Wentao Luo

GREAT3 results - I. Systematic errors in shear estimation and the impact of real galaxy morphology

We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space- and ground-based data with constant or cosmologically varying shear fields. The simplest (control) experiment included parametric galaxies with a realistic distribution of signal-to-noise, size, and ellipticity, and a complex point spread function (PSF). The other experiments tested the additional impact of realistic galaxy morphology, multiple exposure imaging, and the uncertainty about a spatially varying PSF; the last two questions will be explored in Paper II. The 24 participating teams competed to estimate lensing shears to within systematic error tolerances for upcoming Stage-IV dark energy surveys, making 1525 submissions overall. GREAT3 saw considerable variety and innovation in the types of methods applied. Several teams now meet or exceed the targets in many of the tests conducted (to within the statistical errors). We conclude that the presence of realistic galaxy morphology in simulations changes shear calibration biases by similar to 1 per cent for a wide range of methods. Other effects such as truncation biases due to finite galaxy postage stamps, and the impact of galaxy type as measured by the S,rsic index, are quantified for the first time. Our results generalize previous studies regarding sensitivities to galaxy size and signal-to-noise, and to PSF properties such as seeing and defocus. Almost all methods results support the simple model in which additive shear biases depend linearly on PSF ellipticity.

Spin Alignments of Spiral Galaxies within the Large-scale Structure from SDSS DR7

Using a sample of spiral galaxies selected from the Sloan Digital Sky Survey Data Release 7 and Galaxy Zoo 2, we investigate the alignment of spin axes of spiral galaxies with their surrounding large-scale structure, which is characterized by the large-scale tidal field reconstructed from the data using galaxy groups above a certain mass threshold. We find that the spin axes only have weak tendencies to be aligned with (or perpendicular to) the intermediate (or minor) axis of the local tidal tensor. The signal is the strongest in a cluster environment where all three eigenvalues of the local tidal tensor are positive. Compared to the alignments between halo spins and the local tidal field obtained in N-body simulations, the above observational results are in best agreement with those for the spins of inner regions of halos, suggesting that the disk material traces the angular momentum of dark matter halos in the inner regions.

First galaxy–galaxy lensing measurement of satellite halo mass in the CFHT Stripe-82 Survey

We select satellite galaxies from the galaxy group catalogue constructed with the Sloan Digital Sky Survey spectroscopic galaxies and measure the tangential shear around these galaxies with the source catalogue extracted from the Canada-France-Hawaii Telescope Stripe-82 Survey. Using the tangential shear, we constrain the mass of subhaloes associated with these satellites. The lensing signal is measured around satellites in groups with masses in the range 10(13)-5 x 10(14) h (1) M-circle dot, and is found to agree well with theoretical expectations. Fitting the data with a truncated NFW profile, we obtain an average subhalo mass of log (M-sub/h (-1) M-circle dot) = 11.68 +/- 0.67 for satellites whose projected distances to central galaxies are in the range 0.1-0.3 h (1) Mpc and log (M-sub/h (1) M-circle dot) = 11.68 +/- 0.76 for satellites with projected halo-centric distance in [0.3, 0.5]h (1) Mpc. The best-fitting subhalo masses are comparable to the truncated subhalomasses assigned to satellite galaxies using abundance matching and are about 5-10 times higher than the average stellar mass of the lensing satellite galaxies.

CONNECTIONS BETWEEN GALAXY MERGERS AND STARBURST: EVIDENCE FROM THE LOCAL UNIVERSE

Major mergers and interactions between gas-rich galaxies with comparable masses are thought to be the main triggers of starburst. In this work, we study, for a large stellar mass range, the interaction rate of the starburst galaxies in the local universe. We focus independently on central and satellite star forming galaxies extracted from the Sloan Digital Sky Survey. Here the starburst galaxies are selected in the star formation rate (SFR) stellar mass plane with SFR five times larger than the median value found for star forming galaxies of the same stellar mass. Through visual inspection of their images together with close companions determined using spectroscopic redshifts, we find that ~50% of the starburst populations show evident merger features, i.e., tidal tails, bridges between galaxies, double cores and close companions. In contrast, in the control sample we selected from the normal star forming galaxies, only ~19% of galaxies are associated with evident mergers. The interaction rates may increase by ~5% for the starburst sample and 2% for the control sample if close companions determined using photometric redshifts are considered. The contrast of the merger rate between the two samples strengthens the hypothesis that mergers and interactions are indeed the main causes of starburst.

APPROACHING THE CRAMÉR–RAO BOUND IN WEAK LENSING WITH PDF SYMMETRIZATION

Weak lensing statistics is typically measured as weighted sum of shear estimators or their products (shear-shear correlation). The weighting schemes are designed in the hope of minimizing the statistical error without introducing systematic errors. It would be ideal to approach the Cramer-Rao bound (the lower bound of the statistical uncertainty) in shear statistics, though it is generally difficult to do so in practice. The reasons may include: difficulties in galaxy shape measurement, inaccurate knowledge of the probability-distribution-function (PDF) of the shear estimator, misidentification of point sources as galaxies, etc.. Using the shear estimators defined in Zhang et al. (2015), we show that one can overcome all these problems, and allow shear measurement accuracy to approach the Cramer-Rao bound. This can be achieved by symmetrizing the PDF of the shear estimator, or the joint PDF of shear estimator pairs (for shear-shear correlation), without any prior knowledge of the PDF. Using simulated galaxy images, we demonstrate that under general observing conditions, this idea works as expected: it minimizes the statistical uncertainty without introducing systematic error.

An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories (SFH) of galaxies in halos of different masses, we develop an empirical model to grow galaxies in dark mattet halos. This model has very few ingredients, any of which can be associated to observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological

Galaxy–Galaxy Weak-lensing Measurements from SDSS. II. Host Halo Properties of Galaxy Groups

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An Accurate Centroiding Algorithm for PSF Reconstruction

-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass function (SMF) up to redshift

Galaxy-Galaxy Weak Lensing Measurements from SDSS: I. Image Processing and Lensing signals

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Cross identification between X-ray and Optical Clusters of Galaxies in the SDSS DR7 Field

and the conditional stellar mass functions (CSMF) in the local universe are in good agreement with observations. In addition, the 2-point correlation is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in