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1.
NGC 1365     
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   

2.
Rich and massive clusters of galaxies at intermediate redshift are capable of magnifying and distorting the images of background galaxies. A comparison of different mass estimators among these clusters can provide useful information about the distribution and composition of cluster matter and its dynamical evolution. Using the hitherto largest sample of lensing clusters drawn from the literature, we compare the gravitating masses of clusters derived from the strong/weak gravitational lensing phenomena, from the X-ray measurements based on the assumption of hydrostatic equilibrium, and from the conventional isothermal sphere model for the dark matter profile characterized by the velocity dispersion and core radius of galaxy distributions in clusters. While there is excellent agreement between the weak lensing, X-ray and isothermal sphere model-determined cluster masses, these methods are likely to underestimate the gravitating masses enclosed within the central cores of clusters by a factor of 2–4 as compared with the strong lensing results. Such a mass discrepancy has probably arisen from the inappropriate applications of the weak lensing technique and the hydrostatic equilibrium hypothesis to the central regions of clusters, as well as from assuming an unreasonably large core radius for both luminous and dark matter profiles. Nevertheless, it is pointed out that these cluster mass estimators may be safely applied on scales greater than the core sizes. Namely, the overall clusters of galaxies at intermediate redshift can still be regarded as the dynamically relaxed systems, in which the velocity dispersion of galaxies and the temperature of X-ray emitting gas are good indicators of the underlying gravitational potentials of clusters.  相似文献   

3.
We detect a positive angular correlation between bright, high-redshift QSOs and foreground galaxies. The QSOs are taken from the optically selected LBQS Catalogue, while the galaxies are from the APM Survey. The correlation amplitude is about a few per cent on angular scales of over a degree. It is a function of QSO redshift and apparent magnitude, in a way expected from weak lensing, and inconsistent with QSO–galaxy correlations being caused by physical associations, or uneven obscuration by Galactic dust. The correlations are ascribed to the weak lensing effect of the foreground dark matter, which is traced by the APM galaxies. The amplitude of the effect found here is compared to the analytical predictions from the literature, and to the predictions of a phenomenological model, which is based on the observed counts-in-cells distribution of APM galaxies. While the latter agree reasonably well with the analytical predictions (namely those of Dolag &38; Bartelmann and Sanz et al.), both underpredict the observed correlation amplitude on degree angular scales. We consider the possible ways to reconcile these observations with theory, and discuss the implications that these observations have on some aspects of extragalactic astronomy.  相似文献   

4.
Summary. As of today, seven X-ray sources have been tentatively identified as radio-quiet, isolated neutron stars. The family appears to be a rapidly growing one, although not all the objects have been identified with the same degree of certainty. The most convincing example of radio quiet pulsar is certainly Geminga, the neutron star nature of which, proposed in 1983 on the basis of its similarity with the Vela pulsar, has been firmly established with the discovery of its X and pulsation. Four more neutron star candidates, originally found in the Einstein data, have been confirmed by ROSAT, which has added to the list two more entries. All this is not the result of an unbiased search. The seven sources were not selected at random: four are inside supernova remnants, an obvious place to search for isolated neutron stars, while the remaining three were singled out because of some peculiarity. Intense -ray emission in the case of Geminga, very high X-ray counting rate for RXJ185635-3754, or being the brightest unidentified source in the Einstein medium sensitivity survey, MS 0317-6647. In spite of the limited number of objects and of the observational biases, these seven radio quiet neutron star candidates add valuable pieces of information to the observational panorama of known pulsars. Their properties, inferred from the X-ray emission, offer a coherent picture, pointing towards thermally emitting, cooling neutron stars. Received: April 1, 1996  相似文献   

5.
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   

6.
The angular cross-correlation between two galaxy samples separated in redshift is shown to be a useful measure of weak lensing by large-scale structure. Angular correlations in faint galaxies arise as a result of spatial clustering of the galaxies as well as gravitational lensing by dark matter along the line of sight. The lensing contribution to the two-point autocorrelation function is typically small compared with the gravitational clustering. However, the cross-correlation between two galaxy samples is almost unaffected by gravitational clustering provided that their redshift distributions do not overlap. The cross-correlation is then induced by magnification bias resulting from lensing by large-scale structure. We compute the expected amplitude of the cross-correlation for popular theoretical models of structure formation. For two populations with mean redshifts of ≃0.3 and 1, we find a cross-correlation signal of ≃1 per cent on arcmin scales and ≃3 per cent on scales of a few arcsec. The dependence on the cosmological parameters Ω and Λ, the dark matter power spectrum and the bias factor of the foreground galaxy population is explored.  相似文献   

