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261.
262.
Margrethe Wold Mark Lacy Per B. Lilje Stephen Serjeant 《Monthly notices of the Royal Astronomical Society》2000,316(2):267-282
We have observed the galaxy environments around a sample of 21 radio-loud, steep-spectrum quasars at 0.5≤ z ≤0.82, spanning several orders of magnitude in radio luminosity. The observations also include background control fields used to obtain the excess number of galaxies in each quasar field. The galaxy excess was quantified using the spatial galaxy–quasar correlation amplitude, B gq , and an Abell-type measurement, N 0.5 . A few quasars are found in relatively rich clusters, but on average, they seem to prefer galaxy groups or clusters of approximately Abell class 0. We have combined our sample with literature samples extending down to z ≈0.2 and covering the same range in radio luminosity. By using the Spearman statistic to disentangle redshift and luminosity dependences, we detect a weak, but significant, positive correlation between the richness of the quasar environment and the radio luminosity of the quasar. However, we do not find any epoch dependence in B gq , as has previously been reported for radio quasars and galaxies. We discuss the radio luminosity–cluster richness link and possible explanations for the weak correlation that is seen. 相似文献
263.
Per Olof Lindblad 《Astronomy and Astrophysics Review》1999,9(3-4):221-271
Summary. The aim of the present review is to give a global picture of the supergiant barred galaxy NGC 1365. This galaxy with its
strong bar and prominent spiral structure displays a variety of nuclear activity and ongoing star formation.
The kinematics of the galaxy has been mapped in detail by optical long slit and Fabry–Perot observations as well as radio
observations of Hi and CO interstellar lines. From these observations a combined velocity field has been derived, describing the circulation
of interstellar gas in the symmetry plane of the galaxy. With a gravitational potential based on near infrared photometry
of the bar and the shape of the apparent rotation curve, computer simulations of the dynamics of the interstellar gas have
been made with the aim to reproduce both the morphology of the interstellar matter as well as the observed velocity field.
The simulations demonstrate the role of the bar and the importance of resonances between the bar rotation and the rotation
of the galaxy for the formation of the spiral structure. Polarization of radio radiation reveals magnetic fields concentrated
to the dust lanes along and across the bar, where they are aligned with the flow pattern of the gas, and along the spiral
arms. The kinematics of the outer region of the galaxy with a fairly unique decline of the rotation curve leads to the conclusion
that NGC 1365 lacks a very massive dark matter halo, which may permit the formation of a very strong bar.
The galaxy contains an active nucleus with both broad and narrow components of the permitted spectral emission lines. The
nucleus is surrounded by a molecular torus, numerous star forming regions and continuum radio sources. The star forming regions
are, as seen with the Hubble Space Telescope (HST), resolved into a large number of super star clusters suggested to be young
globular clusters. A very compact radio source, seen at high spatial resolution with the Very Large Array (VLA), has been
claimed to coincide with one of the super star clusters. This compact source has a radio brightness of the order of 100 times
that of the bright galactic supernova remnant Cas A and is suggested to be a so called ‘radio supernova’. Two other such compact
radio sources, positioned in the prominent dark dust lane penetrating the nuclear region, are identified as strong infrared
sources by observations with the Very Large Telescope (VLT). The cause of this infrared radiation may be dust heated by the
objects that drive the radio sources.
The X-ray radiation from the nucleus is interpreted to consist of hard continuum radiation from the active nucleus itself,
Fe-K line emission from a rotating disk, and thermal emission from the surrounding star burst activity. A secondary, highly
variable source has been discovered close to the nuclear region. It seems to be one of the most luminous and most highly variable
off-nuclear X-ray sources known.
The higher excitation optical emission lines in the nuclear region, primarily from [Oiii], reveal a velocity field quite different from that described by the galactic rotation. The deviating [Oiii] morphology and velocity field in the nuclear region is interpreted in terms of a high excitation outflow double-cone with
its apex at the nucleus and symmetry axis perpendicular to the symmetry plane of the galaxy. One of the circumnuclear radio
sources seems to be a one-sided jet emerging from the nucleus aligned with the cone axis. According to the model, the outward
flow within the cone is accelerated and the flow velocity highest at the cone axis.
Received 15 January 1999 相似文献
264.
We examine how the stable isotope composition of meteoric water is transmitted through soil and epikarst to dripwaters in a cave in western Romania. δ2H and δ18O in precipitation at this site are influenced by temperature and moisture sources (Atlantic and Mediterranean), with lower δ18O in winter and higher in summer. The stable isotope composition of cave dripwaters mimics this seasonal pattern of low and high δ18O, but the onset and end of freezing conditions in the winter season are marked by sharp transitions in the isotopic signature of cave dripwaters of approximately 1 ‰. We interpret these shifts as the result of kinetic isotopic fractionation during the transition phase from water to ice at the onset of freezing conditions and the input of meltwater to the cave at the beginning of the spring season. This process is captured in dripwaters and therefore speleothems from Ur?ilor Cave, which grew under such dripping points, may have the potential to record past changes in the severity of winters. Similar isotopic changes in dripwaters driven by freeze–thaw processes can affect other caves in areas with winter snow cover, and cave monitoring during such changes is essential in linking the isotopic variability in dripwaters and speleothems to surface climate. 相似文献
265.
