共查询到20条相似文献，搜索用时 78 毫秒
1.
Yu. L. Bukhmastova 《Astrophysics》2002,45(2):191-197
The distribution function of quasars with respect to apparent brightness is given, found under the assumption that quasars are, at least partially, the gravitationally enhanced images of the active nuclei of distant galaxies. A Schechter law and a two-power law for the luminosity function of the sources are used, as well as a probabilistic law of image enhancement for various models of gravitational lenses. To find the theoretical distribution function of quasars with respect to apparent brightness we use a theorem on the probability density of a product of random quantities. It is shown that the slope of this function ranges from -1 to -2 for faint quasars, like that for ordinary galaxies. In the case of bright quasars, the slope of the apparent brightness distribution function is determined mainly by the lensing effect and has a lower limit of -3. The good agreement between theory and observations suggests that statistically quasars are gravitationally enhanced images of the active nuclei of distant galaxies. If the initial assumptions are correct, then the luminosity function of galaxies and the apparent brightness function for quasars are not independent but are related by means of the differential lensing probability. 相似文献
2.
Yu. L. Bukhmastova 《Astronomy Letters》2007,33(6):355-367
Based on the SDSS catalog, we have found new close quasar—galaxy pairs. We analyze the radial distribution of quasars from pairs around galaxies of different types. We show that the quasars from pairs follow the density profile of halo globular clusters. This is new observational evidence that the quasars projected onto the halos of galaxies are magnified by gravitational lensing by halo globular clusters. 相似文献
3.
Hongsheng Zhao David J. Bacon y N. Taylor Keith Horne 《Monthly notices of the Royal Astronomical Society》2006,368(1):171-186
We propose to use multiple-imaged gravitational lenses to set limits on gravity theories without dark matter, specifically tensor–vector–scalar (TeVeS) theory, a theory which is consistent with fundamental relativistic principles and the phenomenology of Modified Newtonian Dynamics (MOND) theory. After setting the framework for lensing and cosmology, we analytically derive the deflection angle for the point lens and the Hernquist galaxy profile, and study their patterns in convergence, shear and amplification. Applying our analytical lensing models, we fit galaxy-quasar lenses in the CfA-Arizona Space Telescope Lens Survey (CASTLES) sample. We do this with three methods, fitting the observed Einstein ring sizes, the image positions, or the flux ratios. In all the cases, we consistently find that stars in galaxies in MOND/TeVeS provide adequate lensing. Bekenstein's toy μ function provides more efficient lensing than the standard MOND μ function. But for a handful of lenses, a good fit would require a lens mass orders of magnitude larger/smaller than the stellar mass derived from luminosity unless the modification function μ and modification scale a 0 for the universal gravity were allowed to be very different from what spiral galaxy rotation curves normally imply. We discuss the limitation of present data and summarize constraints on the MOND μ function. We also show that the simplest TeVeS 'minimal-matter' cosmology, a baryonic universe with a cosmological constant, can fit the distance–redshift relation from the supernova data, but underpredicts the sound horizon size at the last scattering. We conclude that lensing is a promising approach to differentiate laws of gravity. 相似文献
4.
S. J. Warren P. C. Hewett C. B. Foltz 《Monthly notices of the Royal Astronomical Society》2000,312(4):827-832
The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly, and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterize fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K -band, flux-limited samples of quasars, termed 'KX' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly α galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera. 相似文献
5.
M. V. Sazhin O. S. Khovanskaya M. Capaccioli G. Longo J. M. Alcalá R. Silvotti M. V. Pavlov 《Astronomy Letters》2005,31(2):73-79
We consider additional arguments in favor of the first observable cosmic string. We discuss candidates for gravitational lensing events near the extragalactic double source CSL-1 (Capodimonte-Sternberg-Lens Candidate no. 1) discovered in the Osservatorio Astronomico di Capodimonte Deep Field (OACDF). The detected excess of candidates for such events cannot be explained in terms of the theory of gravitational lensing by standard extragalactic objects (galaxies, groups of galaxies, etc.) and is in close agreement with the proposed model of gravitational lensing by a cosmic string. 相似文献
6.