7.
The surface density of populations of galaxies with steep/shallow source counts is increased/decreased by gravitational lensing magnification. These effects are usually called 'magnification bias' and 'depletion', respectively. However, if sources are demagnified by lensing, then the situation is reversed, and the detectable surface density of galaxies with a shallow source count, as expected at the faintest flux densities, is increased. In general, demagnified sources are difficult to detect and study: exquisite subarcsec angular resolution and surface brightness sensitivity are required, and emission from the lensing object must not dominate the image. These unusual conditions are expected to be satisfied for observations made of the dense swarm of demagnified images that could form very close to the line of sight through the centre of a rich cluster of galaxies using the forthcoming submillimetre-wave Atacama Large Millimeter Array (ALMA) interferometer. The demagnified images of most of the background galaxies lying within about 1 arcmin of a rich cluster of galaxies could be detected in a single 18-arcsec-diameter ALMA field centred on the cluster core, providing an effective increase in the ALMA field of view. This technique could allow a representative sample of faint,  10–100 μJy  submillimetre galaxies to be detected several times more rapidly than in a blank field.  相似文献   

8.
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   

9.
We cross-correlate the sample of type Ia supernovae from Riess A. G. et al. with the SDSS DR2 photometric galaxy catalogue. In contrast to recent work, we find no detectable correlation between supernova magnitude and galaxy overdensity on scales ranging between 1 and 10 arcmin. Our results are in accord with theoretical expectations for gravitational lensing of supernovae by large-scale structure. Future supernova surveys such as SNAP will be capable of detecting unambiguously the predicted lensing signal.  相似文献   

10.
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   

11.
引力透镜效应是探测星系团物质分布的有效方法之一.目前,利用引力透镜数据重构星系团质量分布的主流方法可以分为两大类,即参数法和非参数法.在实际研究工作中,受限于质量模型假设和计算分辨率等方面的影响,现有的重构算法仍有诸多亟需解决的问题.基于Shapelets基函数的引力透镜质量重构方法通过基函数来实现引力透镜质量重构,使用Shapelets基函数分解引力透镜势,以引力透镜中多重像的位置和背景星系椭率畸变为限制条件来迭代求解基函数系数从而得到透镜体的质量分布.通过拟合一个模拟的NFW (Navarro,Frenk and White)透镜系统测试了新方法的可行性,结果表明新方法可以在整体上重构出透镜体的质量分布,并拟合出接近真实的源位置,能够为星系团质量测量提供一套灵活且高效的重构算法.  相似文献   

12.
Gravitational lensing provides an efficient tool for the investigation of matter structures, independent of the dynamical or the hydrostatic equilibrium properties of the deflecting system. However, it depends on the kinematic status. In fact, either a translational motion or a coherent rotation of the mass distribution can affect the lensing properties. Here, light deflection by galaxy clusters in motion is considered. Even if gravitational lensing mass measurements of galaxy clusters are regarded as very reliable estimates, the kinematic effect should be considered. A typical peculiar motion with respect to the Hubble flow brings about a systematic error ≲0.3 per cent, independent of the mass of the cluster. On the other hand, the effect of the spin increases with the total mass. For cluster masses  ∼1015 M⊙  , the effect of the gravitomagnetic term is ≲0.04 per cent on strong lensing estimates and ≲0.5 per cent in the weak-lensing analyses. The total kinematic effect on the mass estimate is then ≲1 per cent, which is negligible in current statistical studies. In the weak-lensing regime, the rotation imprints a typical angular modulation in the tangential shear distortion. This would allow, in principle, a detection of the gravitomagnetic field and a direct measurement of the angular velocity of the cluster but the required background source densities are well beyond current technological capabilities.  相似文献   

13.
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   

14.
Sunspots are the most readily visible manifestations of solar magnetic field concentrations and of their interaction with the Sun's plasma. Although sunspots have been extensively studied for almost 400 years and their magnetic nature has been known since 1908, our understanding of a number of their basic properties is still evolving, with the last decades producing considerable advances. In the present review I outline our current empirical knowledge and physical understanding of these fascinating structures. I concentrate on the internal structure of sunspots, in particular their magnetic and thermal properties and on some of their dynamical aspects. Received 27 September 2002 / Published online 3 March 2003  相似文献   

