Search

We present an investigation into a hitherto unexplored systematic that affects the accuracy of galaxy cluster mass estimates with weak gravitational lensing. Specifically, we study the covariance between the weak lensing signal, ΔΣ, and the 'true' cluster galaxy number count, N gal, as measured within a spherical volume that is void of projection effects. By quantifying the impact of this covariance on mass calibration, this work reveals a significant source of systematic uncertainty. Using the MDPL2 simulation with galaxies traced by the SAGE semi-analytic model, we measure the intrinsic property covariance between these observables within the three-dimensional vicinity of the cluster, spanning a range of dynamical mass and redshift values relevant for optical cluster surveys. Our results reveal a negative covariance at small radial scales (R ≲ R 200c) and a null covariance at large scales (R ≳ R 200c) across most mass and redshift bins. We also find that this covariance results in a |$2{\!-\!}3~{{\ \rm per\ cent}}$| bias in the halo mass estimates in most bins. Furthermore, by modelling N gal and ΔΣ as multi-(log)-linear equations of secondary halo properties, we provide a quantitative explanation for the physical origin of the negative covariance at small scales. Specifically, we demonstrate that the N gal–ΔΣ covariance can be explained by the secondary properties of haloes that probe their formation history. We attribute the difference between our results and the positive bias seen in other works with (mock)-cluster finders to projection effects. These findings highlight the importance of accounting for the covariance between observables in cluster mass estimation, which is crucial for obtaining accurate constraints on cosmological parameters. [ABSTRACT FROM AUTHOR]

The mass distribution in massive elliptical galaxies encodes their evolutionary history, thus providing an avenue to constrain the baryonic astrophysics in their evolution. The power-law assumption for the radial mass profile in ellipticals has been sufficient to describe several observables to the noise level, including strong lensing and stellar dynamics. In this paper, we quantitatively constrained any deviation, or the lack thereof, from the power-law mass profile in massive ellipticals through joint lensing–dynamics analysis of a large statistical sample with 77 galaxy–galaxy lens systems. We performed an improved and uniform lens modelling of these systems from archival Hubble Space Telescope imaging using the automated lens modelling pipeline dolphin. We combined the lens model posteriors with the stellar dynamics to constrain the deviation from the power law after accounting for the line-of-sight lensing effects, a first for analyses on galaxy–galaxy lenses. We find that the Sloan Lens ACS Survey lens galaxies with a mean redshift of 0.2 are consistent with the power-law profile within 1.1σ (2.8σ) and the Strong Lensing Legacy Survey lens galaxies with a mean redshift of 0.6 are consistent within 0.8σ (2.1σ), for a spatially constant (Osipkov–Merritt) stellar anisotropy profile. We adopted the spatially constant anisotropy profile as our baseline choice based on previous dynamical observables of local ellipticals. However, spatially resolved stellar kinematics of lens galaxies are necessary to differentiate between the two anisotropy models. Future studies will use our lens models to constrain the mass distribution individually in the dark matter and baryonic components. [ABSTRACT FROM AUTHOR]

Counts of galaxy clusters offer a high-precision probe of cosmology, but control of systematic errors will determine the accuracy of this measurement. Using Buzzard simulations, we quantify one such systematic, the triaxiality distribution of clusters identified with the redMaPPer optical cluster finding algorithm, which was used in the Dark Energy Survey Year-1 (DES Y1) cluster cosmology analysis. We test whether redMaPPer selection biases the clusters' shape and orientation and find that it only biases orientation, preferentially selecting clusters with their major axes oriented along the line of sight. Modelling the richness–mass relation as log-linear, we find that the log-richness amplitude ln (A) is boosted from the lowest to highest orientation bin with a significance of 14σ, while the orientation dependence of the richness-mass slope and intrinsic scatter is minimal. We also find that the weak lensing shear-profile ratios of cluster-associated dark haloes in different orientation bins resemble a 'bottleneck' shape that can be quantified with a Cauchy function. We test the correlation of orientation with two other leading systematics in cluster cosmology – miscentering and projection – and find a null correlation. The resulting mass bias predicted from our templates confirms the DES Y1 finding that triaxiality is a leading source of bias in cluster cosmology. However, the richness-dependence of the bias confirms that triaxiality does not fully resolve the tension at low-richness between DES Y1 cluster cosmology and other probes. Our model can be used for quantifying the impact of triaxiality bias on cosmological constraints for upcoming weak lensing surveys of galaxy clusters. [ABSTRACT FROM AUTHOR]