Strong-lensing optical depths in a ΛCDM universe – II. The influence of the stellar mass in galaxies
Stefan Hilbert Simon D. M. White Jan Hartlap Peter Schneider 《Monthly notices of the Royal Astronomical Society》2008,386(4):1845-1854
We investigate how strong gravitational lensing in the concordance ΛCDM cosmology is affected by the stellar mass in galaxies. We extend our previous studies, based on ray tracing through the Millennium Simulation, by including the stellar components predicted by galaxy formation models. We find that the inclusion of these components greatly enhances the probability for strong lensing compared to a 'dark matter only' universe. The identification of the 'lenses' associated with strong-lensing events reveals that the stellar mass of galaxies (i) significantly enhances the strong-lensing cross-sections of group and cluster haloes and (ii) gives rise to strong lensing in smaller haloes, which would not produce noticeable effects in the absence of the stars. Even if we consider only image splittings ≳10 arcsec, the luminous matter can enhance the strong-lensing optical depths by up to a factor of 2. 相似文献
7.
The gravitational radiation of n = 1 polytropes undergoing quasiradial pulsations is examined. The intensity of the gravitational
radiation and the gravitational wave amplitudes are calculated for polytropic models of white dwarfs and neutron stars when
the energy of rotation of the object serves as the source of the radiated energy. Calculations of h0 show that objects with a polytropic equation of state can describe the expected gravitational radiation from white dwarfs
and neutron stars. The gravitational radiation of polytropic models of galactic nuclei and quasars is also examined. These
objects can create a high enough background of gravitational radiation at frequencies of 10-8–10-11 Hz for gravitational wave detectors operating in this frequency range.
__________
Translated from Astrofizika, Vol. 48, No. 4, pp. 603–612 (November 2005). 相似文献
8.
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
9.
R. J. Ivison Ian Smail Misty Bentz J. A. Stevens K. Menéndez-Delmestre S. C. Chapman A. W. Blain 《Monthly notices of the Royal Astronomical Society》2005,362(2):535-541
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
10.
11.
Marina Gibilisco 《Astrophysics and Space Science》1997,249(2):189-210
In this paper, I investigate a local effect of polarization of the Cosmic Microwave Background (CMB) in clusters of galaxies,
induced by the Thomson scattering of an anisotropic radiation. A local anisotropy of the CMB is produced by some scattering
and gravitational effects, as, for instance, the Sunyaev Zel‘dovich effect, the Doppler shift due to the cluster motion and
the gravitational lensing. The resulting anisotropy ΔI/I depends on the physical properties of the clusters, in particular
on their emissivity in the X band on their size, on their gravitational potential and on the peculiar conditions characterizing
the gas they contain. By solving the Boltzmann radiative transfer equation in presence of such anisotropies I calculate the
average polarization at the centre of some clusters, namelyA2218, A576 and A2163, whose properties are quite well known. I
prove that the gravitational effects due to the contraction or to the expansion have some importance, particularly for high
density structures; moreover, the peculiar motion of the cluster, considered as a gravitational lens, influences the propagation
of the CMB photons by introducing a particular angular dependence in the gravitational anisotropy and in the scattering integrals.
Thus, the gravitational and the scattering effects overally produce an appreciable local average polarization of the CMB,
may be observable through a careful polarization measurements towards the centres of the galaxy clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
Xiang-Ping Wu Tzihong Chiueh Li-Zhi Fang & Yan-Jie Xue 《Monthly notices of the Royal Astronomical Society》1998,301(3):861-871
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
13.
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
14.
15.
A.W.Blain 《Monthly notices of the Royal Astronomical Society》2002,330(1):219-224
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
16.
Eran O. Ofek Stella Seitz Felix Klein 《Monthly notices of the Royal Astronomical Society》2008,389(1):311-314
Highly magnified lensed galaxies allow us to probe the morphological and spectroscopic properties of high-redshift stellar systems in great detail. However, such objects are rare, and there are only a handful of lensed galaxies that are bright enough for a high-resolution spectroscopic study with current instrumentation. We report the discovery of a new massive lensing cluster, SDSS J120923.7+264047, at z = 0.558. Present around the cluster core, at angular distances of up to ∼40 arcsec, are many arcs and arc candidates, presumably due to lensing of background galaxies by the cluster gravitational potential. One of the arcs, 21 arcsec long, has an r -band magnitude of 20, making it one of the brightest known lensed galaxies. We obtained a low-resolution spectrum of this galaxy, using the Keck-I telescope, and found it is at redshift of z = 1.018. 相似文献
17.
