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1.
The chance that the pair of X-ray sources observed across NGC 4258 is accidental can be calculated as 5×10–6. The recent confirmation as quasars, and determination of the redshifts of the pair, atz=0.40 and 0.65 by E.M. Burbidge enables the final accidental probability of the configuration to be calculated as <4×10–7. In addition there are a number of observations which indicate the central Seyfert galaxy is ejecting material from its active nucleus.The NGC 4258 association is compared to four other examples of close association of pairs of X-ray quasars with low redshift galaxies. It is concluded that in each of these five cases the chance of accidental association is less than one in a million. The ejection speed calculated from the redshift differences of the X-ray quasars is 0.12c. This agrees with the ejection velocity of 0.1c calculated in 1968 from radio quasars associated with low redshift peaks become narrower—simultaneously strengthening the ejection origin for quasars and the quantization property of their redshift  相似文献   

2.
Despite a general consensus in the astronomical community that all quasars are located at the distances implied by their redshifts, a number of observations still challenge this interpretation, possibly indicating that some subpopulation of quasars may harbour significant redshift components not related to the expansion of the Universe. It has been suggested that these objects may have been ejected from local galaxies and are likely to evolve into new galaxies themselves. Here, a test of such exotic scenarios is proposed, based on the spectral energy distribution of the galaxies hosting quasars with suspected ejection origin. Provided that the time-scales over which the ejected objects manifest themselves as quasars is short, one would in the framework of the ejection scenarios expect to find either no quasar host galaxy, a pseudo-host consisting of gas ionized by the quasar, or a host galaxy consisting of young stars only. It is argued that the spectral energy distributions corresponding to the last two options should differ significantly from that of most quasar host galaxies detected at low redshift so far, thus providing a potential test of the claimed existence of ejected quasars. A minimal implementation of this test, involving optical and near-infrared broad-band photometry, is suggested.  相似文献   

3.
We present spectroscopic observations of a sample of faint gigahertz peaked‐spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS). Redshifts have been determined for 19 (40 per cent) of the objects. The optical spectra of the GPS sources identified with low‐redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by active galactic nucleus-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep‐spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z ∼2–3. It is unlikely that a significant population of low-redshift GPS quasars is missed as a result of selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift–luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio spectral properties, and hypothesize that GPS quasars are a subclass of flat-spectrum quasars.  相似文献   

4.
We investigate the hypothesis that quasars formed together with the stellar populations of early-type galaxies. This hypothesis – in conjunction with the stellar ages of early-type galaxies from population synthesis models, the relation of black hole mass to bulge velocity dispersion, and the velocity dispersion distribution of spheroids from the Sloan Digital Sky Survey – completely determines the cosmic accretion history of supermassive black holes and the redshift evolution of the characteristic luminosity. On the other hand, the precise shape of the luminosity function of quasars depends on the light curve of quasars and – in the optical, but not so much in X-rays – on the covering factor of the dust surrounding the active nucleus. We find a plausible set of assumptions for which the coeval formation of supermassive black holes and elliptical galaxies is in good agreement with the observed B -band and X-ray luminosity functions of quasars.  相似文献   

5.
塞佛特星系ＮＧＣ1 0 6 8是一个非常令人注目的特殊天体 ,它是Ｓｅｙｆｅｒｔ[1]发现的第一批此类星系中离我们最近 (1 9Ｍｐｃ,取Ｈ0 =6 0ｋｍｓ- 1Ｍｐｃ- 1)和最亮的一个塞佛特星系 ,因此天文学家从射电到Ｘ 射线整个电磁波段对它进行了详尽的观测和研究。ＮＧＣ1 0 6 8最初被归类为塞佛特ＩＩ型天体 ,但是通过对其偏振光的光谱观测 ,发现了认为被遮挡住的塞佛特Ｉ型核发出的± 432 0ｋｍ/ｓ的宽Ｈβ发射线 ,因此被认为是活动星系核统一模型的一个范例 (见文 [2 ] )。射电与光学的观测资料都表明ＮＧＣ1 0 6 8的中心部分有激烈的喷射活动…  相似文献   

6.
We quantify the galaxy environments around a sample of 0.5≤ z ≤0.8 radio-quiet quasars using the amplitude of the spatial galaxy–quasar correlation function, B gq. The quasars exist in a wide variety of environments; some sources are located in clusters as rich as Abell class 1–2 clusters, whereas others exist in environments comparable to the field. We find that, on average, the quasars prefer poorer clusters of ≈Abell class 0, which suggests that quasars are biased tracers of mass compared with galaxies. The mean B gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments.  相似文献   

7.
An X-ray source close to the classical Seyfert galaxy NGC 1068 is identified as a quasar with a redshift of 0.63. The very high surface density of quasars around NGC 1068 suggests that the quasars may be physically associated with this active galaxy.  相似文献   

