Abstract
Subpercent level accuracy in shear measurement is required by the Stage-IV weak lensing surveys. One important challenge is suppressing the shear bias on source images of low signal-to-noise ...ratios (S/N ≲ 10). Previously, it has been demonstrated that the shear estimators defined in the Fourier_Quad (FQ) method can achieve subpercent accuracy at the very faint end (S/N ≲ 5) through ensemble averaging. Later, it was found that we can approach the minimum statistical error (the Cramer–Rao Bound) by symmetrizing the full probability distribution function (PDF) of the FQ shear estimators (the PDF_SYM approach), instead of taking ensemble averages. Recently, with a large amount of mock galaxy images, we were able to identify some small amount of shear biases in the PDF_SYM approach at the faint end. The multiplicative bias goes up to (1–2) × 10
−2
at S/N ≲ 10, and the anisotropy of the point-spread function causes an additive bias that can reach a few times 10
−4
. We find that these biases originate from the noise-source coupling in the galaxy power spectrum. It turns out that this problem can be largely fixed by adding additional terms to the FQ shear estimators. The resulting multiplicative and additive biases can be significantly suppressed to the level of 10
−3
and 10
−5
, respectively. These corrections substantially extend the available S/N range for accurate shear measurement with the PDF_SYM approach.
Abstract
Weak lensing provides a direct way of mapping the density distribution in the Universe. To reconstruct the density field from the shear catalog, an important step is to build the shear field ...from the shear catalog, which can be quite nontrivial due to the inhomogeneity of the background galaxy distribution and the shape noise. We propose the PDF-folding method as a statistically optimal way of reconstructing the shear field. It is an extention of the PDF-SYM method, which was previously designed for optimizing the stacked shear signal as well as the shear-shear correlation for the Fourier_Quad shear estimators. PDF-folding does not require smoothing kernels as in traditional methods, therefore it suffers less information loss on small scales and avoids possible biases due to the spatial variation in the shear on the scale of the kernel. We show with analytic reasoning as well as numerical examples that the new method can reach the optimal signal-to-noise ratio on the reconstructed shear map under general observing conditions, i.e., with inhomogeneous background densities or masks. We also show the performance of the new method on real data around foreground galaxy clusters.
Abstract
Based on independent shear measurements using the Dark Energy Camera Legacy Survey/DR8 imaging data, we measure the weak lensing signals around isolated central galaxies (ICGs) from Sloan ...Digital Sky Survey/DR7 at
z
∼ 0.1. The projected stellar mass density profiles of satellite galaxies are further deduced, using photometric sources from the Hyper Suprime-cam survey. The signals of ICGs + their extended stellar halos are taken from Wang et al. All measurements are compared with predictions by the IllustrisTNG300-1 simulation. We find, overall, a good agreement between observation and TNG300. In particular, a correction to the stellar mass of massive observed ICGs is applied based on the calibration of He et al., which brings a much better agreement with TNG300 predicted lensing signals at
log
10
M
*
/
M
⊙
>
11.1
. In real observation, red ICGs are hosted by more massive dark matter halos and have more satellites and more extended stellar halos than blue ICGs at fixed stellar mass. However, in TNG300 there are more satellites around blue ICGs at fixed stellar mass, and the outer stellar halos of red and blue ICGs are similar. The stellar halos of TNG galaxies are more extended compared with real observed galaxies, especially for blue ICGs with
log
10
M
*
/
M
⊙
>
10.8
. We find the same trend for TNG100 galaxies and for true halo central galaxies. The tensions between TNG and real galaxies indicate that satellite disruptions are stronger in TNG. In both TNG300 and observation, satellites approximately trace the underlying dark matter distribution beyond 0.1
R
200
, but the fraction of total stellar mass in TNG300 does not show the same radial distribution as real galaxies.
In this work, we present a novel centroiding method based on Fourier space Phase Fitting (FPF) for Point Spread Function (PSF) reconstruction. We generate two sets of simulations to test our method. ...The first set is generated by GalSim with an elliptical Moffat profile and strong anisotropy that shifts the center of the PSF. The second set of simulations is drawn from CFHT i band stellar imaging data. We find non-negligible anisotropy from CFHT stellar images, which leads to ∼0.08 scatter in units of pixels using a polynomial fitting method (Vakili & Hogg). When we apply the FPF method to estimate the centroid in real space, the scatter reduces to ∼0.04 in S/N = 200 CFHT-like sample. In low signal-to-noise ratio (S/N; 50 and 100) CFHT-like samples, the background noise dominates the shifting of the centroid; therefore, the scatter estimated from different methods is similar. We compare polynomial fitting and FPF using GalSim simulation with optical anisotropy. We find that in all S/N (50, 100, and 200) samples, FPF performs better than polynomial fitting by a factor of ∼3. In general, we suggest that in real observations there exists anisotropy that shifts the centroid, and thus, the FPF method provides a better way to accurately locate it.
Abstract
In image processing, source detections are inevitably affected by the presence of the geometric boundaries in the images, including the physical boundaries of the CCD, and the boundaries of ...masked regions due to column defects, bright diffraction spikes, etc. These boundary conditions make the source detection process not statistically isotropic and can lead to additive shear bias near the boundaries. We build a phenomenological model to understand the bias, and propose a simple method to effectively eliminate the influence of geometric boundaries on shear measurement. We demonstrate the accuracy and efficiency of this method using both simulations and the
z
-band imaging data from the third data release of the DECam Legacy Survey.
