共查询到20条相似文献,搜索用时 14 毫秒
1.
2.
Marek J. Kukula James S. Dunlop David H. Hughes & Steve Rawlings 《Monthly notices of the Royal Astronomical Society》1998,297(2):366-382
Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S ∝ ν−α ), and high brightness temperatures ( T B ≥ 105 K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample. 相似文献
3.
The numerical method for obtaining the luminosity-dependent evolution function of radio sources is presented and applied to the flat spectrum radio sources selected at 5 GHz.The method constructed on the basis of the iterative one given by Robertson, which requires assuming a specific type of the searched function enables one to obtain the evolution function of sources with radio flux densityS
5.01 mJy.The influence of the shape of the local RLF, of the luminosity distributionN(P) and of the spectral index function on the resulting evolution function is investigated. The results presented show that the luminosity dependence of the evolution function of flat-spectrum radio sources is significant and should not be neglected. The obtained evolution function, which allows one to reproduce the source counts curve, the redshift distribution, and the spectral index distribution, may exhibit the redshift cut-off at z03. 相似文献
4.
It is shown that the luminosity-volume test for optically selected objects has an in-built bias towards increasing the average
value ofV/V
m above the Euclidean value of 1/2. A more satisfactory bias-free statistic is suggested in the form of In (V/V
m), The result of applying the test to a sample from the Bright Quasar Survey (BQS) shows that the local hypothesis of quasars
is consistent with the data. 相似文献
5.
6.
7.
H.-R. Klöckner A. Martínez-Sansigre S. Rawlings M. A. Garrett 《Monthly notices of the Royal Astronomical Society》2009,398(1):176-188
The radio properties of 11 obscured 'radio-intermediate' quasars at redshifts z ≳ 2 have been investigated using the European Very-Long-Baseline-Interferometry Network (EVN) at 1.66 GHz. A sensitivity of ∼25 μJy per 17 × 14 mas2 beam was achieved, and in seven out of 11 sources unresolved radio emission was securely detected. The detected radio emission of each source accounts for ∼30–100 per cent of the total source flux density. The physical extent of this emission is ≲150 pc, and the derived properties indicate that this emission originates from an active galactic nucleus (AGN). The missing flux density is difficult to account for by star formation alone, so radio components associated with jets of physical size ≳150 pc and ≲40 kpc are likely to be present in most of the sources. Amongst the observed sample steep, flat, gigahertz-peaked and compact-steep spectrum sources are all present. Hence, as well as extended and compact jets, examples of beamed jets are also inferred, suggesting that in these sources, the obscuration must be due to dust in the host galaxy, rather than the torus invoked by the unified schemes. Comparing the total to core (≲150 pc) radio luminosities of this sample with different types of AGN suggests that this sample of z ≳ 2 radio-intermediate obscured quasars shows radio properties that are more similar to those of the high-radio-luminosity end of the low-redshift radio-quiet quasar population than those of Fanaroff–Riley type I (FR I) radio galaxies. This conclusion may reflect intrinsic differences, but could be strongly influenced by the increasing effect of inverse-Compton cooling of extended radio jets at high redshift. 相似文献
8.
M. J. Hardcastle L. W. Looney 《Monthly notices of the Royal Astronomical Society》2008,388(1):176-186
We have observed a small sample of powerful double radio sources (radio galaxies and quasars) at frequencies around 90 GHz with the Berkeley Illinois Maryland Association (BIMA) millimetre array, with the intention of constraining the resolved high-frequency spectra of radio galaxies. When combined with other sources we have previously observed and with data from the BIMA archive, these observations allow us for the first time to make general statements about the high-frequency behaviour of compact components of radio galaxies – cores, jets and hotspots. We find that cores in our sample remain flat-spectrum up to 90 GHz; jets in some of our targets are detected at 90 GHz for the first time in our new observations and hotspots are found to be almost universal, but show a wide range of spectral properties. Emission from the extended lobes of radio galaxies is detected in a few cases and shows rough consistency with the expectations from standard spectral ageing models, though our ability to probe this in detail is limited by the sensitivity of BIMA. We briefly discuss the prospects for radio galaxy astrophysics with Atacama Large Millimeter Array. 相似文献
9.