8.
In this paper we propose a scenario for the formation of a population of baryon-rich, dark-matter-deficient dwarf galaxies at high redshift from the mass swept out in the intergalactic medium (IGM) by energetic outflows from luminous quasars. We predict the intrinsic properties of these galaxies, and examine the prospects for their observational detection in the optical, X-ray and radio wavebands. Detectable thermal Sunyaev–Zel'dovich decrements on arcminute scales in the cosmic microwave background radiation maps are expected during the shock-heated expanding phase from these hot bubbles. We conclude that the optimal detection strategy for these dwarfs is via narrow-band Lyman-α imaging of regions around high-redshift quasars. A scaled-down (in the energetics) version of the same model is speculated upon as a possible mechanism for forming pre-galactic globular clusters.  相似文献   

9.
Building on evidence starting from 1966, X-ray observations have once again confirmed the association of quasars with low redshift galaxies. Enough examples of quasar-like objects ejected in opposite directions from nearby, active galaxies have accumulated so that an empirical evolutionary sequence can be outlined. The quasars start out with low luminosity and high (z > 2) redshift. As they travel away from their galaxy of origin they grow in size and decay in redshift. The redshifts drop in steps and near the quantized values of z = 0.6, 0.3, and 0.06 the quasars become particularly active, ejecting or breaking up into many objects which evolve finally into groups and clusters of galaxies. The observations massively violate the assumptions of the Big Bang and require continuous, episodic creation in a non expanding universe of indefinitely large size and age.  相似文献   

10.
We present the results of multiwavelength observations of cores and hotspots, at L , C , X and U bands with the Very Large Array, of a matched sample of radio galaxies and quasars selected from the Molonglo Reference Catalogue . We use these observations to determine the spectra of cores and hotspots, and test the unified scheme for radio galaxies and quasars. Radio cores have been detected at all wavelengths in all of the quasars in our sample, whereas only ∼50 per cent of the galaxies have cores detected in at least one of the wavelengths . The degree of core prominence in this sample is consistent with the unified scheme for radio galaxies and quasars. A comparison of the distributions of the two-point spectral index of the cores in our sample of lobe-dominated quasars, with the distributions in a matched sample of core-dominated quasars, shows that the distributions for these two classes are significantly different, and this is consistent with the expectations of the unified scheme. The difference in the spectral indices of the two hotspots on opposite sides is also significantly larger for quasars than for radio galaxies, as is expected in the unified scheme. We also investigate the relationship between the spectral index of the hotspots and the redshift or luminosity for our sample of sources.  相似文献   

11.
A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS) has been imaged in the optical and near-infrared, resulting in an identification fraction of 87 per cent. The R  −  I R  −  K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R -band Hubble diagram as determined for radio-bright GPS galaxies. We find evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies. The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift that has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are generally at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.  相似文献   

12.
We present optical (∼3200 to ∼9000 Å) off-nuclear spectra of 26 powerful active galaxies in the redshift range 0.1≤ z ≤0.3, obtained with the Mayall and William Herschel 4-m class telescopes. The sample consists of radio-quiet quasars, radio-loud quasars (all with −23≥ M V ≥−26) and radio galaxies of Fanaroff–Riley Type II (with extended radio luminosities and spectral indices comparable to those of the radio-loud quasars). The spectra were all taken approximately 5 arcsec off-nucleus, with offsets carefully selected so as to maximize the amount of galaxy light falling into the slit, whilst simultaneously minimizing the amount of scattered nuclear light. The majority of the resulting spectra appear to be dominated by the integrated stellar continuum of the underlying galaxies rather than by light from the non-stellar processes occurring in the active nuclei, and in many cases a 4000-Å break feature can be identified. The individual spectra are described in detail, and the importance of the various spectral components is discussed. Stellar population synthesis modelling of the spectra will follow in a subsequent paper.  相似文献   

13.
Three distinct and representative sources of quasar data are used in this paper: 1) the Revised Optical Catalog of QSOs, 2) the quasars in the ?40° CTIO Curtis Schmidt Survey, and 3) the quasars in the Virgo Cluster region. Their redshift and magnitude distributions on various scales and their possible association with galaxies are analysed statistically.The preliminary conclusions are as follows. 1) The redshift and magnitude distributions on various scales are non-uniform. 2) The non-uniformity appears to be greater in the two local samples. 3) There is no evidence of a general association of quasars with field galaxies. 4) A general association of quasars with brighter cluster galaxies is statistically significant. 5) It seems likely that not all quasars have the same origin, some being local, some cosmological.  相似文献   

14.
The VLBA has been used over a period of four years to study the internal motions within a sample of quasars and AGN. In most sources, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c, with some evidence for apparent accelerations and decelerations. The distribution of apparent velocity is not consistent with any simple ballistic model and appears to require either a spread in intrinsic velocity or a difference between the bulk velocity and pattern velocity. The dependence of apparent angular velocity with redshift is consistent with standard Friedmann world models. Further observations of a larger source sample, especially at large redshift may lead to meaningful constraints on world models.  相似文献   

