排序方式： 共有74条查询结果，搜索用时 15 毫秒
11.
V. R. Eke Carlos S. Frenk Carlton M. Baugh Shaun Cole Peder Norberg John A. Peacock Ivan K. Baldry Joss Bland-Hawthorn Terry Bridges Russell Cannon Matthew Colless Chris Collins Warrick Couch Gavin Dalton Roberto de Propris Simon P. Driver George Efstathiou Richard S. Ellis Karl Glazebrook Carole A. Jackson Ofer Lahav Ian Lewis Stuart Lumsden Steve J. Maddox Darren Madgwick Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2004,355(3):769-784
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William S. Burgett Michael M. Vick David S. Davis Matthew Colless Roberto De Propris Ivan Baldry Carlton Baugh Joss Bland-Hawthorn Terry Bridges Russell Cannon Shaun Cole Chris Collins Warrick Couch Nicholas Cross Gavin Dalton Simon Driver George Efstathiou Richard Ellis Carlos S. Frenk Karl Glazebrook Edward Hawkins Carole Jackson Ofer Lahav Ian Lewis Stuart Lumsden Steve Maddox Darren Madgwick Peder Norberg John A. Peacock Will Percival Bruce Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2004,352(2):605-654
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Late Holocene upper bounds of flood magnitudes and twentieth century large floods in the ungauged, hyperarid alluvial Nahal Arava, Israel 总被引：1，自引：0，他引：1
Yael Jacoby Tamir Grodek Yehouda Enzel Naomi Porat Eric V. McDonald Ofer Dahan 《Geomorphology》2008,95(3-4):274-294
The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m3s− 1. This is a larger magnitude than the largest gauged flood of 1150 m3s− 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene ( 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of 2000 m3s− 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning. 相似文献
15.
Summary. Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic
Equator, creating a “zone of avoidance” of about 25% in the distribution of optically visible galaxies. A “whole-sky” map
of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity
of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor
region.
Various dynamically important structures behind the Milky Way have only recently been made “visible” through dedicated deep
surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and
X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects
of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction
methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational
evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the
density and biasing parameters and b.
Received 4 April 2000 / Published online 18 July 2000 相似文献
16.
Sreekumar T. Balan Ofer Lahav 《Monthly notices of the Royal Astronomical Society》2009,394(4):1936-1944
Retrieval of orbital parameters of extrasolar planets poses considerable statistical challenges. Due to sparse sampling, measurement errors, parameters degeneracy and modelling limitations, there are no unique values of basic parameters, such as period and eccentricity. Here, we estimate the orbital parameters from radial velocity data in a Bayesian framework by utilizing Markov Chain Monte Carlo (MCMC) simulations with the Metropolis–Hastings algorithm. We follow a methodology recently proposed by Gregory and Ford. Our implementation of MCMC is based on the object-oriented approach outlined by Graves. We make our resulting code, exofit , publicly available with this paper. It can search for either one or two planets as illustrated on mock data. As an example we re-analysed the orbital solution of companions to HD 187085 and HD 159868 from the published radial velocity data. We confirm the degeneracy reported for orbital parameters of the companion to HD 187085, and show that a low-eccentricity orbit is more probable for this planet. For HD 159868, we obtained slightly different orbital solution and a relatively high 'noise' factor indicating the presence of an unaccounted signal in the radial velocity data. exofit is designed in such a way that it can be extended for a variety of probability models, including different Bayesian priors. 相似文献
17.
Adrian Collister Ofer Lahav Chris Blake Russell Cannon Scott Croom Michael Drinkwater Alastair Edge Daniel Eisenstein Jon Loveday Robert Nichol Kevin Pimbblet Roberto De Propris Isaac Roseboom Nic Ross Donald P. Schneider Tom Shanks David Wake 《Monthly notices of the Royal Astronomical Society》2007,375(1):68-76
We describe the construction of MegaZ-LRG, a photometric redshift catalogue of over one million luminous red galaxies (LRGs) in the redshift range 0.4 < z < 0.7 with limiting magnitude i < 20 . The catalogue is selected from the imaging data of the Sloan Digital Sky Survey (SDSS) Data Release 4. The 2dF-SDSS LRG and Quasar (2SLAQ) spectroscopic redshift catalogue of 13 000 intermediate-redshift LRGs provides a photometric redshift training set, allowing use of ann z, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is σ z = 0.049 averaged over all galaxies, and σ z = 0.040 for a brighter subsample ( i < 19.0) . The catalogue is expected to contain ∼5 per cent stellar contamination. The ann z code is used to compute a refined star/galaxy probability based on a range of photometric parameters; this allows the contamination fraction to be reduced to 2 per cent with negligible loss of genuine galaxies. The MegaZ-LRG catalogue is publicly available on the World Wide Web from http://www.2slaq.info . 相似文献
18.
