J. Bankert

SIMULATION OF ASTRONOMICAL IMAGES FROM OPTICAL SURVEY TELESCOPES USING A COMPREHENSIVE PHOTON MONTE CARLO APPROACH

We present a comprehensive methodology for the simulation of astronomical images from optical survey telescopes. We use a photon Monte Carlo approach to construct images by sampling photons from models of astronomical source populations, and then simulating those photons through the system as they interact with the atmosphere, telescope, and camera. We demonstrate that all physical effects for optical light that determine the shapes, locations, and brightnesses of individual stars and galaxies can be accurately represented in this formalism. By using large scale grid computing, modern processors, and an efficient implementation that can produce 400,000 photons/second, we demonstrate that even very large optical surveys can be now be simulated. We demonstrate that we are able to: 1) construct kilometer scale phase screens necessary for wide-field telescopes, 2) reproduce atmospheric point-spread-function moments using a fast novel hybrid geometric/Fourier technique for non-diffraction limited telescopes, 3) accurately reproduce the expected spot diagrams for complex aspheric optical designs, and 4) recover system effective area predicted from analytic photometry integrals. This new code, the photon simulator (PhoSim), is publicly available. We have implemented the Large Synoptic Survey Telescope (LSST) design, and it can be extended to other telescopes. We expect that because of the comprehensive physics implemented in PhoSim, it will be used by the community to plan future observations, interpret detailed existing observations, and quantify systematics related to various astronomical measurements. Future development and validation by comparisons with real data will continue to improve the fidelity and usability of the code.

Spurious shear in weak lensing with the large synoptic survey telescope

The complete 10-year survey from the Large Synoptic Survey Telescope (LSST) will image {approx} 20,000 square degrees of sky in six filter bands every few nights, bringing the final survey depth to r {approx} 27.5, with over 4 billion well measured galaxies. To take full advantage of this unprecedented statistical power, the systematic errors associated with weak lensing measurements need to be controlled to a level similar to the statistical errors. This work is the first attempt to quantitatively estimate the absolute level and statistical properties of the systematic errors on weak lensing shear measurements due to the most important physical effects in the LSST system via high fidelity ray-tracing simulations. We identify and isolate the different sources of algorithm-independent, additive systematic errors on shear measurements for LSST and predict their impact on the final cosmic shear measurements using conventional weak lensing analysis techniques. We find that the main source of the errors comes from an inability to adequately characterise the atmospheric point spread function (PSF) due to its high frequency spatial variation on angular scales smaller than {approx} 10{prime} in the single short exposures, which propagates into a spurious shear correlation function at the 10{sup -4}-10{sup -3} level on these scales. With the large multi-epoch dataset that will be acquired by LSST, the stochastic errors average out, bringing the final spurious shear correlation function to a level very close to the statistical errors. Our results imply that the cosmological constraints from LSST will not be severely limited by these algorithm-independent, additive systematic effects.

EFFECT OF MEASUREMENT ERRORS ON PREDICTED COSMOLOGICAL CONSTRAINTS FROM SHEAR PEAK STATISTICS WITH LARGE SYNOPTIC SURVEY TELESCOPE

We study the effect of galaxy shape measurement errors on predicted cosmological constraints from the statistics of shear peak counts with the Large Synoptic Survey Telescope (LSST). We use the LSST Image Simulator in combination with cosmological N-body simulations to model realistic shear maps for different cosmological models. We include both galaxy shape noise and, for the first time, measurement errors on galaxy shapes. We find that the measurement errors considered have relatively little impact on the constraining power of shear peak counts for LSST.

Simulating the LSST system

Extracting science from the LSST data stream requires a detailed knowledge of the properties of the LSST catalogs and images (from their detection limits to the accuracy of the calibration to how well galaxy shapes can be characterized). These properties will depend on many of the LSST components including the design of the telescope, the conditions under which the data are taken and the overall survey strategy. To understand how these components impact the nature of the LSST data the simulations group is developing a framework for high fidelity simulations that scale to the volume of data expected from the LSST. This framework comprises galaxy, stellar and solar system catalogs designed to match the depths and properties of the LSST (to r=28), transient and moving sources, and image simulations that ray-trace the photons from above the atmosphere through the optics and to the camera. We describe here the state of the current simulation framework and its computational challenges.

EVIDENCE FOR NONLINEAR GROWTH OF STRUCTURE FROM AN X-RAY-SELECTED CLUSTER SURVEY USING A NOVEL JOINT ANALYSIS OF THE CHANDRA AND XMM-NEWTON ARCHIVES

We present a large X-ray-selected serendipitous cluster survey based on a novel joint analysis of archival Chandra and XMM-Newton data. The survey provides enough depth to reach clusters of flux of ≈10–14ergcm–2s–1 near z≈ 1 and simultaneously a large-enough sample to find evidence for the strong evolution of clusters expected from structure formation theory. We detected a total of 723 clusters of which 462 are newly discovered clusters with greater than 6σ significance. In addition, we also detect and measure 261 previously known clusters and groups that can be used to calibrate the survey. The survey exploits a technique that combines the exquisite Chandra imaging quality with the high throughput of the XMM-Newton telescopes using overlapping survey regions. A large fraction of the contamination from active galactic nucleus point sources is mitigated by using this technique. This results in a higher sensitivity for finding clusters of galaxies with relatively few photons and a large part of our survey has a flux sensitivity between 10–14 and 10–15ergcm–2s–1. The survey covers 41.2 deg2 of overlapping Chandra and XMM-Newton fields and 122.2 deg2 of non-overlapping Chandra data. We measure the log N-log S distribution and fit it with a redshift-dependent model characterized by a luminosity distribution proportional to . We find z 0 to be in the range 0.7-1.3, indicative of rapid cluster evolution, as expected for cosmic structure formation using parameters appropriate to the concordance cosmological model. Confirmation of our cluster detection efficiency through optical follow-up studies currently in progress will help to strengthen this conclusion and eventually allow to use these data to derive tight constraints on cosmological parameters.