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1.
Aprajita Verma Vassilis Charmandaris Ulrich Klaas Dieter Lutz Martin Haas 《Space Science Reviews》2005,119(1-4):355-407
Some of the most ‘active’ galaxies in the Universe are obscured by large quantities of dust and emit a substantial fraction
of their bolometric luminosity in the infrared. Observations of these infrared luminous galaxies with the Infrared Space Observatory
(ISO) have provided a relatively unabsorbed view to the sources fuelling this active emission. The improved sensitivity, spatial
resolution and spectroscopic capability of ISO over its predecessor Infrared Astronomical Satellite (IRAS) of enabled significant
advances in the understanding of the infrared properties of active galaxies. ISO surveyed a wide range of active galaxies
which, in the context of this review, includes those powered by intense bursts of star formation as well as those containing
a dominant active galactic nucleus (AGN). Mid-infrared imaging resolved for the first time the dust enshrouded nuclei in many
nearby galaxies, while a new era in infrared spectroscopy was opened by probing a wealth of atomic, ionic and molecular lines
as well as broad band features in the mid- and far-infrared. This was particularly useful, since it resulted in the understanding
of the power production, excitation and fuelling mechanisms in the nuclei of active galaxies including the intriguing but
so far elusive ultraluminous infrared galaxies. Detailed studies of various classes of AGN and quasars greatly improved our
understanding of the unification scenario. Far-infrared imaging and photometry revealed the presence of a new very cold dust
component in galaxies and furthered our knowledge of the far-infrared properties of faint starbursts, ULIGs and quasars. We
summarise almost nine years of key results based on ISO data spanning the full range of luminosity and type of active galaxies. 相似文献
2.
This paper presents an application of the recent relativistic HLLC approximate Riemann solver by Mignone & Bodo to magnetized
flows with vanishing normal component of the magnetic field. The numerical scheme is validated in two dimensions by investigating
the propagation of axisymmetric jets with toroidal magnetic fields. The selected jet models show that the HLLC solver yields
sharper resolution of contact and shear waves and better convergence properties over the traditional HLL approach. 相似文献
3.
The fastest pulsar and the slowest nova; the oldest galaxies and the youngest stars; the weirdest life forms and the commonest
dwarfs; the highest energy particles and the lowest energy photons. These were some of the extremes of Astrophysics 2006.
We attempt also to bring you updates on things of which there is currently only one (habitable planets, the Sun, and the Universe)
and others of which there are always many, like meteors and molecules, black holes and binaries. 相似文献
4.
T. Sbarrato L. Foschini G. Ghisellini F. Tavecchio 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The γ-ray emission of blazar jets shows a pronounced variability and this feature provides limits to the size and to the speed of the emitting region. We study the γ-ray variability of bright blazars using data from the first 18 months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space Telescope. From the daily light-curves of the blazars characterized by a remarkable activity, we firstly determine the minimum variability time-scale, giving an upper limit for the size of the emitting region of the sources, assumed to be spheroidal blobs in relativistic motion. These regions must be smaller than ∼10−3 parsec. Another interesting time-scale is the duration of the outbursts. We conclude that they cannot correspond to radiation produced by a single blob moving relativistically along the jet, but they are either the signature of emission from a standing shock extracting energy from a modulated jet, or the superposition of a number of flares occurring on a shorter time-scale. We also derive lower limits on the bulk Lorentz factor needed to make the emitting region transparent for gamma-rays interacting through photon–photon collisions. 相似文献
5.
We describe the formation of hot intergalactic gas along with baryonic remnants in galaxy halos. In this scenario, the mass and metallicity of the hot intracluster and intragroup gas relates directly to the production of baryonic remnants during the collapse of galactic halos. We construct a schematic but self-consistent model in which early bursts of star formation lead to a large remnant population in the halo, and to the outflow of stellar ejecta into the halo and ultimately the Local Group. We consider local as well as high redshift constraints on this scenario. This study suggests that the microlensing objects in the Galactic halo may predominantly be 0.5M white dwarfs, assuming that the initial mass function for early star formation favored the formation of intermediate mass stars with m 1M. However, the bulk of the baryonic dark matter in this scenario is associated with the ejecta of the white dwarf progenitors, and resides in the hot intergalactic medium. 相似文献
6.
