constant.
1507.05360
Galaxy clustering, photometric redshifts and diagnosis of systematics in the DES science verification data
Crocce, et al
Study the clustering of galaxies detected at i<22.5 in the SV observations of DES. 2PCFs are measured using 2.3e6 galaxies over a contiguous 116 deg^2 region in 5 bins of photo-z width Delta z=0.2 in the range 0.2<z<1.2. The impact of photo-z errors are assessed by comparing results using a template-based photo-z algorithm (BPZ) to a machine-learning algorithm (TPZ). A companion paper (Leistedt + 2015) presents maps of several observational variables (e.g. seeing, sky brightness) which could modulate the galaxy density. Characterize and mitigate systematic errors on the measured clustering which arise from these observational variables, in addition to others such as Galactic dust and stellar contamination. After correcting for systematic effects measured galaxy bias over a broad range of linear scales relative to mass clustering predicted from the Planck LCDM models, finding agreement with CFHTLS measurements with chi^2 of 4.0(8.7) with 5 degrees of freedom for the TPZ (BPZ) redshifts. Test a "near bias" model, in which the galaxy clustering is a fixed multiple of the predicted NL DM clustering. The precision of the data allow determination that the linear bias model describes the observed galaxy clustering to 2.5% accuracy down to scales at least 4 to 10x smaller than those on which linear theory is expected to be sufficient.
1507.05460
redMaGiC: selecting luminous red galaxies from the DES Science Verification Data
Rozo, et al
Introduce redMaGiC, an automated algorithm for selecting LRGs. The algorithm was specifically developed to minimize photometric redshift uncertainties in photometric LSS studies. redMaGiC achieves this by self-training the color-cuts necessary to produce a luminosity-thresholded LRG sample of constant comoving density. Demonstrate that redMaGiC photo-zs are very nearly as accurate as the best machine-learning based methods, yet they require minimal spectroscopic training, do not suffer from extrapolation biases, and are very nearly Gaussian. Apply the algorithm to DES SV data to produce a redMaGiC catalog sampling the z range 0.2<z<0.8. The fiducial sample has a comoving space density of 1e-3 (Mpc/h)^3, and a median photo-z bias (z_spec-z_photo) and scatter (sigma_z/(1+z)) of 0.005 and 0.017 respectively. The corresponding 5 sigma outlier fraction is 1.4%. Also test the algorithm with SDSS DR8 and Stripe 82 data, and discuss how spectroscopic training can be used to control photo-z biases at the 0.1% level.
1507.05551
CMB lensing tomography with the DES science verification galaxies
Giannantonio, et al
Measure the cross-correlation between the galaxy density in DES SV data and the lensing of the CMB as reconstructed with Planck and SPT. When using the DES main galaxy sample over the full z range 0.2<z<1.2, a cross-correlation signal is detected at 6sigma and 4sigma with SPT and Planck, respectively. Then divide the DES galaxies into 5 photo-z bins, finding significant (>2sigma) detections in all bins. Comparing to the fiducial Planck cosmology, find the z evolution of the signal matches expectation, although the amplitude is consistently lower than predicted across redshift bins. Test for possible systematics that could affect the realist and find no evidence for significant contamination. Finally, demonstrate how these measurements can be used to constrain the growth of structure across cosmic time. Find the data are fit by a model in which the amplitude of structure in z<1.2 universe is 0.73±0.16 times as large as predicted in the LCDM Planck cosmology, a 1.7sigma deviation.
1507.05552
Cosmology from cosmic shear with DES Science Verification data
The Dark Energy Survey Collaboration, et al
Present the first constraints on cosmology from DES, using WL measurements from preliminary SV data. Use 139 sq deg of SV data, <3% of the full DES survey area. Using cosmic shear 2 point measurements over 3 z bins, find sigma8(Omega_m/0.3)^0.5=0.81±0.06 (68% CL), after marginalizing over 7 systematics parameters and 3 other cosmological parameters. Examine the robustness of the results to the choice of data vector and systematics assumed, and find them to be stable. About 20% of the error bar comes from marginalizing over shear and photo-z calibration uncertainties. The current state-of-the-art cosmic shear measurements from CFHTLenS are mildly discrepant with the cosmological constraints from Planck CMB data; the DES results are consistent with both datasets. The uncertainties are ~30% larger than those from CFHTLenS when carrying out a comparable analysis of the two datasets, which is attributed largely to the lower number density of the shear catalog. Investigate constraints on DE and find that, with this small fraction of the full survey, the DES SV constraints make negligible impact on the Planck constraints. The moderate disagreement between the CFHTLenS and Planck values of sigma8(Omega_m/0.3)^0.5 is present regardless of the value of w.
1507.05598
Cosmic shear measurements with DES Science Verification data
Becker et al
Present measurements of WL lensing cosmic shear 2pt statistics using DES SV data. Demonstrate that the results are robust to the choice of shear measurement pipeline, either ngmix or im3shape, and robust to the choice of 2pt statistic, including both real and Fourier-space statistics. The results pass a suite of null tests including tests for B-mode contamination and direct tests for any dependence of the 2pt functions on a set of 16 observing conditions and galaxy properties, such as seeing, airmass, galaxy color, galaxy magnitude, etc. Furthermore use a large suite of simulation to compute the covariance matrix of cosmic shear measurements and assign statistical significance to the null tests. Find that the covariance matrix is consistent with the halo model prediction, indicating that it has the appropriate level of halo sample variance. Compare the same jackknife procedure applied to the data and the simulations in order to search for additional sources of noise no capture by the simulations. Find no statistically significant extra sources of noise in the data. The overall detection significance with tomography for the highest source density catalog is 9.7 sigma. Cosmological constrains from the measurements in this work are presented in a companion paper.
1507.05603
The DES Science Verification weak lensing shear catalogs
Jarvis, Sheldon, Zuntz, Kacprzak, Bridle, et al
Present WL shear catalogs for DES SV from DECam. Describe object detection, PSF estimation and shear measurement procedures using 2 independent shear pipelines, IM3HSAPE and NGMIX, which produce catalogs of 2.12 M and 3.44 M galaxies, respectively. Detail a set of null tests for the shear measurements and find that they pass the requirements for systematic errors at the level necessary for WL science applications using the SV data. Also discuss some of the planned algorithmic improvements that will be necessary to produce sufficiently accurate shear catalogs for the full 5-yr DES, which is expected to cover 5000 sq deg.
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