The Aspen–Amsterdam void finder comparison project     

Jörg M. Colberg  Frazer Pearce  Caroline Foster  Erwin Platen  Riccardo Brunino  Mark Neyrinck  Spyros Basilakos  Anthony Fairall  Hume Feldman  Stefan Gottlöber  Oliver Hahn  Fiona Hoyle  Volker Müller  Lorne Nelson  Manolis Plionis  Cristiano Porciani  Sergei Shandarin  Michael S. Vogeley  Rien van de Weygaert 《Monthly notices of the Royal Astronomical Society》2008,387(2):933-944

Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than   m _B =−20  is found to be smaller than about −0.8 by all our void finding algorithms.  相似文献   

Photometric redshifts with surface brightness priors     

Hans F. Stabenau  rew Connolly  Bhuvnesh Jain 《Monthly notices of the Royal Astronomical Society》2008,387(3):1215-1226

We use galaxy surface brightness as prior information to improve photometric redshift (photo- z ) estimation. We apply our template-based photo- z method to imaging data from the ground-based VVDS survey and the space-based GOODS field from HST , and use spectroscopic redshifts to test our photometric redshifts for different galaxy types and redshifts. We find that the surface brightness prior eliminates a large fraction of outliers by lifting the degeneracy between the Lyman and 4000-Å breaks. Bias and scatter are improved by about a factor of 2 with the prior in each redshift bin in the range  0.4 < z < 1.3  , for both the ground and space data. Ongoing and planned surveys from the ground and space will benefit, provided that care is taken in measurements of galaxy sizes and in the application of the prior. We discuss the image quality and signal-to-noise ratio requirements that enable the surface brightness prior to be successfully applied.  相似文献   

Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon     

Timmons?M.?EricksonEmail author  Mark?A.?Pearce  Steven?M.?Reddy  Nicholas?E.?Timms  Aaron?J.?Cavosie  Julien?Bourdet  William?D.?A.?Rickard  Alexander?A.?Nemchin 《Contributions to Mineralogy and Petrology》2017,172(1):6

Zircon (ZrSiO₄) is used to study impact structures because it responds to shock loading and unloading in unique, crystallographically controlled manners. One such phenomenon is the transformation of zircon to the high-pressure polymorph, reidite. This study quantifies the geometric and crystallographic orientation relationships between these two phases using naturally shocked zircon grains. Reidite has been characterized in 32 shocked zircon grains (shocked to stages II and III) using a combination of electron backscatter diffraction (EBSD) and focused ion beam cross-sectional imaging techniques. The zircon-bearing clasts were obtained from within suevite breccia from the Nördlingen 1973 borehole, close to the center of the 14.4 Ma Ries impact crater, in Bavaria, Germany. We have determined that multiple sets (up to 4) of reidite lamellae can form in a variety of non-rational habit planes within the parent zircon. However, EBSD mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misorientation relationship with the host zircon that is characterized by an approximate alignment of a {100}_zircon with a {112}_reidite and alignment of a {112}_zircon with a conjugate {112}_reidite. Given the tetragonal symmetry of zircon and reidite, we predict that there are eight possible variants of this interphase relationship for reidite transformation within a single zircon grain. Furthermore, laser Raman mapping of one reidite-bearing grain shows that moderate metamictization can inhibit reidite formation, thereby highlighting that the transformation is controlled by zircon crystallinity. In addition to lamellar reidite, submicrometer-scale granules of reidite were observed in one zircon. The majority of reidite granules have a topotaxial alignment that is similar to the lamellar reidite, with some additional orientation dispersion. We confirm that lamellar reidite likely forms via a deviatoric transformation mechanism in highly crystalline zircon, whereas granular reidite forms via a reconstructive transformation from low-crystallinity ZrSiO₄ within the reidite stability field. The results of this study further refine the formation mechanisms and conditions of reidite transformation in naturally shocked zircon.  相似文献   

Hydrology and related changes after harvesting native forest catchments and establishing pinus radiata plantations. Part 1. Introduction to study     

L. K. Rowe  A. J. Pearce  C. L. O'Loughlin 《水文研究》1994,8(3):263-279

The hydrology of eight small catchments (1.63–8.26 ha) has been monitored in Westland, New Zealand since 1975. Two of these catchments were left in indigenous beech-podocarp-hardwood forest and the rest were subjected to various harvesting and land preparation treatments before being planted with Pinus radiata. This paper introduces a series of papers on the hydrology of the indigenous forest catchments and the changes that occurred after treatment. The catchments, experimental programme, climate of the area and the rainfall regime experienced between 1975 and 1987 are described.  相似文献   

