共查询到20条相似文献，搜索用时 93 毫秒
1.
Summary. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review
we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial r?le in the
cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity
can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve
many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields,
outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor
galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and
the physical processes that are at work in these objects. The question on how they are related and may evolve from one class
of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion
of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe,
we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in
the universe. We discuss what do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance
of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator
and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly emission.
Received 19 August 1999 / Published online: 15 February 2000 相似文献
2.
M.-H. Ulrich 《Astronomy and Astrophysics Review》2000,10(1-2):135-178
Summary. The Seyfert galaxy NGC 4151 harbors in its nucleus the most intensively studied AGN (Active Galactic Nucleus). Among the
brightest AGN (in apparent luminosity) it is the most widely variable and the variations of its ultraviolet and X-ray spectrum
have been studied on time scales ranging from hours to decades. These observations have formed the basis of methods and models
which have been found to generally apply to broad emission line AGN: the rich and complex relation between the X-ray and UV
variations, the comptonization model of the X-ray spectrum from medium X-ray to -rays, the reverberation mapping, the stratification in velocity and physical conditions of the gas in the broad line region,
and a method to estimate the black hole mass from emission line variability.
The large barred spiral which hosts this nucleus has been extensively studied especially in the central region. Inflow of
gas along the and possibly also the orbits have been detected, but since the accretion disk is not in the galactic plane (as evidenced by the significant angle
separating the radio axis and the rotation axis of the galaxy) the incoming gas seen on kpcs scale must, as it flows further
inward, move out of the galactic plane, along trajectories which are entirely unknown.
There is more to learn on NGC 4151. In fact, the best is yet to come. Three avenues of investigation appear particularly promising:
1) The variations in flux and spectral shape of the X-ray continuum and its relationship with the UV variations are the key
to understanding the specifics of the Comptonization model. Progress on this point will come from repeated simultaneous observations
of the UV spectrum and of the entire X-ray and -ray spectrum. This will also give insights on the structure of the disk in the last stable orbits, the formation and structure
of the corona and in the end, the process of energy production. Exciting results on these topics are expected in the near
future from Chandra-AXAF, XMM and INTEGRAL. The Chandra and XMM (which have short energy range) main contributions will, however,
be line diagnostics and for Chandra, imaging of the soft diffuse emission.
2) The search for the gas inflow which merges into and/or forms the torus could finally be successful. Several powerful approaches
are possible: observing molecular lines in emission with millimeter arrays of increasing baseline and collecting area; using
the nuclear radio structure as background source to observe free-free and atomic or molecular lines in absorption.
3) The observations of NGC 4151 during a state of deep minimum will provide a unique oportunity to observe the X-ray spectrum
of a Seyfert 1 nucleus at epochs of very low accretion rate, to identify the nature of the narrow variable lines, to determine
the stellar population of a currently active nucleus, and measure the mass of the black hole from the stellar lines. NGC 4151
at minimum states should be a target of opportunity for all space missions. In addition, observations on time scales of 10
years or more, especially following a deep minimum, will allow one to map emitting regions of size up to 1pc, thereby overlapping with the linear scale directly mapped with large radio telescopes.
Received 30 October 1999 / Published online: 24 March 2000 相似文献
3.