本文描述了砷在耕作土壤中的作用过程中，有关砷的物质平衡的数学表述形式。需要用许多模型参数定义所涉及砷的物质平衡的数学表述形式，即使是简化的数学形式。根据参数值的范围和源于出版的文献的初始条件进行实例模拟。研究结果表明，由于对耕作土壤施肥和灌溉水，植被对砷的摄入量引起根部区总砷含量逐渐增加。搞清楚植物对砷的吸收和析出与弄清楚对增加砷的去除途径同等重要。反过来，矿物相的溶解动力学和吸附相的分布系数影响植物吸收和浸出的可用性。根据实验室砷矿相的分解、As（III）的矿化和氧化推导出参数，而砷植物的吸收似乎对砷在土壤中的传输估计过高。数学模型的研究是一个简明的过程，而用自信度定义的模型参数值的不同阻碍了它在实际情况中的应用。目前对土壤一一植被系统中砷的传输过程和作用的了解是不充分的，所以要校准或者验证模型。研究必须了解土壤中砷矿物分解和沉淀的动力学原理和土壤中根部生长以及植被砷吸收的动力学原理。 相似文献
19.
Audra Baleisis Ofer Lahav rew J. Loan & Jasper V. Wall 《Monthly notices of the Royal Astronomical Society》1998,297(2):545-558
We calculate the expected amplitude of the dipole and higher spherical harmonics in the angular distribution of radio galaxies. The median redshift of radio sources in existing catalogues is z ∼ 1, which allows us to study large-scale structure on scales between those accessible to present optical and infrared surveys, and that of the cosmic microwave background (CMB). The dipole is a result of two effects which turn out to be of comparable magnitude: (i) our motion with respect to the CMB, and (ii) large-scale structure, parametrized here by a family of cold dark matter power-spectra. We make specific predictions for the Green Bank 1987 (87GB) and Parkes–MIT–NRAO (PMN) catalogues, which in our combined catalogue include ∼ 40 000 sources brighter than 50 mJy at 4.85 GHz, over about 70 per cent of the sky. For these relatively sparse catalogues both the motion and large-scale structure dipole effects are expected to be smaller than the Poisson shot noise. However, we detect dipole and higher harmonics in the combined 87GB–PMNraw catalogue which are far larger than expected. We attribute this to a 2 per cent flux mismatch between the two catalogues. Ad hoc corrections made in an effort to match the catalogues may suggest a marginal detection of a dipole. To detect a dipole and higher harmonics unambiguously, a catalogue with full sky coverage and ∼ 106 sources is required. We also investigate the existence and extent of the supergalactic plane in the above catalogues. In a strip of ± 10° of the standard supergalactic equator, we find a 3 σ detection in PMNraw , but only 1 σ in 87 GBraw . We briefly discuss the implications of ongoing surveys such as FIRST and NVSS and follow-up redshift surveys. 相似文献
20.
Electron spin resonance (ESR) measurements of heated flint from Kebara cave show the presence of two radiation sensitive signals: E' and Al. Both are strongly interfered with by organic signals produced during heating of the flint. The signal subtraction method of Porat and Schwarcz (1991) was used to determine equivalent doses for flint samples from levels VII-XII, for which thermoluminescence (TL) ages on the same samples are known. Ages were determined using dose rates based on analyses of the flint and TLD measurements of external dose rates. The average E' age of 48.5 ± 5.1 ka is much less than the average TL age (62.2 ± 4.3 ka), whereas the average age of 64.6 ± 12 ka given by the Al signal is in good agreement with the TL age, but with much larger dispersion. © 1994 John Wiley & Sons, Inc. 相似文献