Red giants in the blue compact galaxy IZw18     

N. A. Tikhonov 《Astronomy Letters》2007,33(3):137-148

Archival Hubble Space Telescope (HST) data have been used for the photometry of stars in blue compact dwarf (BCD) galaxies of the IZw18 system. Applying the spatial selection of stars, we have detected red giants, stars older than 1–2 Gyr, in the galaxies. These red giants have allowed the distance to IZw18 to be reliably determined for the first time: D = 13.9 ± 1.2 Mpc. The presence of old stars in the galaxies of the IZw18 system refutes the hypothesis about the observed primary star formation in these galaxies.  相似文献   

Effect of food quality on energy uptake     

Jeffrey S. Ren   《Journal of Sea Research》2009,62(2-3):72

The dynamic energy budget (DEB) theory assumes that the feeding of an organism in relation to food density follows the type-II functional response when food consists of purely digestible organic matter. However when faced with a food source high in inorganic content, an organism needs to adapt physiologically to maximise organic extraction. Indeed filter-feeding marine bivalves inhabiting turbid coastal waters cope with large fluctuations in food quality by selectively ingesting and pseudofaeces production. Data from feeding experiments with the New Zealand greenshell mussel Perna canaliculus have demonstrated that inorganic particles contribute some uncertainty to the estimate of the half-saturation coefficient. Incorporating inorganic matter into the functional response would improve the prediction of energy uptake.  相似文献   

The method of a two-point conditional column density for estimating the fractal dimension of the distribution of galaxies     

Yu. V. Baryshev  Yu. L. Bukhmastova 《Astronomy Letters》2004,30(7):444-450

We suggest a new method for estimating the fractal dimension of the spatial distribution of galaxies: the method of selected cylinders. We show the capabilities of this method by constructing a two-point conditional column density for galaxies with known redshifts from the LEDA database. The fractal dimension of a sample of LEDA and EDR SDSS galaxies has been estimated to be 2.1±0.1 for cylinder lengths of 200 Mpc. A major advantage of the suggested method is that it allows scales comparable to the catalog depth to be analyzed for galaxy surveys in the form of conical sectors and small fields in the sky.  相似文献   

Morphology and Luminosity Segregations in Galaxy Groups     

M. Girardi  F. Mardirossian  M. Mezzetti  E. Rigoni 《Astrophysics and Space Science》2003,285(1):31-35

We study morphology and luminosity segregation of galaxies in groups. We analyze the two catalogs of (∼2×400) groups which have been identified in the Nearby Optical Galaxy sample, by means of hierarchical and percolation `friends-of-friends' methods. We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects are generally detected at the ≳3-sigma level, with the exception of morphological segregation in velocity, which is the weakest effect. Our main results are confirmed by the analysis of statistically more reliable groups (with at least five members), and are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Luminosity segregation is shown to be independent of morphology segregation. Our conclusions agree with a continuum of segregation properties of galaxies in systems, from low-mass groups to massive clusters. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Ab initio prediction of 29Si-NMR chemical shifts     

Chu Shidong  Li Yingxia  Song Ni  Guan Huashi 《中国海洋大学学报(英文版)》2002,1(1):63-65

The ability of several ab initio models to predict experimental ²⁹Si-NMR chemical shift is examined. The shielding values of trimethylsilyl chloride (A), t-butyldimethylsilyl chloride (B) and allytrimethylsilane (C) are calculated by GIAO, CSGT and IGAIM methods, using HF/6–31G^*, B3LYP/6-31G^*, HF/6–311+G^**, B3LYP/6–311+G^** and MPW1PW91/6–311+G^** models respectively. The ²⁹Si chemical shifts calculated by GIAO method using HF/6–311+G^** model are highly in agreement with those obtained experimentally. All of the models above reproduce the trends of chemical shifts in all cases studied, suggesting that the models are of practical value.  相似文献   

鄂尔多斯盆地西南部上三叠统延长组长8、长6油层组 的沉积体系与物源方向   总被引：14，自引：0，他引：14  

罗静兰  李忠兴  史成恩  李建  韩永林  王海宏  李进步  王成玉 《地质通报》2008,27(1):101-111

野外露头剖面的岩石学与岩相组合,沉积特征与相标志,古流向测定,室内砂岩的骨架矿物、重矿物组合及其平面分布规律的研究结果表明：鄂尔多斯盆地西南部上三叠统延长组长8油层组是以线状或点状物源为特征的一套近源快速堆积的冲积扇与扇三角洲沉积体系,形成于盆地由快速拗陷转入逆冲负荷沉降期间。长8沉积期盆地西南部的古水流与物源主要来自盆地西南方向,其次为西北和东南方向。长6油层组沉积期,盆地内部底床下沉作用减缓,湖盆开始收缩,湖盆西岸除北部有少量扇三角洲沉积外,主要为辫状河三角洲沉积：而盆地东北部与东部的沉积作用大大加强,致使在盆地东部形成一系列大型曲流河三角洲沉积体系,至盆地西南部相变为半深湖-深湖相与浊流相沉积。长6沉积期古水流除来自西南、西北和东南方向外．还有来自北东和正东方向的次要物源,它们在盆地西南部悦乐-玄马-板桥-固城-合水-带汇合,使该地带成为混合物源区。  相似文献   

