分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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 $Q\lesssim 2$. The conclusion holds for galaxies much smaller than the PSF, and those with a significant level of noise.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Weak lensing studies typically require excellent seeing conditions for the purpose of maximizing the number density of well-resolved galaxy images. It is interesting to ask to what extent the seeing size limits the usefulness of the astronomical images in weak lensing. In this work, we study this issue with the data of the DECam Legacy Survey (DECaLS), which is a part of the target selection program for the Dark Energy Spectroscopic Instrument (DESI). Using the Fourier Quad shear measurement pipeline, we demonstrate that images with relatively poor seeing conditions (around 1.5 arcsec) can still yield accurate shear estimators. We do not find any correlation between systematic shear error and the image resolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jiaqi Wang
Xiaohu Yang
Jun Zhang
Hekun Li
Matthew Fong
Haojie Xu
Min He
Yizhou Gu
Wentao Luo
Fuyu Dong
Yirong Wang
Qingyang Li
Antonios Katsianis
Haoran Wang
Zhi Shen
Pedro Alonso
Cong Liu
Yiqi Huang
Zhenjie Liu
摘要： Based on a large group/cluster catalog recently constructed from the DESI Legacy Imaging Surveys DR9 using an extended halo-based group finder, we measure and model the group-galaxy weak lensing signals for groups/clusters in a few redshift bins within redshift range $0.1 \leqslant z<0.6$. Here, the background shear signals are obtained based on the DECaLS survey shape catalog derived with the \textsc{Fourier\_Quad} method. We divide the lens samples into 5 equispaced redshift bins and 7 mass bins, which allow us to probe the redshift and mass dependence of the lensing signals and hence the resulting halo properties. In addition to these sample selections, we have also checked the signals around different group centers, e.g., brightest central galaxy (BCG), luminosity weighted center and number weighted center. We use a lensing model that includes off-centering to describe the lensing signals we measure for all mass and redshift bins. The results demonstrate that our model predictions for the halo masses, bias and concentrations are stable and self-consistent among different samples for different group centers. Taking advantage of the very large and complete sample of groups/clusters, as well as the reliable estimation of their halo masses, we provide measurements of the cumulative halo mass functions up to redshift $z=0.6$, with a mass precision at $0.03\sim0.09$ dex.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Bloch waves in 1D periodic systems carry Zak phase, which plays a key role in determining the band topology. In general, for systems that possess inversion symmetry, the Zak phase of an isolated band is quantized as 0 or Pi and is associated with the spatial field symmetries at the Brillouin zone center and boundary. The phase is Pi if the field symmetries are different but is 0 when they are the same. Since the radiation losses from leaky systems are strongly associated with the Bloch waves, one may probe the far-field continuum to determine the Zak phases. Here, we formulate the diffractions from photonic systems at the zone center and boundary and find their spectral profiles reveal the Bloch wave symmetries and thereby the corresponding Zak phase. The field symmetries also generalize the occurrence of bound states in the continuum at high symmetry points. For verification, we have studied the Zak phases of one-dimensional TM plasmonic and TE photonic crystals by electrodynamic simulations and measuring the optical properties of plasmonic crystals using Fourier space diffraction spectroscopy and common path interferometry. In addition, a topological protected interface state is demonstrated when two 0 and Pi systems are joined together. The results prove our method provides a simple way for characterizing the band topology of non-Hermitian systems via far-fields.
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