Sidney van den Bergh 《Astronomy and Astrophysics Review》1999,9(3-4):273-318
Summary. Hubble's (1936, p. 125) view that the Local Group (LG) is “a typical, small group of nebulae which is isolated in the general
field” is confirmed by modern data. The total number of certain and probable Group members presently stands at 35. The half-mass
radius of the Local Group is found to be kpc. The zero-velocity surface, which separates the Local Group from the field that is expanding with the Hubble flow, has
a radius Mpc. The total mass of the LG is . Most of this mass appears to be concentrated in the Andromeda and Milky Way subgroups of the LG. The total luminosity of
the Local Group is found to be :. This yields a mass-to-light ratio (in solar units) of . The solar motion with respect to the LG is \,km s, directed towards an apex at , and . The velocity dispersion within the LG is km s. The galaxies NGC 3109, Antlia, Sextans A and Sextans B appear to form a distinct grouping with kpc relative to the LG, that is located beyond the LG zero-velocity surface at a distance of 1.7 Mpc from the Local Group
centroid. The luminosity distribution of the LG has a slope . This value is significantly less negative than that which is found in rich clusters of galaxies. The luminosity distribution
of the dwarf spheroidal galaxies is steeper than that for dwarf irregulars. Furthermore the dSph galaxies are strongly concentrated
within the Andromeda and Milky Way subclusters of the Local Group, whereas the majority of dIr galaxies appear to be free-floating
members of the LG as a whole. With the possible exception of Leo I and Leo A, most LG members appear to have started forming
stars simultaneously Gyr ago. Many of the galaxies, for which evolutionary data are available, appear to have shrunk with time. This result is
unexpected because Hubble Space Telescope observations appear to show galaxies at to be smaller than they are at . In the Large Magellanic Cloud the rate of cluster formation was low for a period that extended from Gyr to Gyr ago. The rate of cluster formation may have increased more rapidly 3–5 Gyr ago, than did the rate of star formation.
The reason for the sudden burst of cluster formation in the LMC Gyr ago remains obscure. None of the dwarf galaxies in the LG appears to have experienced a starburst strong enough to have
produced a “boojum”.
Received 14 April 1999 相似文献
4.
F.P. Israel 《Astronomy and Astrophysics Review》1998,8(4):237-278
Summary. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be
the prototype Fanaroff-Riley Class I ‘low-luminosity’ radio galaxy, and as such it plays an important role in our understanding
of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding
unrivalled but still incomplete knowledge of its structure and dynamics.
The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and
molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical
images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are
generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past.
The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths,
probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption
spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into
the nucleus, presumably ‘feeding the monster’. Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic
at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond
the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although
at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected
to the circumnuclear disk, appears to precess on timescales of order a few times 10 years.
This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying
physical processes are outside its scope: they are briefly referred to, but not discussed.
Received 30 December 1997 相似文献
5.
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
6.
Summary. Recent papers dealing with the most controversial aspects of AGNs are reviewed. They suggest interesting conclusions: all
Seyferts can be described by a single parameter, the X-ray column density; radio loud AGNs may host a rapidly spinning black
hole and radio quiet AGNs a slowly spinning black hole; high-ionization AGNs (Seyfert galaxies and QSOs) contain an optically
thick, geometrically thin accretion disk, while low-ionization AGNs (Liners) contain an optically thin, geometrically thick
accretion disk; a number of blazars have been classified as BLLs on the basis of insufficient data; most objects with weak
broad emission lines are in fact HPQs; many objects have been called Liners although they are not AGNs but rather the result
of stellar activity; type 2 QSOs exist, but are quite inconspicuous if radio quiet.
Received 16 November 1999 / Published online: 15 February 2000 相似文献
7.
We present results of an investigation of clustering evolution of field galaxies between a redshift of z ∼ 1 and the present epoch. The current analysis relies on a sample of ∼ 14000 galaxies in two fields of the COMBO 17 survey.
The redshift distribution extends to z ∼ 1. The amplitude of the three-dimensional correlation function can be estimated by means of the projected correlation function
w(r
p
). The validity of the deprojection was tested on the Las Campanas Redshift Survey (LCRS). In a flat cosmology with non-zero
cosmological constant for bright galaxies (M
B ≤-18) the clustering growth is proportional to (1+z)
-2. However, the measured clustering evolution clearly depends on Hubble type. While locally the clustering strength of early
type galaxies is equal to that of the bright galaxies, at high redshifts they are much stronger clustered, and thus the clustering
has to evolve much more slowly.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
8.