The Primordial Helium-4 Abundance from Observations of a Large Sample of Blue Compact Dwarf Galaxies
We use a sample of 45 low-metallicity H II regions in blue compact dwarf (BCD) galaxies to determine the primordial helium abundance YP with a precision better than 5%. We have carefully investigated the physical effects which may make the He I line intensities deviate from their recombination values such as collisional and fluorescent enhancements, underlying He I stellar absorption and absorption by Galactic interstellar Na I. By extrapolating the Y vs. O/H and Y vs. N/H linear regressions to O/H = N/H = 0, we obtain YP = 0.244±0.002 and 0.245±0.001, respectively, higher than previous determinations (YP = 0.230 - 0.234). Part of the difference comes from the fact that previous investigators have not taken into account underlying He I stellar absorption, especially in the NW component of the BCD I Zw 18 which, because of its extremely low metallicity plays a key role in the determination of YP. We derive a slope dY/dZ = 2.3±1.0, considerably smaller than those derived before. With this smaller slope and taking into account the errors, chemical evolution models with an outflow of well-mixed material can be built for star-forming dwarf galaxies which satisfy all the observational constraints. Our YP gives bh
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= 0.058±0.007,f consistent with the lower limit set by dynamical measurements and X-ray observations of clusters of galaxies. It is also consistent, within the framework of standard big bang nucleosynthesis theory, with measurements of primordial 7Li in galactic halo stars and with the D/H abundance measured in absorption systems toward quasars by Burles and Tytler (1997). 相似文献
7.
J.S. Kaastra R. Lieu T. Tamura F.B.S. Paerels J.W. den Herder 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(12):2504-2508
We discuss the detection of soft excess X-ray emission in a sample of 19 clusters of galaxies observed by XMM-Newton. In 6/19 clusters evidence for a soft X-ray excess is found. Four of these clusters show soft X-ray and O VII line emission from gas with a temperature of 0.2 keV. The centroid of this oxygen line is consistent with the redshift of the cluster. The intensity and spatial extend of the soft excess agrees with previous PSPC measurements. These observations are interpreted as emission from warm-hot intergalactic medium filaments, with density enhancements near the cluster centers, consistent with theoretical predictions. In the other two soft excess clusters a non-thermal origin is consistent with the data. 相似文献
8.
We have obtained the first UV high resolution spectra of hot luminous stars in M31 with the FOS onHubble Space Telescope. The spectra, combined with optical spectroscopic and photometric observations, enable us to study their stellar winds and photospheric parameters. We derive mass-loss rates and velocity laws from the wind line profiles, with the SEI method, as well as information on abundances. The wind lines and photospheric spectra are compared with galactic stars of the same spectral type.The spectra analyzed so far indicate that the stars have mass-loss rates comparable or slightly lower than galactic stars of the same spectral type, but possibly different velocity laws in their winds. The spectra of two stars are discussed here. 相似文献
9.
Diane Sonya Wong Ryan Chornock Alexei V. Filippenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2813-2815
We present results of optical follow-up observations of candidate ultra-luminous X-ray sources (ULXs). Using Keck optical spectroscopy, 17 of the candidates from the Colbert and Ptak [Colbert, E.J.M., Ptak, A.F. A catalog of candidate intermediate-luminosity X-ray objects. ApJS 143, 25–45, 2002] catalog have been identified; this is one of the largest sets of optical identifications of such objects thus far. Fifteen are background active galactic nuclei (AGN); two are foreground stars in our Galaxy. These findings are consistent with background and foreground object expectations, as derived from log N–log S relations. Also, the results are briefly discussed in terms of the spiral-galaxy/ULX connection. 相似文献
10.
Z.I. Tsvetanov S.A. Zhekov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,3(10-12)
The kinematics of gas clouds in broad-line region of active galactic nuclei and quasars is considered. The motion of the clouds is governed by three forces — gravitational influence from the central supermassive body, radiational pressure from the continuum radiation, and resistance of the intercloud medium. Clouds moves radial but only outward motion gives a velocity field, which is in accordance with the observational data. The profiles of the permited lines are obtained in some simplify assumptions for the emissive capacity of the gas in clouds, and are in good agreement with the observational data. In the framework of the model under consideration there is a possibility to estimate some physical parameters of the nuclei such as mass of the central body and density of the intercloud medium. 相似文献