When analysing galaxy clustering in multiband imaging surveys, there is a trade-off between selecting the largest galaxy samples (to minimize the shot noise) and selecting samples with the best photometric redshift (photo- z) precision, which generally includes only a small subset of galaxies. In this paper, we systematically explore this trade-off. Our analysis is targeted towards the third-year data of the Dark Energy Survey (DES), but our methods hold generally for other data sets. Using a simple Gaussian model for the redshift uncertainties, we carry out a Fisher matrix forecast for cosmological constraints from angular clustering in the redshift range z  = 0.2–0.95. We quantify the cosmological constraints using a figure of merit (FoM) that measures the combined constraints on Ωm and σ 8 in the context of Λ cold dark matter (ΛCDM) cosmology. We find that the trade-off between sample size and photo- z precision is sensitive to (1) whether cross-correlations between redshift bins are included or not, and (2) the ratio of the redshift bin width δ z to the photo- z precision σ z . When cross-correlations are included and the redshift bin width is allowed to vary, the highest FoM is achieved when δ z ∼ σ z . We find that for the typical case of 5−10 redshift bins, optimal results are reached when we use larger, less precise photo- z samples, provided that we include cross-correlations. For samples with higher σ z , the overlap between redshift bins is larger, leading to higher cross-correlation amplitudes. This leads to the self-calibration of the photo- z parameters and therefore tighter cosmological constraints. These results can be used to help guide galaxy sample selection for clustering analysis in ongoing and future photometric surveys. [ABSTRACT FROM AUTHOR]

The combination of multiple cosmological probes can produce measurements of cosmological parameters much more stringent than those possible with any individual probe. We examine the combination of two highly correlated probes of late-time structure growth: (i) weak gravitational lensing from a survey with photometric redshifts and (ii) galaxy clustering and redshift space distortions from a survey with spectroscopic redshifts. We choose generic survey designs so that our results are applicable to a range of current and future photometric and spectroscopic redshift surveys. Combining the surveys greatly improves their power to measure both dark energy and modified gravity. An independent, non-overlapping combination sees a dark energy figure of merit more than four times larger than that produced by either survey alone. The powerful synergies between the surveys are strongest for modified gravity, where their constraints are orthogonal, producing a non-overlapping joint figure of merit nearly two orders of magnitude larger than either alone. Our projected angular power spectrum formalism makes it easy to model the cross-correlation observable when the surveys overlap on the sky, producing a joint data vector and full covariance matrix. We calculate a same-sky improvement factor, from the inclusion of these cross-correlations, relative to non-overlapping surveys. We find nearly a factor of 4 for dark energy and more than a factor of 2 for modified gravity. We have also forecast constraints on the linear growth factor, finding a factor of 3.4 improvement for an independent combination of probes and a modest 20 per cent same-sky benefit. [ABSTRACT FROM AUTHOR]

Telescope point spread function (PSF) quality is critical for realizing the potential of cosmic weak lensing observations to constrain dark energy and test general relativity. In this paper, we use quantitative weak gravitational lensing measures to inform the precision of lens optical alignment, with specific reference to the Dark Energy Survey (DES). We compute optics spot diagrams and calculate the shear and flexion of the PSF as a function of position on the focal plane. For perfect optical alignment, we verify the high quality of the DES optical design, finding a maximum PSF contribution to the weak lensing shear of 0.04 near the edge of the focal plane. However, this can be increased by a factor of approximately 3 if the lenses are only just aligned within their maximum specified tolerances. We calculate the E- and B-mode shear and flexion variance as a function of the decentre or tilt of each lens in turn. We find tilt accuracy to be a few times more important than decentre, depending on the lens considered. Finally, we consider the compound effect of decentre and tilt of multiple lenses simultaneously, by sampling from a plausible range of values of each parameter. We find that the compound effect can be around twice as detrimental as when considering any one lens alone. Furthermore, this combined effect changes the conclusions about which lens is most important to align accurately. For DES, the tilt of the first two lenses is the most important. [ABSTRACT FROM AUTHOR]