X-ray spectroscopy offers an opportunity to study the complex mixture of emitting and absorbing components in the circumnuclear
regions of active galactic nuclei (AGN), and to learn about the accretion process that fuels AGN and the feedback of material
to their host galaxies. We describe the spectral signatures that may be studied and review the X-ray spectra and spectral
variability of active galaxies, concentrating on progress from recent Chandra, XMM-Newton and Suzaku data for local type 1 AGN. We describe the evidence for absorption covering a wide range of column densities, ionization
and dynamics, and discuss the growing evidence for partial-covering absorption from data at energies ≳ 10 keV. Such absorption
can also explain the observed X-ray spectral curvature and variability in AGN at lower energies and is likely an important
factor in shaping the observed properties of this class of source. Consideration of self-consistent models for local AGN indicates
that X-ray spectra likely comprise a combination of absorption and reflection effects from material originating within a few
light days of the black hole as well as on larger scales. It is likely that AGN X-ray spectra may be strongly affected by
the presence of disk-wind outflows that are expected in systems with high accretion rates, and we describe models that attempt
to predict the effects of radiative transfer through such winds, and discuss the prospects for new data to test and address
these ideas. 相似文献
9.
Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved ‘star-like’ objects with red-shifts
appropriate to the Fornax-Cluster. These objects have intrinsic sizes of ≈ 100 pc and absolute B-band magnitudes in the range
- 14 < MB < -11.5 mag and lower limits for the central surface brightness μB ≥ 23 mag/arcsec2 (Phillipps et al., 2001, Hilker et al., 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies
(UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star clusters;
not to confuse with super stellar clusters (SSC)) by P. Kroupa (1998), which are rich aggregates of young massive star clusters
(YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field.
The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as Knot
S in the interacting Antennae galaxies (Whitmore et al., 1999) evolve to merger objects that are long-lived and show properties
comparable to the newly discovered UCDs.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
10.
Summary. Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar
systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe.
In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation
from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize
the tremendous developments, as much theoretical as observational, that have taken place during the last two decades, and
which have led to a quantum jump in our understanding of these beautiful dynamical systems.
Received 3 August 1996 相似文献
11.
The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This
phenomenon had been predicted some 70 years earlier as a consequence of General Relativity. We present here a summary of what
we have learnt since. The applications are so numerous that we had to concentrate on a few selected aspects of this new field
of research.
This review is focused on strong gravitational lensing, i.e. the formation of multiple images, in QSO samples. It is intended
to give the reader an up-to-date status of the observations and to present an overview of its most interesting potential applications
in cosmology and astrophysics, as well as numerous important results achieved so far.
The first section follows an intuitive approach to the basics of gravitational lensing and is developed in view of our interest
in multiply imaged quasars. The astrophysical and cosmological applications of gravitational lensing are outlined in Sect.
2 and the most important results are presented in Sect. 5. Sections 3 and 4 are devoted to the observations. Finally, conclusions
are summarized in the last section.
We have tried to avoid duplication with existing (and excellent) introductions to the field of gravitational lensing. For
this reason, we did not concentrate on the individual properties of specific lens models, as these are already well presented
in Narayan and Bartelmann (1996) and on a more intuitive ground in Refsdal and Surdej (1994). Wambsganss (1998) proposes a
broad view on gravitational lensing in astronomy; the reviews by Fort and Mellier (1994) and Hattori et al. (1999) deal with
lensing by galaxy clusters; microlensing in the Galaxy and the local group is reviewed by Paczyński (1996) and a general panorama
on weak lensing is given by Bartelmann and Schneider (1999) and Mellier (1999). The monograph on the theory of gravitational
lensing by Schneider, Ehlers and Falco (1992) also remains a reference in the field.
Received 4 April 2000 / Published online 9 August 2000 相似文献
12.
We discuss the implications of the recent X-ray and TeV γ-ray observations of the PSR B1259–63 system (a young rotation powered pulsar orbiting a Be star) for the theoretical models
of interaction of pulsar and stellar winds. We show that previously considered models have problems to account for the observed
behaviour of the system. We develop a model in which the broad band emission from the binary system is produced in result
of collisions of GeV–TeV energy protons accelerated by the pulsar wind and interacting with the stellar disk. In this model
the high energy γ-rays are produced in the decays of secondary neutral pions, while radio and X-ray emission are synchrotron and inverse Compton
emission produced by low-energy (≤100 MeV) electrons from the decays of secondary charged π
± mesons. This model can explain not only the observed energy spectra, but also the correlations between TeV, X-ray and radio
emission components.
