The Universe behind the Milky Way

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     

Massive star formation in galaxies: radiative transfer models of the UV to millimetre emission of starburst galaxies

We present illustrative models for the UV to millimetre emission of starburst galaxies which are treated as an ensemble of optically thick giant molecular clouds (GMCs) centrally illuminated by recently formed stars. The models follow the evolution of the GMCs owing to the ionization-induced expansion of the H  ii regions and the evolution of the stellar population within the GMC according to the Bruzual & Charlot stellar population synthesis models. The effect of transiently heated dust grains/PAHs on the radiative transfer, as well as multiple scattering, is taken into account.
The expansion of the H  ii regions and the formation of a narrow neutral shell naturally explain why the emission from PAHs dominates over that from hot dust in the near- to mid-infrared, an emerging characteristic of the infrared spectra of starburst galaxies.
The models allow us to relate the observed properties of a galaxy to its age and star formation history. We find that exponentially decaying 10⁷–10⁸ yr old bursts can explain the IRAS colours of starburst galaxies. The models are also shown to account satisfactorily for the multiwavelength data on the prototypical starburst galaxy M82 and NGC 6090, a starburst galaxy recently observed by ISO . In M82 we find evidence for two bursts separated by 10⁷ yr. In NGC 6090 we find that at least part of the far-infrared excess may be due to the age of the burst (6.4×10⁷ yr). We also make predictions about the evolution of the luminosity of starbursts at different wavelengths which indicate that far-infrared surveys may preferentially detect older starbursts than mid-infrared surveys.     

Optical properties of the carbon dust grains in the envelopes around asymptotic giant branch stars

We have investigated the optical properties of the carbon dust grains in the envelopes around carbon-rich asymptotic giant branch stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including IRAS Point Source Catalog and IRAS Low Resolution Spectrograph data. We have deduced an opacity function of amorphous carbon dust grains from model fitting with infrared carbon stars. From the opacity function, we have derived the optical constants of the AMC grains. The optical constants satisfy the Kramers–Kronig relation and produce the opacity function that fits the observations of infrared carbon stars better than previous works in the wide wavelength range 1–1000 μm. We have used simple mixtures of the AMC and silicon carbide grains for modelling. We have compared the contributions that AMC and SiC grains make to the opacity for the cases of simple mixtures of them and spherical core–mantle type grains consisting of a SiC core and an AMC mantle .     

基于连续消光特征分析的岩石薄片分析系统

连续消光是指在正交偏光显微镜下,岩石薄片随载物台旋转而发生消光强弱连续变化的现象.作为人工鉴定岩石薄片的重要依据,提取后的连续消光特征可以用于实现岩石薄片的自动分析.结合数字图像处理技术与聚类划分算法,开发出一套能够分割出孔隙、颗粒轮廓,实现颗粒类别划分,并对结果做定量分析的岩石薄片分析系统.从矿物的消光特征本质出发,...     

激光雷达反演参数k值的研究

采用532 nm激光雷达2006年在兰州市不同天气状况下的观测数据,结合能见度因子、CE318太阳光度计观测数据,对利用Klett法求解时,k在不同天气状况下的取值进行了初步研究,结果表明:只有当0.7≤k≤1.0时,气溶胶消光系数的大小与能见度估算出的值相接近。但在k=0.7时,计算出的气溶胶消光系数正、负参半;k=1.0时,消光系数廓线在晴天无云的天气状况下同实际情况不符。通过进一步分析研究激光雷达和光度计的同期观测资料发现:k=0.8时,较合理的数据所占比例为100%,k=0.9为84%,k=1.0仅为12%。对兰州市区而言,利用激光雷达分析气溶胶光学特性时,取k=0.8或k=0.9较为合理。     