Cosmic shear statistics, such as the two-point correlation function (2PCF), can be evaluated with the PDF-SYM method instead of the traditional weighted-sum approach. It makes use of the full PDF ...information of the shear estimators, and does not require weightings on the shear estimators, which can in principle introduce additional systematic biases. This work presents our constraints on
S
8
and Ω
m
from the shear-shear correlations using the PDF-SYM method. The data we use is from the
z
-band images of the Dark Energy Camera Legacy Survey (DECaLS), which covers about 10000 deg
2
with more than 100 million galaxies. The shear catalog is produced by the Fourier_Quad method, and well tested on the real data itself with the field-distortion effect. Our main approach is called quasi-2D as we do use the photo-
z
information of each individual galaxy, but without dividing the galaxies into redshift bins. We mainly use galaxy pairs within the redshift interval between 0.2 and 1.3, and the angular range from 4.7 to 180 arcmin. Our analysis yields
S
8
= 0.762 ± 0.026 and Ω
m
= 0.234 ± 0.075, with the baryon effects and the intrinsic alignments included. The results are robust against redshift uncertainties. We check the consistency of our results by deriving the cosmological constraints from auto-correlations of
γ
1
and
γ
2
separately, and find that they are consistent with each other, but the constraints from the
γ
1
component are much weaker than that from
γ
2
. It implies a much worse data quality of
γ
1
, which is likely due to additional shear uncertainties caused by CCD electronics (according to the survey strategy of DECaLS). We also perform a pure 2D analysis, which gives
S
8
= 0.81
−0.04
+0.03
and Ω
m
= 0.25
−0.05
+0.06
. Our findings demonstrate the potential of the PDF-SYM method for precision cosmology.
Sample selection is a necessary preparation for weak lensing measurement. It is well-known that selection itself may introduce bias to the measured shear signal. Using image simulation and the ...Fourier_Quad shear measurement pipeline, we quantify the selection bias in various commonly used selection criteria (signal-to-noise ratio, magnitude, etc.). We propose a new selection criterion defined in the power spectrum of the galaxy image. This new selection criterion has a low selection bias, and it is particularly convenient for shear measurement pipelines based on Fourier transformation.
ABSTRACT
We use weak lensing observations to make the first measurement of the characteristic depletion radius, one of the three radii that characterize the region where matter is being depleted by ...growing haloes. The lenses are taken from the halo catalogue produced by the extended halo-based group/cluster finder applied to DESI Legacy Imaging Surveys DR9, while the sources are extracted from the DECaLS DR8 imaging data with the fourier_quad pipeline. We study halo masses $12 \lt \log (M_{\rm grp} \,{\rm M_{\odot }}\, h^{-1}) \le 15.3$ within redshifts 0.2 ≤ z ≤ 0.3. The virial and splashback radii are also measured and used to test the original findings on the depletion region. When binning haloes by mass, we find consistency between most of our measurements and predictions from the cosmicgrowth simulation, with exceptions to the lowest mass bins. The characteristic depletion radius is found to be roughly 2.5 times the virial radius and 1.7–3 times the splashback radius, in line with an approximately universal outer density profile, and the average enclosed density within the characteristic depletion radius is found to be roughly 29 times the mean matter density of the Universe in our sample. When binning haloes by both mass and a proxy for halo concentration, we do not detect a significant variation of the depletion radius with concentration, on which the simulation prediction is also sensitive to the choice of concentration proxy. We also confirm that the measured splashback radius varies with concentration differently from simulation predictions.
The tilt, rotation, or offset of each CCD with respect to the focal plane, as well as the distortion of the focal plane itself, causes shape distortions in the observed objects, an effect typically ...known as field distortion (FD). We point out that FD provides a unique way of quantifying the accuracy of cosmic shear measurement. The idea is to stack the shear estimators from galaxies that share similar FD-induced shape distortions. Given that the latter can be calculated with parameters from astrometric calibrations, the accuracy of the shear estimator can be directly tested on real images. It provides a way to calibrate the multiplicative and additive shear recovery biases within the scientific data itself, without requiring simulations or any external data sets. We use the CFHTLenS images to test the Fourier_Quad shear recovery method. We highlight some details in our image-processing pipeline, including background removal, source identification and deblending, astrometric calibration, star selection for point-spread function reconstruction, noise reduction, etc. We show that in the shear ranges of −0.005 g1 0.005 and −0.008 g2 0.008, the multiplicative biases are at the level of 0.04. Slight additive biases on the order of ∼5 × 10−4 (6 ) are identified for sources provided by the official CFHTLenS catalog (not using its shear catalog), but are minor (4 ) for the source catalog generated by our Fourier_Quad pipeline.
Abstract
Images taken by space telescopes typically have a superb spatial resolution, but a relatively poor sampling rate due to the finite CCD pixel size. Beyond the Nyquist limit, it becomes ...uncertain how much the pixelation effect may affect the accuracy of galaxy shape measurement. It is timely to study this issue given that a number of space-based large-scale weak lensing surveys are planned. Using the Fourier_Quad method, we quantify the shear recovery error as a function of the sampling factor
Q
, i.e., the ratio between the FWHM of the point-spread function (PSF) and the pixel size of the CCD, for different PSFs and galaxies of different sizes and noise levels. We show that sub-percent-level accuracy in shear recovery is achievable with single-exposure images for
Q
≲ 2. The conclusion holds for galaxies much smaller than the PSF, and those with a significant level of noise.
PDF