相似文献
13.
Observations of active galactic nuclei imply that shocks must be an essential and important part of their structure. We outline the basic observations, and discuss those features which must be addressed by any physical model of active nuclei. These features, in particular the observed spectrum of strong emission and absorption lines, lead naturally to the conclusion that shocks are present. The velocity widths of these lines, which range from hundreds to many thousands of kilometres per second, are most readily explained by models in which shocks play an important role in the generation of cool gas. The extreme parameters of the shocks in and around active nuclei provide a unique application for the physics discussed in this meeting. 相似文献
14.
We determine the mass profile of a synthetic cluster built from the combination of 59 nearby clusters observed in the ESO
Nearby Abell Cluster Survey (ENACS). We use ellipticals and S0s as tracers of the cluster potential, and solve the Jeans equation
assuming isotropic orbits. Such an assumption is justified by the analysis of the shape of the velocity distribution of ellipticals
and S0s. We find that the cluster mass profile is consistent with the Navarro, Frenk and White(NFW) model. We use this cluster
mass profile to search for equilibrium solutions for the other cluster galaxy populations: very bright ellipticals (M
R
≤–22+5 log h),early-type spirals (Sa-Sb), and late-type spirals and irregulars (Sbc-Ir), together with emission-line galaxies. We find
equilibrium solutions for both the early- and the late-spirals, but not for the very bright ellipticals. The dynamics of very
bright ellipticals is probably affected by dissipative processes which invalidate the use of the collisionless Jeans equation.
The equilibrium solution found for the early-spirals implies them to move on nearly-isotropic orbits. Late-spirals are instead
found to be on mildly radial orbits, with the radial anisotropy increasing outwards. We discuss the implications of these
results for the evolutionary histories of the different populations of cluster galaxies.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
We describe the present state of massive star research seen from the viewpoint of stellar evolution, with special emphasis
on close binaries. Statistics of massive close binaries are reasonably complete for the Solar neighbourhood. We defend the
thesis that within our knowledge, many scientific results where the effects of binaries are not included, have an academic
value, but may be far from reality. In chapter I, we summarize general observations of massive stars where we focus on the
HR diagram, stellar wind mass loss rates, the stellar surface chemistry, rotation, circumstellar environments, supernovae.
Close binaries can not be studied separately from single stars and vice versa. First, the evolution of single stars is discussed
(chapter I). We refer to new calculations with updated stellar wind mass loss rate formalisms and conclusions are proposed
resulting from a comparison with representative observations. Massive binaries are considered in chapter II. Basic processes
are briefly described, i.e. the Roche lobe overflow and mass transfer, the common envelope process, the spiral-in process
in binaries with extreme mass ratio, the effects of mass accretion and the merging process, the implications of the (asymmetric)
supernova explosion of one of the components on the orbital parameters of the binary. Evolutionary computations of interacting
close binaries are discussed and general conclusions are drawn. The enormous amount of observational data of massive binaries
is summarized. We separately consider the non-evolved and evolved systems. The latter class includes the semi-detached and
contact binaries, the WR binaries, the X-ray binaries, the runaways, the single and binary pulsars. A general comparison between
theoretical evolution and observations is combined with a discussion of specially interesting binaries: the evolved binaries
HD 163181, HD 12323, HD 14633, HD 193516, HD 25638, HD 209481, Per and Sgr; the WR+OB binary V444 Cyg; the high mass X-ray binaries Vela X-1, Wray 977, Cyg X-1; the low mass X-ray binaries Her
X-1 and those with a black hole candidate; the runaway Pup, the WR+compact companion candidates Cyg X-3, HD 50896 and HD 197406. We finally propose an overall evolutionary model
of massive close binaries as a function of primary mass, mass ratio and orbital period. Chapter III deals with massive star
population synthesis with a realistic population of binaries. We discuss the massive close binary frequency, mass ratio and
period distribution, the observations that allow to constrain possible asymmetries during the supernova explosion of a massive
star. We focuss on the comparison between observed star numbers (as a function of metallicity) and theoretically predicted
numbers of stellar populations in regions of continuous star formation and in starburst regions. Special attention is given
to the O-type star/WR star/red supergiant star population, the pulsar and binary pulsar population, the supernova rates.