Alan Heavens Alexandre Refregier Catherine Heymans 《Monthly notices of the Royal Astronomical Society》2000,319(2):649-656
Weak gravitational lensing is now established as a powerful method to measure mass fluctuations in the universe. It relies on the measurement of small coherent distortions of the images of background galaxies. Even low-level correlations in the intrinsic shapes of galaxies could however produce a significant spurious lensing signal. These correlations are also interesting in their own right, since their detection would constrain models of galaxy formation. Using haloes found in N -body simulations, we compute the correlation functions of the intrinsic ellipticity of spiral galaxies assuming that the disc is perpendicular to the angular momentum of the dark matter halo. We also consider a simple model for elliptical galaxies, in which the shape of the dark matter halo is assumed to be the same as that of the light. For deep lensing surveys with median redshifts ∼1, we find that intrinsic correlations of ∼10−4 on angular scales are generally below the expected lensing signal, and contribute only a small fraction of the excess signals reported on these scales. On larger scales we find limits to the intrinsic correlation function at a level ∼10−5 , which gives a (model-dependent) range of separations for which the intrinsic signal is about an order of magnitude below the ellipticity correlation function expected from weak lensing. Intrinsic correlations are thus negligible on these scales for dedicated weak lensing surveys. For wider but shallower surveys such as SuperCOSMOS, APM and SDSS, we cannot exclude the possibility that intrinsic correlations could dominate the lensing signal. We discuss how such surveys could be used to calibrate the importance of this effect, as well as study spin–spin correlations of spiral galaxies. 相似文献
18.
Evgeny Griv Michael Gedalin David Eichler Chi Yuan 《Astrophysics and Space Science》2000,271(1):21-58
Linear kinetic theory is developed to describe collective oscillations (and their instabilities) propagating in a rapidly
rotating disk of stars, representing a highly flattened galaxy. The analysis is carried out for the special case of a self-gravitating,
infinitesimally thin, and spatially inhomogeneous system, taking into account the effects both of thermal movements of stars
and of gravitational encounters between stars and giant molecular clouds of an interstellar medium. The star–cloud encounters
are described with the use of the Landau collision integral. The dynamics of gravity perturbations with rare interparticle
encounters is considered. Such a disk is treated by employing the well elaborated mathematical formalisms from plasma perturbation
theory using normal-mode analysis. In particular, the method of solving the Boltzmann equation is applied by integration along
paths, neglecting the influence of star–cloud encounters on the distribution of stars in the zeroth-order approximation. We
are especially interested in important kinetic effects due to wave–star resonances, which we have little knowledge about.
The kinetic effects are introduced via a minor drift motion of stars which is computed from the equations of stellar motion
in an unperturbed central force field of a galaxy. The dispersion laws for two main branches of disk's oscillations, that
is the classical Jeans branch and an additional gradient branch, are deduced. The resonant Landau-type instabilities of hydrodynamically
stable Jeans and gradient gravity perturbations is considered to be a long-term generating mechanism for propagating density
waves, thereby leading to spiral-like and/or ring-like patterns in the flat galaxies.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
Ramesh Chander Kapoor 《Astrophysics and Space Science》1983,93(1):79-85
It is argued that accreting supermassive black holes ejected from centers of galaxies are the likely models for the quasars
observed in association with galaxies. Also pointed out are the implications of a recent suggestion by Horák (1982) to account
for the excess redshifts of such quasars due to a combined effect of peculiar Doppler-motion and the gravitational field. 相似文献
20.
In the first part of the paper the known results on the gravitational interaction of a massive black hole with the surrounding stars in a galactic nucleus are discussed. The tidal disruption of stars in close encounters with a black hole is reviewed. Expressions for the flux of stars on a black hole are given, taking into account energy and angular momentum diffusion of stellar orbits. The scenario of star disruption and accretion of the released stellar matter is depicted. The growth of a black hole in a typical galactic nucleus on account of gas accretion from disrupted stars is discussed. A comparison with the upper limit to the luminosity of the nucleus of our Galaxy puts rather severe constraints on the mass of a hypothetical black hole at the galactic centre. Possible mechanisms preventing the formation and growth of black holes in normal galactic nuclei are discussed.The second part of the paper (Section 8) deals with the hypothesis that massive black holes are the primary energy sources in active galaxies and quasars. The luminosity requirements of bright quasars and weak Seyferts can probably be accounted for in such a model, but there are difficulties in explaining the intermediate range. Mass ejection from Seyferts and quasars is not a severe problem. The same applies to the spectrum. A much more serious objection is the observed periodic and quasi-periodic variability. Another unsatisfactory feature of this hypothesis is that one needs two different evolutionary tracks for quasars and active galaxies, and for normal galaxies. 相似文献