Colliding and merging galaxies

Aided by advances in computer technology and observations from space, astronomers have begun to unravel the mysteries of galaxy formation and evolution. Galaxies evolve by interacting with their environment and especially with each other. During brief but often fierce galactic encounters, gravitational forces generate strong tides that survive as telltale signatures for billions of years. Because these so-called collisions dissipate orbital energy, galaxies on bound orbits may eventually merge. Collisions and mergers are responsible for a great variety of phenomena, including the triggering of widespread star formation in galaxies and the fueling of nuclear activity in quasars. Evidence is accumulating that not all galaxies formed shortly after the Big Bang. A sizable fraction of them may have formed later, and many are still experiencing significant dynamical evolution.     

Dynamics of elliptical galaxies

Elliptical galaxies were once thought to be similar in their structure and dynamics to rotationally flattened bodies like stars. The discovery that elliptical galaxies rotate much more slowly than a fluid body with the same shape has led to a qualitative change in our understanding of the dynamics of these systems. It is now believed that elliptical galaxies are fully triaxial in shape. Self-consistent triaxial equilibria have been constructed and appear to be long-lived; they are made possible by the existence of conserved quantities, or integrals of motion, for galactic potentials without rotational symmetry. Many self-consistent equilibria are unstable; the nonexistence of elliptical galaxies with axis ratios more extreme than 3:1 is probably the result of such an instability. There is evidence for strong central mass concentrations, perhaps massive black holes, at the centers of some nearby galaxies. Recent observations suggest that many elliptical galaxies formed through the merger of two or more spiral galaxies.     

Percolation of core melts at lower mantle conditions

Experiments at high pressure and temperature to determine the dihedral angle of core melts in lower mantle phases yielded a value of approximately 71 degrees for perovskite-dominated matrices. This angle, although greater than the 60 degrees required for completely efficient percolation, is considerably less than the angles observed in mineral matrices at upper mantle pressure-temperature conditions in experiments. In other words, molten iron alloy can flow much more easily in lower mantle mineralogies than in upper mantle mineralogies. Accordingly, although segregation of core material by melt percolation is probably not feasible in the upper mantle, core formation by percolation may be possible in the lower mantle.     

地榆鞣质的渗滤提取工艺研究

[目的]筛选地榆鞣质渗滤提取的工艺条件。[方法]以地榆鞣质含量及浸膏得率作为评价指标,以没食子酸为对照品,用紫外分光光度法测定地榆鞣质的含量,并对药材粒度、溶剂用量、渗漉速度和径高比等因素条件进行优化,优选出地榆鞣质渗滤提取工艺的最佳工艺条件。[结果]最佳提取条件为:地榆药材粉碎并过1号筛后,装入渗滤桶(径高比为1∶1),料液比为1∶6(g/ml),乙醇浓度为70%,渗滤速度为5 ml/min.kg;在此条件下,得到的平均收膏率为30.48%,鞣质提取率为15.5%。[结论]该工艺合理可行,重复性好,地榆鞣质的提取率高,可为制备地榆鞣质半成品提供依据。     

The most distant known galaxies

Ever since the proposal of the idea of an expanding universe more than 50 years ago, each generation of investigators has found that some current theory could be (marginally) tested by the properties of the most distant known galaxies. There has consequently been a continuing effort to identify very remote objects, especially to confront theories of the evolution of galaxies (since galaxies are seen as they were at prior epochs) and to confront cosmological theories (which make predictions about the overall dynamics of the expansion of the universe). These theories have yet to be definitively tested, but a new generation of optical telescopes and detectors provides hope for significant progress during this decade.     

The morphological evolution of galaxies

Many galaxies have taken on their familiar appearance relatively recently. In the distant Universe, galaxy morphology deviates significantly (and systematically) from that of nearby galaxies at redshifts (z) as low as 0.3. This corresponds to a time approximately 3.5 x 10(9) years in the past, which is only approximately 25% of the present age of the Universe. Beyond z = 0.5 (5 x 10(9) years in the past), spiral arms are less well developed and more chaotic, and barred spiral galaxies may become rarer. At z = 1, around 30% of the galaxy population is sufficiently peculiar that classification on Hubble's traditional "tuning fork" system is meaningless. On the other hand, some characteristics of galaxies have not changed much over time. The space density of luminous disk galaxies has not changed significantly since z = 1, indicating that although the general appearance of these galaxies has continuously changed over time, their overall numbers have been conserved.     

Dwarfs and dim galaxies mark limits of knowledge

When 250 astronomers met in Cardiff early this month at the U.K. National Astronomy Meeting, sponsored by the Royal Astronomical Society and the Particle Physics and Astronomy Research Council, they grappled with some big problems, among them the age of the universe and its mass. They also took on an even bigger metaproblem: How confident can they be in their knowledge about cosmic origins?     

渗灌在日光温室樱桃番茄上的应用效果研究

以樱桃番茄为试材,在日光温室内研究了渗灌对樱桃番茄生育期、产量、品质和用水量的影响.结果表明:渗灌可有效提高樱桃番茄产量和改善果实品质,并降低用水量.渗灌条件下,樱桃番茄提前上市5 d,用水量比膜下灌溉少10.96%,水分生产率提高5.61 kg/m3;产量提高2878.5 kg/hm2,平均单果重提高1.5989 g,可溶性固形物含量提高0.365%.