Kenneth Bredesen Per Avseth Tor Arne Johansen Richard Olstad 《Geophysical Prospecting》2019,67(4):825-842
Understanding how physical properties and seismic signatures of present day rocks are related to ancient geological processes is important for enhanced reservoir characterization. In this paper, we have studied this relationship for the Kobbe Formation sandstone in the Barents Sea. These rocks show anomalous low shear velocities and high VP/VS ratios, which does not agree well with conventional rock physics models for moderately to well consolidated sandstones. These sandstones have been buried relatively deeply and subsequently uplifted 1–2 km. We compared well log data of the Kobbe sandstone with velocity–depth trends modelled by integrating basin modelling principles and rock physics. We found that more accurate velocity predictions were obtained when first honouring mechanical and chemical compaction during burial, followed by generation of micro-cracks during uplift. We suspect that these micro-cracks are formed as overburden is eroded, leading to changes in the subsurface stress-field. Moreover, the Kobbe Formation is typically heterogeneous and characterized by structural clays and mica that can reduce the rigidity of grain contacts. By accounting for depositional and burial history, our velocity predictions become more consistent with geophysical observables. Our approach yields more robust velocity predictions, which are important in prospect risking and net erosion estimates. 相似文献
266.
Kristoffer T. Nielsen Per Moldrup Sren Thorndahl Jesper E. Nielsen Lene B. Duus Sren H. Rasmussen Mads Uggerby Michael R. Rasmussen 《水文研究》2019,33(26):3364-3377
Rainfall simulators can enhance our understanding of the hydrologic processes affecting the total runoff to urban drainage systems. This knowledge can be used to improve urban drainage designs. In this study, a rainfall simulator is developed to simulate rainfall on urban green surfaces. The rainfall simulator is controlled by a microcomputer programmed to replicate the temporal variations in rainfall intensity of both historical and synthetic rainfall events with constant rainfall intensity on an area of 1 m2. The performance of the rainfall simulator is tested under laboratory conditions with regard to spatial uniformity of the rainfall, the kinetic energy of the raindrops, and the ability to replicate historical and synthetic rainfall events with temporally varying intensity. The rainfall simulator is applied in the field to evaluate its functionality under field conditions and the influence of wind on simulated rainfall. Finally, a field study is carried out on the relationship between runoff, soil volumetric water content, and surface slope. Performance and field tests show that the simulated rainfall has a uniform spatial distribution, whereas the kinetic energy of the raindrops is slightly higher than that of other comparable rainfall simulators. The rainfall simulator performs best in low wind speed conditions. The simulator performs well in replicating historical and synthetic rainfall events by matching both intensity variations and accumulated rainfall depth. The field study shows good correlation between rainfall, runoff, infiltration, soil water content, and surface slope. 相似文献
267.
In this paper, a time series from 1999 to 2007 of absolute total electron content (TEC) values has been computed and analyzed
using singular value decomposition (SVD). The data set has been computed using a Kalman Filter and is based on dual frequency
GPS data from three reference stations in Denmark located in the midlatitude region. The station separation between the three
stations is 132–208 km (the time series of the TEC can be freely downloaded at ). For each year, a SVD has been performed on the TEC time series in order to identify the three time varying (daily, yearly,
and 11 yearly) characteristics of the ionosphere. The applied SVD analysis provides a new method for separating the daily
from the yearly components. The first singular value is very dominant (approximately six times larger than the second singular
value), and this singular value corresponds clearly to the variation of the daily cycle over the year. The second singular
value corresponds to variations of the width of the daily peak over the year, and the third singular value shows a clear yearly
variation of the daily signal with peaks around the equinoxes. The singular values for each year show a very strong correlation
with the sunspot number for all the singular values. The correlation coefficients for the first 5 sets of singular values
are all above 0.96. Based on the SVD analysis yearly models of the TEC in the ionosphere can be recomposed and illustrate
the three time varying characteristics of the ionosphere very clearly. By prediction of the yearly mean sunspot number, future
yearly models can also be predicted. These can serve as a priori information for a real time space weather service providing
information of the current status of the ionosphere. They will improve the Kalman filter processing making it more robust,
but can also be used as starting values in the initialization phase in case of gaps in the data stream. Furthermore, the models
can be used to detect variations from the normal local ionospheric activity. 相似文献