The mass assembly of fossil groups of galaxies in the Millennium simulation     

Ali Dariush  Habib G. Khosroshahi  Trevor J. Ponman  Frazer Pearce  Somak Raychaudhury  Will Hartley 《Monthly notices of the Royal Astronomical Society》2007,382(1):433-442

  相似文献   

Hydrology and related changes after harvesting native forest catchments and establishing pinus radiata plantations. Part 2. The native forest water balance and changes in streamflow after harvesting     

L. K. Rowe  A. J. Pearce 《水文研究》1994,8(4):281-297

Six small, steep, south-west facing catchments (1.63–4.62 ha) have been monitored in Westland, New Zealand since 1974. Two catchments were retained in native mixed evergreen forest and the rest were subjected to various harvesting and land preparation techniques before being planted with Pinus radiata between 1977 and 1980. The 11-year water balance for the native forest catchments was: rain = streamflow + interception loss + transpiration + seepage (2370mm = 1290mm + 620mm + 360mm + 100mm). In the year after treatment streamflow generally increased by 200–250 mm, except for one treatment (clearfelling, herbicide application, no riparian reserve) where the increase was 550 mm. The catchments were planted with Pinus radiata, but rapid colonization by bracken (Pteridium esculentum) and Himalayan honeysuckle (Leycesteria formosa) led to a rapid decline in streamflow, which returned to pre-treatment levels after an average of about five years. Streamflow yields then continued to decline for another two to three years before stabilizing at a level about 250mm yr^?1 lower than pre-treatment levels. At this time the catchments had a dense bracken/honeysuckle understorey beneath 5 m tall pine trees.  相似文献   

The identification of runoff-production mechanisms using environmental isotopes in a tussock grassland catchment,eastern otago,New Zealand     

Mike Bonell  Andrew J. Pearce  Michael K. Stewart 《水文研究》1990,4(1):15-34

A previous hydrometric study of runoff production in tussock grassland drainage basins in Otago (45°50′S, 169°45′E), New Zealand, revealed a marked change of slope in storm hydrograph recessions. An environmental isotope study was initiated to investigate the runoff mechanisms operating and to test specific hypotheses to explain this break in the hydrograph recession. The results indicated that for quickflow volumes in excess of 10mm, the first part of the storm hydrograph can be attributed to two separate sources, namely, ‘old’ water from a shallow, unconfined groundwater reservoir and ‘new’ water from saturation overland flow on the lower wetlands of concave slopes. Despite the extensive area of wetlands, ‘old’ water runoff from the unconfined groundwater reservoir is delivered more rapidly to the stream than ‘new’ water from saturation overland flow. Substantial surface storage in the wetlands has first to be exceeded before rain becomes a significant part of stream discharge. For quickflow volumes less than 10mm, only ‘old’ water from groundwater contributes to the first part of the hydrograph recession. This means that only the largest 7 per cent of storms (in terms of quickflow volume) generate quickflow containing significant amounts of ‘new water’. The second part of the recession of the storm hydrograph consists of ‘old’ water derived from a remarkably well-mixed shallow unconfined groundwater body.  相似文献   

The spectacular Supernova SN1987A: Grain evaporation and condensation     

K. Turner  G. Pearce 《Astrophysics and Space Science》1992,188(1):1-8

Non-similarity solutions of the equations governing the motion of a perfect gas behind a cylindrical shock wave of variable strength have been obtained. These solutions are applicable to both the weak and the strong shocks. The nature of flow and field variables are illustrated through graphs. The total energy of the wave is taken to be constant.  相似文献   

Complex P waveforms from a Gulf of Aden earthquake     

R. G. Pearce 《Geophysical Journal International》1981,64(1):187-200

Summary. It is shown that complex teleseismic P waveforms from a shallow earthquake in a tectonic area can be interpreted using a simple source model embedded in a plane layer velocity structure (with sea layer) whose details are based upon independent evidence. This gives hope that structural complexity in tectonic regions may not always make distant P -wave seismograms impossible to interpret, and that, instead, source complexity may be responsible for some of the many complex waveforms observed, even for earthquakes below magnitude m _b 5.5.  相似文献   

Genus statistics of the Virgo N-body simulations and the 1.2-Jy redshift survey     

Volker Springel  Simon D. M. White  Jo¨rg M. Colberg  Hugh M. P. Couchman  George P. Efstathiou  Carlos S. Frenk  Adrian R. Jenkins  Frazer R. Pearce  Alistair H. Nelson  John A. Peacock  & Peter A. Thomas 《Monthly notices of the Royal Astronomical Society》1998,298(4):1169-1188

  相似文献