10.
I. Waddington R. A. Windhorst J. S. Dunlop D. C. Koo J. A. Peacock 《Monthly notices of the Royal Astronomical Society》2000,317(4):801-824
The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden–Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S 1.4 1.0 mJy, in a 1.2 deg2 region of Hercules. This sample is almost completely identified in the g , r , i and K bands, with some additional data available at J and H . The magnitude distributions peak at r ≃22 mag, K ≃16 mag and extend down to r ≃26 mag, K ≃21 mag. The K -band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S 1.4 GHz ≲1 Jy, the K -band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red ( r − K ≳6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts. 相似文献
11.
T. J. Cornwell D. J. Saikia P. Shastri L. Feretti G. Giovannini P. Parma C. J. Salter 《Journal of Astrophysics and Astronomy》1986,7(2):119-129
We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299,
which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of
radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The
present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a
distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the
north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension
towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent
source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to
help clarify the situation. 相似文献
12.
Robert S. Priddey Richard G. McMahon 《Monthly notices of the Royal Astronomical Society》2001,324(1):L17-L22
We combine photometric observations of high-redshift quasars, obtained at submillimetre to millimetre wavelengths, to obtain a mean far-infrared (rest-frame) spectral energy distribution (SED) of the thermal emission from dust, parametrized by a single temperature ( T ) and power-law emissivity index ( β ). The best-fitting values are and . Our method exploits the redshift spread of this set of quasars, which allows us to sample the SED at a larger number of rest wavelengths than is possible for a single object: the wavelength range extends down to ∼60 μm, and therefore samples the turnover in the greybody curve for these temperatures. This parametrization is of use to any studies that extrapolate from a flux at a single wavelength, for example to infer dust masses and far-infrared luminosities.
We interpret the cool, submillimetre component as arising from dust heated by star formation in the host galaxy of the quasar, although we do not exclude the presence of dust heated directly by the active galactic nucleus (AGN). Applying the mean SED to the data, we derive consistent star formation rates ∼1000 M⊙ yr−1 and dust masses ∼109 M⊙ , and investigate a simple scheme of AGN and host galaxy co-evolution to account for these quantities. The time-scale for formation of the host galaxy is , and the luminous quasar phase occurs towards the end of this period, just before the reservoir of cold gas is depleted. Given the youth of the Universe at (1.6 Gyr), the coexistence of a massive black hole and a luminous starburst at high redshifts is a powerful constraint on models of quasar host galaxy formation. 相似文献
We interpret the cool, submillimetre component as arising from dust heated by star formation in the host galaxy of the quasar, although we do not exclude the presence of dust heated directly by the active galactic nucleus (AGN). Applying the mean SED to the data, we derive consistent star formation rates ∼1000 M
13.
Margrethe Wold Mark Lacy Per B. Lilje Stephen Serjeant 《Monthly notices of the Royal Astronomical Society》2000,316(2):267-282
We have observed the galaxy environments around a sample of 21 radio-loud, steep-spectrum quasars at 0.5≤ z ≤0.82, spanning several orders of magnitude in radio luminosity. The observations also include background control fields used to obtain the excess number of galaxies in each quasar field. The galaxy excess was quantified using the spatial galaxy–quasar correlation amplitude, B gq , and an Abell-type measurement, N 0.5 . A few quasars are found in relatively rich clusters, but on average, they seem to prefer galaxy groups or clusters of approximately Abell class 0. We have combined our sample with literature samples extending down to z ≈0.2 and covering the same range in radio luminosity. By using the Spearman statistic to disentangle redshift and luminosity dependences, we detect a weak, but significant, positive correlation between the richness of the quasar environment and the radio luminosity of the quasar. However, we do not find any epoch dependence in B gq , as has previously been reported for radio quasars and galaxies. We discuss the radio luminosity–cluster richness link and possible explanations for the weak correlation that is seen. 相似文献
14.