Received 17 July 1998 相似文献
16.
17.
We discuss the contribution of the blazar population to the extragalactic background radiation across the electromagnetic (e.m.) spectrum with particular reference to the microwave, hard-X-ray and γ-ray bands. Our estimates are based on a recently derived blazar radio LogN-LogS that was built by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that dominates the extragalactic brightness in the high-energy part of the e.m. spectrum. We also estimate the number of blazars that are expected to be detected by future planned or hypothetical missions operating in the X-ray and γ-ray energy bands. 相似文献
18.
Peter G. Mezger Wolfgang J. Duschl Robert Zylka 《Astronomy and Astrophysics Review》1996,7(4):289-388
Summary. This paper reviews the physical state of stars and Interstellar Matter in the Galactic Bulge (radius kpc from the dynamical center of the Galaxy), in the Nuclear Bulge (kpc) and in the Sgr A Radio and GMC Complex, i.e. the central \,pc of our Galaxy. The Galactic Bulge is devoid of cold Interstellar Matter and consists mainly of old stars, while the Nuclear
Bulge accounts for of the mass of all of the Interstellar Matter in the Galaxy. A similar ratio holds for the formation rate of medium and high
mass stars in Bulge and Disk. The metal abundance of the Interstellar Matter in the Galactic Bulge is found to be . The H-to-CO conversion factors to be applied to molecular gas in the Central Region are by factors 3 (Arimoto et al. 1996) to 10 (Sodroski
et al. 1995) lower than in the solar vicinity. Hence, most H masses derived for the Central Region appear to be considerably overestimated. The Nuclear Bulge is pervaded by a thermal
plasma (K) which is responsible for the diffuse free-free emission. Lyman continuum photon and dust IR luminosity of the Nuclear Bulge
again account for of the respective total luminosities of the Galaxy. Magnetic fields in the Nuclear Bulge are strong (up to mG) as compared with the Galactic Disk (a few tens of G). The field lines are oriented parallel to the galactic plane inside giant molecular clouds and perpendicular to the plane
in the intercloud medium. The compact source Sgr A* is close to or at the dynamical center of the Galaxy. Its radio spectrum
with a high frequency cut-off at GHz, a low frequency turnover at GHz and a flux density dependence in between can be explained by synchrotron emission from quasi-monoenergetic relativistic electrons.
Due to an extinction between Sun and Galactic Center corresponding to , an intrinsic weakness of this source in the near infrared, and a strong background emission from warm dust there are only
upper limits available for the flux density of Sgr A* in the far, mid and near infrared and X-ray regime. The size of Sgr
A* in the radio regime is cm, its dereddened K-band flux density is mJy, its luminosity has upper limits of (if radiation comes from an Accretion Disk) and (if black-body radiation from an object with a single temperature of K is assumed). If anyone of the soft X-ray sources detected by ROSAT actually coincides with Sgr A*, its X-ray luminosity
would be less than a few . With a dark mass of Sgr A* is the best candidate for a starving black hole, although there are no observational indications for the presence
of a (Standard) Accretion Disk. While the radio/IR spectrum of Sgr A* is purely nonthermal, the spectrum integrated over the
central parsec resembles that of a Seyfert galaxy. Sgr A* is embedded in the Hii region Sgr A West with part of the ionized gas forming a minispiral. Sgr A West is surrounded by the Circum Nuclear Disk,
an irregular shaped assembly of molecular gas which extends from pc and rotates around the Galactic Center with an estimated dynamical time scale of \,yr. The total luminosity of of the central parsec is due to the radiation of early-type stars of which have now been directly identified as luminous blue supergiants. It is still debated, however, if these stars can also account
for all of the ionization of Sgr A West. In addition, the central parsec contains red giants, AGB stars, and a few super giants
of which the brightest are now identified by direct imaging. These stars – together with a few million low mass main sequence stars – account for