15.
With a recently constructed composite quasar spectrum and the X2 minimization technique, we describe a general method for estimating the photometric redshifts of a large sample of quasars by deriving theoretical color-redshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in the first major data release of SDSS and compared them with their spectroscopic redshifts. The difference is less than 0.1 for 47% of the quasars and less than 0.2 for 68%. Based on the calculation of the theoretical color-color diagrams of stars, galaxies and quasars both on the SDSS system and on the BATC system, we expect that we would be able to select candidates of high redshift quasars more efficaciously with the latter than with the former, provided the BATC survey can detect objects with magnitudes fainter than 21.  相似文献   

16.
Stellar photometry of images from the ACS and WFPC2 cameras on the Hubble Space Telescope (HST) is used to study the star composition and spatial distribution of stars in three edge-on visible spiral galaxies: NGC 891, NGC 4144, and NGC 4244. Measurements of the surface number density of old stars revealed two stellar subsystems in these galaxies: a thick disk and a halo. The boundaries of these subsystems, which consist mainly of red giants, are determined from the change in the gradient of the number density of the stars. The halos are flattened at the poles of the galaxies and extend to distances of 8–25 kpc from the planes of the galaxies. The present results on the number density distribution of stars with different ages perpendicular to the planes of these galaxies make it possible to improve our model for the stellar structure of spiral galaxies. The distances to these galaxies are calculated using a determination of the tip of the red giant branches (the TRGB method): D = 9.82 Mpc (NGC 891), D = 7.24 Mpc (NGC 4144), and D = 4.29 Mpc (NGC 4244).__________Translated from Astrofizika, Vol. 48, No. 2, pp. 261–280 (May 2005).  相似文献   

17.
For this paper, we collected almost all of the type II quasars so far discovered. Among them, 485 sources have photometric data at JHK bands, mainly from Two-Micron All-Sky Survey observations, 65 sources have IRAS photometric data in at least one of the three IRAS bands at 25, 60 and 100 μm and 15 sources have IRAS photometric data in all three IRAS bands. We find that in nearly half of all type II quasars, both the near- and far-infrared radiations are dominated by starlight or thermal reprocessing of starlight by dust in the underlying galaxy. The infrared radiation of the other group (slightly over half) is dominated by a non-thermal radiation in the near-infrared, and mostly in the far-infrared also (although there is a mixture particularly for the longer wavelengths). It is proposed that for the later group, hidden broad lines may exist in the infrared. On the basis of our and previous results, we also discuss the possibility that there are two distinct classes of type II quasars: 'true' type II quasars without a broad-line region and heavily obscured type I quasars, in full analogy with the case for type II Seyfert galaxies. No relationships can be found for either the near-infrared or the far-infrared colours and the redshift. Correlations between absolute magnitude in the near- and far-infrared with redshift are found, but could be due to a flux limit (Malmquist bias).  相似文献   

18.
We extend our previous analysis which used generalized luminosity functions (GLFs) to predict the number of quasars and galaxies in low-radio-frequency-selected samples as a function of redshift, radio luminosity, narrow-emission-line luminosity and type of unified scheme. Our extended analysis incorporates the observed submillimetre (850-μm) flux densities of radio sources, employs a new method which allows us to deal with non-detections, and focuses on the high-luminosity population. First, we conclude that the submillimetre luminosity L 850 of low-frequency-selected radio sources is correlated with the bolometric luminosity L bol of their quasar nuclei via an approximate scaling relation   L 850∝ L 0.7±0.2bol  . Secondly, we conclude that there is quantitative evidence for a receding-torus-like physical process for the high-luminosity population within a two-population unified scheme for radio sources; this evidence comes from the fact that radio quasars are brighter in both narrow emission lines and submillimetre luminosity than radio galaxies matched in radio luminosity and redshift. Thirdly, we note that the combination of a receding-torus-like scheme and the assumption that the observed submillimetre emission is dominated by quasar-heated dust yields a scaling relation   L 850∝ L 1/2bol  which is within the errors of that determined here for radio-selected quasars, and consistent with that inferred for radio-quiet quasars.  相似文献   

19.
Infrared-Faint Radio Sources represent a new and unexpected class of object which is bright at radio wavelengths but unusually faint at infrared wavelengths. If, like most mJy radio sources, they were either conventional active or star-forming galaxies in the local Universe, we would expect them to be detectable at infrared wavelengths, and so their non-detection by the Spitzer Space Telescope is surprising. Here, we report the detection of one of these sources using very long baseline interferometry, from which we conclude that the sources are driven by active galactic nuclei. We suggest that these sources are either normal radio-loud quasars at high redshift or abnormally obscured radio galaxies.  相似文献   

20.
Near-IR images, obtained at the ESO-VLT during excellent seeing conditions, of a sample of 17 radio-loud and radio-quiet quasars in the redshift range 1 < z < 2 are presented. The host galaxies of both types of quasars appear to follow the expected trend in luminosity of massive ellipticals undergoing simple passive evolution. We find a systematic difference by a factor ∼2 in the host luminosity between RLQs and RQQs that does not change significantly from z = 2 to the present epoch. Quasar hosts appear thus to be already well formed at z ∼ 2 and similar to massive inactive spheroids. These findings are in disagreement with the predictions of models for the joint formation and evolution of galaxies and active nuclei based on the hierarchical structure formation scenario.  相似文献   

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