The observations of 34 extragalactic radio sources with the 22-m Crimean Astrophysical Observatory radio telescope at 36 GHz in 1985–1994 are presented. Intensity variations were detected in 27 objects, which may result from the appearance of new components in their cores. 相似文献
15.
We explore the relationship between the extended radio and line emission for a radio-loud quasar sample including both core-dominated and lobe-dominated quasars. A strong correlation is present between the extended radio and broad-line emission. The core emission is also correlated with the broad-line emission for core-dominated quasars in the sample. The statistical behaviour of the core emission of lobe-dominated quasars is rather different from that of core-dominated quasars. The extended radio luminosity is a good tracer for jet power, while the core luminosity can only be a jet power tracer for core-dominated quasars. 相似文献
16.
17.
R. Moderski M. Sikora & J.-P. Lasota 《Monthly notices of the Royal Astronomical Society》1998,301(1):142-148
We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the 'spin paradigm' models, radio-loud quasars could be objects in which the rotation rate of the black hole corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford–Znajek mechanism. Radio-quiet quasars could be hosting black holes with an average spin much smaller than the equilibrium one. We discuss possible accretion scenarios which can lead to such a bimodal distribution of black hole spins. 相似文献
18.
V. L. Afanasiev S. N. Dodonov A. V. Moiseev V. Chavushyan R. Mujica Y. Juarez A. G. Gorshkov V. K. Konnikova M. G. Mingaliev 《Astronomy Letters》2003,29(9):579-586
We present the classification of optical identifications and radio spectra of six radio sources from a complete (in flux density) sample in the declination range 10° to 12°30′ (J2000.0). The observations were carried out with the 6-m Special Astrophysical Observatory telescope (Russia) in the wavelength range 3600–10000 Å, the 2.1-m GHAO telescope (Mexico) in the range 4200–9000 Å, and the RATAN-600 radio telescope in the frequency range 0.97–21.7 GHz. Three of the six objects under study are classified as quasars, one is a BL Lac object, one is an absorption-line radio galaxy, and one is an emission-line radio galaxy. Five objects have flat radio spectra, and one object has a power-law radio spectrum. All of the radio sources identified as quasars or BL Lac objects show variable radio flux densities. The spectra of three objects were separated into extended and compact components. 相似文献
19.
Ian Heywood Katherine M. Blundell Steve Rawlings 《Monthly notices of the Royal Astronomical Society》2007,381(3):1093-1102
We present deep, multi-Very Large Array configuration radio images for a set of 18 quasars, having redshifts between 0.36 and 2.5, from the 7C quasar survey. Approximately one quarter of these quasars have Fanaroff–Riley type I (FR I) type twin-jet structures and the remainder are a broad range of wide angle tail, fat double, classical double, core-jet and hybrid sources. These images demonstrate that FR I quasars are prevalent in the Universe, rather than non-existent as had been suggested in the literature prior to the serendipitous discovery of the first FR I quasar a few years ago, the optically powerful 'radio-quiet' quasar E 1821+643.
Some of the FR I quasars have radio luminosities exceeding the traditional FR I/FR II break luminosity; however, we find no evidence for FR II quasars with luminosities significantly below the break. We consider whether the existence of such high-luminosity FR I structures is due to the increasingly inhomogeneous environments in the higher redshift Universe. 相似文献
Some of the FR I quasars have radio luminosities exceeding the traditional FR I/FR II break luminosity; however, we find no evidence for FR II quasars with luminosities significantly below the break. We consider whether the existence of such high-luminosity FR I structures is due to the increasingly inhomogeneous environments in the higher redshift Universe. 相似文献