the bulk of the 2.2\,m emission. The spatial distributions of the three stellar populations in the central pc are remarkably different. Sgr A* is – along the line-of-sight – presumably located close to the center of the Hii region Sgr A West, which in turn is located in front of the extended (pc) synchrotron source Sgr A East, which appears to be the remnant of a gigantic explosion (of the order of the energy of a single supernova explosion) which took place yr ago inside the GMC Sgr A East Core. X-ray observations show within pc a pervasive hot (keV) plasma of expansion age of yr. Both phenomena – as well as the formation of the Circum Nuclear Disk – may have the same origin. Influx of material is
observed within the Nuclear Bulge on all distance scales. In the Nuclear Bulge (pc) as well as in the Circum Nuclear Disk (pc) inflow towards the Galactic Center occurs primarily in the galactic plane and amounts to a few . The accretion rate into the central Black Hole, deduced from the luminosity of Sgr A*, however, appears to be lower by at
least five orders of magnitude (assuming standard disk accretion). But in an equilibrium state only part of the infalling
mass which is not accreted by the Black Hole can be consumed by star formation. A mass inflow rate varying with time is a
more natural explanation. Comparing the physical state of the Center of our Galaxy with that of Active Galactic Nuclei derived
from observations and modelling, we find that most of the basic characteristics of an AGN are also present in the Galactic
Center. Lacking are, however, both the evidence for a standard Accretion Disk and a hard UV spectrum with accompanying high
excitation emission lines in the Galactic Center which are characteristic for AGN. The luminosity of the central parsec, , amounts to only of the total luminosity of the Galaxy of . Seen from a distance of M31 (kpc) with an angular resolution of (corresponding to a linear size of pc) the Center of our Galaxy would appear as a mildly active nucleus with some starburst activity and would probably be classified
as a weak Seyfert galaxy. The synchrotron spectrum of Sgr A*, however, would be completely masked by reprocessed stellar light
(i.e. free-free and dust emission).
Received: October 21, 1996 相似文献
19.
S. Campana M. Colpi S. Mereghetti L. Stella M. Tavani 《Astronomy and Astrophysics Review》1998,8(4):279-316
Summary. Soft X–ray Transients (SXRTs) have long been suspected to contain old, weakly magnetic neutron stars that have been spun
up by accretion torques. After reviewing their observational properties, we analyse the different regimes that likely characterise
the neutron stars in these systems across the very large range of mass inflow rates, from the peak of the outbursts to the
quiescent emission. While it is clear that close to the outburst maxima accretion onto the neutron star surface takes place,
as the mass inflow rate decreases, accretion might stop at the magnetospheric boundary because of the centrifugal barrier
provided by the neutron star. For low enough mass inflow rates (and sufficiently short rotation periods), the radio pulsar
mechanism might turn on and sweep the inflowing matter away. The origin of the quiescent emission, observed in a number of
SXRTs at a level of , plays a crucial role in constraining the neutron star magnetic field and spin period. Accretion onto the neutron star surface
is an unlikely mechanism for the quiescent emission of SXRTs, as it requires very low magnetic fields and/or long spin periods.
Thermal radiation from a cooling neutron star surface in between the outbursts can be ruled out as the only cause of the quiescent
emission.
We find that accretion onto the neutron star magnetosphere and shock emission powered by an enshrouded radio pulsar provide
far more plausible models. In the latter case the range of allowed neutron star spin periods and magnetic fields is consistent
with the values recently inferred from the properties of kHz quasi-periodic oscillation in low mass X–ray binaries. If quiescent
SXRTs contain enshrouded radio pulsars, they provide a missing link between X–ray binaries and millisecond pulsars.
Received 4 November 1997; Accepted 15 April 1998 相似文献
20.
Y. M. Pihlstrm J. E. Conway R. S. Booth P. J. Diamond B. Koribalski 《New Astronomy Reviews》1999,43(8-10)
We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×1022 cm−2 assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm−3). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H2O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO. 相似文献