15.
We have investigated the rest-frame optical and far-infrared properties of a sample of extremely bright candidate Lyman-break galaxies (LBGs) identified in the Sloan Digital Sky Survey. Their high ultraviolet luminosities and lack of strong ultraviolet emission lines are suggestive of massive starbursts, although it is possible that they are more typical luminosity LBGs which have been highly magnified by strong gravitational lensing. Alternatively, they may be an unusual class of weak-lined quasars. If the ultraviolet and submillimetre (submm) properties of these objects mirror those of less luminous, starburst LBGs, then they should have detectable rest-frame far-infrared emission. However, our submm photometry fails to detect such emission, indicating that these systems are not merely scaled-up (either intrinsically or as a result of lensing) examples of typical LBGs. In addition we have searched for the morphological signatures of strong lensing, using high-resolution, near-infrared imaging, but we find none. Instead, near-infrared spectroscopy reveals that these systems are, in fact, a rare class of broad absorption line quasars.  相似文献   

16.
We readdress the outstanding cluster mass discrepancy between strong and weak gravitational lensing techniques utilizing updated data of both giant arcs and weak lensing measurements from the literature.We find that the systematically higher values of cluster masses revealed by strong lensing can be attributed to the oversimplification of the lensing model when estimating the cluster mass enclosed within the giant arcs.This arises because inhomogeneities and substructures in the central cores of clusters may invalidate the spherical symmetry assumption used widely in previous applications.When a more realistic modeling of the arcs is used,then the masses by strong lensing agree fairly well with those given by weak lensing when both are extrapolated to the same cluster regions.We conclude that as of now no significant discrepancy has been found among different cluster mass estimators including optical galaxies,X-ray gas and lensing.  相似文献   

17.
We present the results of an unbiased radio search for gravitational lensing events with image separations between 15 and 60 arcsec, which would be associated with clusters of galaxies with masses >1013–14 M⊙. A parent population of 1023 extended radio sources stronger than 35 mJy with stellar optical identifications was selected using the FIRST radio catalogue at 1.4 GHz and the APM optical catalogue. The FIRST catalogue was then searched for companions to the parent sources stronger than 7 mJy and with separation in the range 15 to 60 arcsec. Higher-resolution observations of the resulting 38 lens candidates were made with the VLA at 1.4 and 5 GHz, and with MERLIN at 5 GHz in order to test the lens hypothesis in each case. None of our targets was found to be a gravitational lens system. These results provide the best current constraint on the lensing rate for this angular scale, but improved calculations of lensing rates from realistic simulations of the clustering of matter on the relevant scales are required before cosmologically significant constraints can be derived from this null result. We now have an efficient, tested observational strategy with which it will be possible to make an order-of-magnitude larger unbiased search in the near future.  相似文献   

18.
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   

19.
We simulated both the matter and light (galaxy) distributions in a wedge of the Universe and calculated the gravitational lensing magnification caused by the mass along the line-of-sight of galaxies and galaxy groups identified in sky surveys. A large volume redshift cone containing cold dark matter particles mimics the expected cosmological matter distribution in a flat universe with low matter density and a cosmological constant. We generate a mock galaxy catalogue from the matter distribution and identify thousands of galaxy groups in the luminous sky projection. We calculate the expected magnification around galaxies and galaxy groups and then the induced quasi-stellar object (QSO)–lens angular correlation due to magnification bias. This correlation is observable and can be used both to estimate the average mass of the lens population and to make cosmological inferences. We also use analytical calculations and various analyses to compare the observational results with theoretical expectations for the cross-correlation between faint QSOs from the 2dF Survey and nearby galaxies and groups from the Automated Plate Measurement and Sloan Digital Sky Survey Early Data Release. The observed QSO–lens anticorrelations are stronger than the predictions for the cosmological model used. This suggests that there could be unknown systematic errors in the observations and data reduction, or that the model used is not adequate. If the observed signal is assumed to be solely due to gravitational lensing, then the lensing is stronger than expected, due to more massive galactic structures or more efficient lensing than simulated.  相似文献   

20.
The interaction of fragmented plasma of active galactic nuclei jets with galactic haloes via gravitational scattering and lensing by dark matter subhaloes is studied using analytical calculations and numerical Monte-Carlo method. The lensing of jet radiation by halo masses is found to be negligible and unobservable. Moving through a galactic halo jet plasma fragments are sequentially deflected on hyperbolic orbits by gravitational field of subhaloes and deviates at some angles when leaving halo, causing widening of the jet. Based on this model jet opening angles are calculated numerically for various values of jet and halo characteristics. Though these angles are very small, gravitational scattering by halo masses results in specific radial profile of jet radiation intensity, that does not depend on halo mass distribution and jet properties. The intensity of jet radiation, obeying the derived profile, decreases by reasonable observable factors giving possibility to probe the presence of dark matter subhaloes.  相似文献   

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