牙形石微量元素对生物绝灭事件的响应: 以二叠-三叠系全球层型剖面第一幕绝灭事件为例

首次系统地利用浙江长兴煤山剖面牙形石化石, 依托中国地质大学(武汉) 地质过程与矿产资源国家重点实验室激光剥蚀等离子体质谱仪(LA-ICP-MS) 获取牙形石微区原位元素含量信息.结果显示, 牙形石的Ce异常和稀土总量出现快速的波动, 它们均反映出P/T之交环境(氧化-还原、生态条件等) 的不稳定性.这些微量元素特征可以很好地与古海洋环境和生物绝灭事件耦合.这一研究将为系统探索全球古生代末生物大灭绝及其后生物复苏的过程、时限与古海洋化学及古生态变化性质等问题的研究提供新的途径, 并有望对这些重要科学问题提供有效的制约.      

Fractal Analysis of the Complexity of United States Coastlines

Coastlines have long been used as a principal example of a natural feature that exhibits fractal structure. With the advent of large digitized databases, it has become possible to examine in detail large regions of coast and to examine differences in complexity, as measured by the fractal dimension, among regions. In this study, we have determined the fractal dimension of the Atlantic and Pacific coastlines of the conterminous United States. The traditional divider method was used in obtaining the fractal dimension of each coastline arc from the NOAA Medium-Resolution Shoreline Data Set. On average, the Atlantic coast has much higher fractal dimension than the Pacific coast. The results also indicate that the complexity of the Atlantic coast increases toward low latitudes. These results have implications for the interpretation of species distributions and diversity patterns along the coast and for the understanding of the dynamics of biotic recovery from mass extinctions.     

泥盆系弗拉阶/法门阶之交事件沉积和海平面变化

在华南板块南部被动大陆边缘和西伯利亚板块南部活动大陆边缘弗拉阶/法门阶之交识别出7种物理事件相: 细粒钙屑浊积岩相、钙质角砾岩相、软变形灰岩相、含微球粒的粒泥灰岩相、凝灰质砾岩相、黑色页岩相和火山喷溢相.它们在法门阶下Palmatolepis triangularis带底部和/或上Palmatolepis rhenana带底部具有广泛的分布, 可能是两次陨击事件记录, 是良好的高分辨率年代地层对比标志.弗拉期/法门期之交的海平面变化不具有同步性和一致性, 可分辨出2种型式(阶跃型和渐进型) 和5种状态(弗拉期最末期下降→法门期最初期上升、弗拉期最末期上升→法门期最初期下降、弗拉期最末期上升→法门期最初期上升、弗拉期最末期下降→法门期最初期下降、弗拉期最末期与法门期最初期海平面变化不明显).弗拉阶/法门阶之交生物的阶梯状绝灭可能与从上Palmatolepis rhenana带底部至Palmatolepis crepida带不均匀分布的6次陨击事件密切相关.      

降水现象对大气消光系数和能见度的影响

大气中各种粒子对大气消光系数和能见度有不同程度影响,除气溶胶粒子外,降水粒子对能见度影响也不可忽视。为了解降水粒子对能见度的影响,确定能见度变化与降水现象之间的关系,该文在分析降水粒子的大小、速度、形状、谱分布、光学特性等特征的基础上,忽略气溶胶粒子的影响,建立基于实测谱分布的降水与能见度的理论模型,讨论不同类型降雨、降雪对大气消光系数和能见度的影响。同时选取Parsivel降水粒子谱仪在南京地区的降雨和降雪观测记录,利用实测数据来对比验证本文所建立的降水-能见度理论模型。结果表明:能见度随着降水强度的增大呈指数降低;受降水粒子特性和天气条件等多种因素影响,能见度与降水强度之间的关系并不是唯一对应的;降雨和降雪对能见度的影响各不相同,相比而言,降雨对能见度的影响比较容易确定,而降雪对能见度的影响比较复杂,主要因为雪花或冰晶的类型复杂多变,对大气消光系数有不同程度的影响。结合理论分析和实测数据对比验证,降水现象对能见度的影响得到了证实。