We present the power spectrum of the reconstructed halo density field derived from a sample of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) Seventh Data Release (DR7). The halo power spectrum has a direct connection to the underlying dark matter power for , well into the quasi-linear regime. This enables us to use a factor of ∼8 more modes in the cosmological analysis than an analysis with , as was adopted in the SDSS team analysis of the DR4 LRG sample. The observed halo power spectrum for is well fitted by our model: for 40 degrees of freedom for the best-fitting Λ cold dark matter (ΛCDM) model. We find for a power-law primordial power spectrum with spectral index ns and fixed, consistent with cosmic microwave background measurements. The halo power spectrum also constrains the ratio of the comoving sound horizon at the baryon-drag epoch to an effective distance to . Combining the halo power spectrum measurement with the Wilkinson Microwave Anisotropy Probe ( WMAP) 5 year results, for the flat ΛCDM model we find and . Allowing for massive neutrinos in ΛCDM, we find eV at the 95 per cent confidence level. If we instead consider the effective number of relativistic species Neff as a free parameter, we find . Combining also with the Kowalski et al. supernova sample, we find and for an open cosmology with constant dark energy equation of state w. The power spectrum and a module to calculate the likelihoods are publicly available at . [ABSTRACT FROM AUTHOR]

The spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) galaxy sample represents the final set of galaxies observed using the original SDSS target selection criteria. We analyse the clustering of galaxies within this sample, including both the luminous red galaxy and main samples, and also include the 2-degree Field Galaxy Redshift Survey data. In total, this sample comprises 893 319 galaxies over 9100 deg2. Baryon acoustic oscillations (BAO) are observed in power spectra measured for different slices in redshift; this allows us to constrain the distance–redshift relation at multiple epochs. We achieve a distance measure at redshift , of (2.7 per cent accuracy), where is the comoving sound horizon at the baryon-drag epoch, is the angular diameter distance and is the Hubble parameter. We find an almost independent constraint on the ratio of distances , which is consistent at the 1.1σ level with the best-fitting Λ cold dark matter model obtained when combining our distance constraint with the Wilkinson Microwave Anisotropy Probe 5-year ( WMAP5) data. The offset is similar to that found in previous analyses of the SDSS DR5 sample, but the discrepancy is now of lower significance, a change caused by a revised error analysis and a change in the methodology adopted, as well as the addition of more data. Using WMAP5 constraints on and , and combining our BAO distance measurements with those from the Union supernova sample, places a tight constraint on and that is robust to allowing and . This result is independent of the behaviour of dark energy at redshifts greater than those probed by the BAO and supernova measurements. Combining these data sets with the full WMAP5 likelihood constraints provides tight constraints on both and for a constant dark energy equation of state. [ABSTRACT FROM AUTHOR]

In Lima et al. we presented a new method for estimating the redshift distribution, , of a photometric galaxy sample, using photometric observables and weighted sampling from a spectroscopic subsample of the data. In this paper, we extend this method and explore various applications of it, using both simulations and real data from the Sloan Digital Sky Survey (SDSS). In addition to estimating the redshift distribution for an entire sample, the weighting method enables accurate estimates of the redshift probability distribution, , for each galaxy in a photometric sample. Use of in cosmological analyses can substantially reduce biases associated with traditional photometric redshifts, in which a single redshift estimate is associated with each galaxy. The weighting procedure also naturally indicates which galaxies in the photometric sample are expected to have accurate redshift estimates, namely those that lie in regions of photometric-observable space that are well sampled by the spectroscopic subsample. In addition to providing a method that has some advantages over standard photo- z estimates, the weights method can also be used in conjunction with photo- z estimates e.g. by providing improved estimation of via deconvolution of and improved estimates of photo- z scatter and bias. We present a publicly available catalogue for ∼78 million SDSS DR7 galaxies. [ABSTRACT FROM AUTHOR]

We present an empirical method for estimating the underlying redshift distribution N( z) of galaxy photometric samples from photometric observables. The method does not rely on photometric redshift (photo- z) estimates for individual galaxies, which typically suffer from biases. Instead, it assigns weights to galaxies in a spectroscopic subsample such that the weighted distributions of photometric observables (e.g. multiband magnitudes) match the corresponding distributions for the photometric sample. The weights are estimated using a nearest neighbour technique that ensures stability in sparsely populated regions of colour–magnitude space. The derived weights are then summed in redshift bins to create the redshift distribution. We apply this weighting technique to data from the Sloan Digital Sky Survey as well as to mock catalogues for the Dark Energy Survey, and compare the results to those from the estimation of photo- zs derived by a neural network algorithm. We find that the weighting method accurately recovers the underlying redshift distribution, typically better than the photo- z reconstruction, provided the spectroscopic subsample spans the range of photometric observables covered by the photometric sample. [ABSTRACT FROM AUTHOR]