试析安哥拉卡宾达飞地的跨界族群问题及治理困境     

邓荣秀  刘鸿武 《世界地理研究》2021,30(6):1140-1150

跨界族群以及由此造成的分离主义运动一直是民族学、社会学与国际关系等学科研究的重点议题。本研究对安哥拉卡宾达飞地分离主义运动的研究着重探讨特殊地理位置为安哥拉政府带来的治理困境。对卡宾达分离主义运动的发展历史进行回顾,发现跨界族群问题是卡宾达分离主义运动产生的根源,而飞地属性则从不同角度增加了安哥拉在治理该问题上的难度,主要表现为未能使当地人产生对安哥拉的国家认同,大幅降低了外部干涉难度以及难以满足该地居民的发展诉求。本研究还认为跨界族群的分离主义思潮看似合理,但其文化特殊性并不一定能转化为分离主义运动的政治合理性。虽然非洲国家很多现有边界存在不合理之处,但对它的改变未必能让跨界族群获得更好的社会经济发展。另外,边界对分离主义组织运行模式的影响也是一个值得深入研究的议题。  相似文献   

香港附近海区台风波浪的波群统计分析     

黎满球 《热带海洋学报》1994,(4)

根据香港附近海区１９８５，１９８６年两次台风大浪的实测资料，以有效波高为相应水平，对波群作统计分析，得出可供海洋工程参考的波群连长、波群重复长度、波群出现频率以及波群中最大波高与有效波高的关系等。并将统计值与理论值作比较，结果表明：统计值大于“连”的理论值，而与包线理论中采用合田修正公式的汁算值相近。  相似文献   

Statistics of nonlinear wave crests and groups   总被引：1，自引：0，他引：1  

M. Aziz Tayfun   《Ocean Engineering》2006,33(11-12):1589-1622

Groups of large nonlinear waves with sharper higher crests can pose hazards to ships, induce harbor resonance and cause wave-overtopping of fixed and floating structures. Past interest in wave groups has mostly been focused on the statistics and modeling of linear wave groups. Studies on nonlinear wave groups are surprisingly few, and address deep water waves only. Here, statistics of nonlinear wave crests and wave-crest groups in deep and transitional water depths are considered, using an appropriate second-order representation for crest heights and the continuous wave-envelope approach. In particular, theoretical expressions describing the statistics of nonlinear wave crests and their groups are posed in the form of a simple second-order transformation of well-known results on linear waves. Predictions from the transformation so posed compare well with nonlinear wave data gathered in the North Sea, and demonstrate that nonlinearities do affect the statistics of large wave crests and their groups significantly.  相似文献   

Functional responses for zooplankton feeding on multiple resources: a review of assumptions and biological dynamics     

Wendy Gentleman  Andrew Leising  Bruce Frost  Suzanne Strom  James Murray 《Deep Sea Research Part II: Topical Studies in Oceanography》2003,50(22-26):2847

Modelers often need to quantify the rates at which zooplankton consume a variety of species, size classes and trophic types. Implicit in the equations used to describe the multiple resource functional response (i.e. how nutritional intake varies with resource densities) are assumptions that are not often stated, let alone tested. This is problematic because models are sensitive to the details of these formulations. Here, we enable modelers to make more informed decisions by providing them with a new framework for considering zooplankton feeding on multiple resources. We define a new classification of multiple resource responses that is based on preference, selection and switching, and we develop a set of mathematical diagnostics that elucidate model assumptions. We use these tools to evaluate the assumptions and biological dynamics inherent in published multiple resource responses. These models are shown to simulate different resource preferences, implied single resource responses, changes in intake with changing resource densities, nutritional benefits of generalism, and nutritional costs of selection. Certain formulations are further shown to exhibit anomalous dynamics such as negative switching and sub-optimal feeding. Such varied responses can have vastly different ecological consequences for both zooplankton and their resources; inappropriate choices may incorrectly quantify biologically-mediated fluxes and predict spurious dynamics. We discuss how our classes and diagnostics can help constrain parameters, interpret behaviors, and identify limitations to a formulation's applicability for both regional (e.g. High-Nitrate-Low-Chlorophyll regions comprising large areas of the Pacific) and large-scale applications (e.g. global biogeochemical or climate change models). Strategies for assessing uncertainty and for using the mathematics to guide future experimental investigations are also discussed.  相似文献