共查询到20条相似文献，搜索用时 22 毫秒
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C. H. Ishwara-Chandra D. J. Saikia 《Monthly notices of the Royal Astronomical Society》1999,309(1):100-112
We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437−244 and 1025−229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (|RM|<20 rad m−2 ). The giant sources are defined to be those with overall projected size 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity–size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton loss owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme. 相似文献
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M. J. Hardcastle D. M. Worrall 《Monthly notices of the Royal Astronomical Society》2000,319(2):562-572
Using ROSAT observations, we estimate gas pressures in the X-ray-emitting medium surrounding 63 FRII radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (including 12/13 sources with modelled, spatially resolved X-ray emission) radio sources appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the internal energy density of the sources. We discuss possible departures from the minimum-energy condition and the consequences of our result for models of the dynamics of radio galaxies, in particular self-similar models . 相似文献
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C. H. Ishwara-Chandra D. J. Saikia P. J. McCarthy W. J. M. van Breugel 《Monthly notices of the Royal Astronomical Society》2001,323(2):460-470
We present multi-wavelength radio observations with the Very Large Array, and narrow- and broad-band optical observations with the 2.5-m telescope at the Las Campanas Observatory, of a well-defined sample of high-luminosity Fanaroff–Riley class II radio galaxies and quasars, selected from the Molonglo Reference Catalogue 1-Jy sample. These observations were carried out as part of a programme to investigate the effects of orientation and environment on some of the observed properties of these sources. We examine the dependence of the Liu–Pooley relationship, which shows that radio lobes with flatter radio spectra are less depolarized, on size, identification and redshift, and show that it is significantly stronger for smaller sources, with the strength of the relationship being similar for both radio galaxies and quasars. In addition to Doppler effects, there appear to be intrinsic differences between the lobes on opposite sides. We discuss the asymmetry in brightness and location of the hotspots, and present estimates of the ages and velocities from matched-resolution observations in the L and C bands. Narrow- and broad-band optical images of some of these sources were made to study their environments and correlate with the symmetry parameters. An extended emission-line region is seen in a quasar, and in four of the objects possible companion galaxies are seen close to the radio axis. 相似文献
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P. Alexander 《Monthly notices of the Royal Astronomical Society》2000,319(1):8-16
An analytical model is presented for the evolution of powerful double radio sources on small physical scales less than about 100 kpc when radiative losses can be neglected. The self-similar model of Kaiser & Alexander is extended to allow for expansion in an atmosphere with a King profile. Distribution functions for the number of sources in a logarithmic interval of linear size within a flux-limited sample are calculated and compared with observation. The observational data can be reproduced if it is assumed that there exists a population of sources that evolve and survive to sizes greater than the core radius, together with a population that suffer disruption of their jets before escaping the core radius. The latter population, while they may be regarded as frustrated sources, are not old sources, but just short-lived. 相似文献
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P. Alexander 《Monthly notices of the Royal Astronomical Society》2006,368(3):1404-1410
Observations of compact symmetric double sources suggest that these objects with physical scales of order tens of parsecs to kiloparsecs are young radio active galactic nuclei. There is, in general, a striking similarity between the structures of these compact objects and the structures of large evolved radio galaxies although the latter are two to three orders of magnitude larger. This has led to the use of self-similar models of the evolution of radio sources as a framework for interpreting observational data. However, the assumptions on which the self-similar models are based become increasingly less valid on the physical scales which are probed by the observations of the smallest sources. In this paper, the dynamics of sources on these small scales is examined and a model developed which extends earlier work in a self-consistent way to small physical scales. The limit of applicability of the models is identified as is the transition from an early evolutionary phase to the self-similar phase of expansion. 相似文献
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We present deep near-infrared images, taken with the Subaru Telescope, of the region around the z =1.08 radio source 3C 356 which show it to be associated with a poor cluster of galaxies. We discuss evidence that this cluster comprises two subclusters traced by the two galaxies previously proposed as identifications for 3C 356, which both seem to harbour active galactic nuclei, and which have the disturbed morphologies expected if they underwent an interpenetrating collision at the time the radio jets were triggered. We explain the high luminosity and temperature of the diffuse X-ray emission from this system as the result of shock heating of intracluster gas by the merger of two galaxy groups. Taken together with the results on other well-studied powerful radio sources, we suggest that the key ingredient for triggering a powerful radio source, at least at epochs corresponding to z ∼1 , is a galaxy–galaxy interaction which can be orchestrated by the merger of their parent subclusters. This provides an explanation for the rapid decline in the number density of powerful radio sources since z ∼1 . We argue that attempts to use distant radio-selected clusters to trace the formation and evolution of the general cluster population must address ways in which X-ray properties can be influenced by the radio source, both directly, by mechanisms such as inverse Compton scattering, and indirectly, by the fact that the radio source may be preferentially triggered at a specific time during the formation of the cluster. 相似文献
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On the redshift cut-off for steep-spectrum radio sources 总被引：1，自引：0，他引：1
Matt J. Jarvis Steve Rawlings Chris J. Willott Katherine M. Blundell Steve Eales Mark Lacy 《Monthly notices of the Royal Astronomical Society》2001,327(3):907-917
We use three samples (3CRR, 6CE and 6C*) selected at low radio frequency to constrain the cosmic evolution in the radio luminosity function (RLF) for the 'most luminous' steep-spectrum radio sources. Though intrinsically rare, such sources give the largest possible baseline in redshift for the complete flux-density-limited samples currently available. Using parametric models to describe the RLF, incorporating distributions in radio spectral shape and linear size, as well as the usual luminosity and redshift, we find that the data are consistent with a constant comoving space density between and . We find that this model is favoured over a model with similar evolutionary behaviour to that of optically selected quasars (i.e., a roughly Gaussian distribution in redshift) with probability ratios of and for spatially flat cosmologies with and respectively. Within the uncertainties, this evolutionary behaviour may be reconciled with the shallow decline preferred for the comoving space density of flat-spectrum sources by Dunlop & Peacock and Jarvis & Rawlings, in line with the expectations of unified schemes. 相似文献
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Chris J. Willott Steve Rawlings Katherine M. Blundell 《Monthly notices of the Royal Astronomical Society》2001,324(1):1-17
We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey (7CRS) which lack strong emission lines and we use these data to investigate their spectral energy distributions (SEDs) with models that constrain their redshifts. Six of these seven galaxies have extremely red colours ( R − K >5.5) and we find that almost all of them lie in the redshift range 1< z <2. We also present near-infrared spectroscopy of these galaxies which demonstrate that their SEDs are not dominated by emission lines, although tentative lines, consistent with H α at z =1.45 and z =1.61, are found in two objects. Although the red colours of the 7CRS galaxies can formally be explained by stellar populations that are either very old or young and heavily reddened, independent evidence favours the former hypothesis. At z ∼1.5 at least 1/4 of powerful radio jets are triggered in massive (> L *) galaxies, which formed the bulk of their stars several Gyr earlier, that is at epochs corresponding to redshifts z ≳5. If a similar fraction of all z ∼1.5 radio galaxies are old, then extrapolation of the radio luminosity function shows that, depending on the radio source lifetimes, between 10 and 100 per cent of the near-IR selected extremely red object (ERO) population undergo a radio outburst at epochs corresponding to 1< z <2. An ERO found serendipitously in the field of one of the 7CRS radio sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission line likely to be [O ii ] at z =1.20. The implication is that some of the most massive elliptical galaxies formed the bulk of their stars at z ≳5 and these objects probably undergo at least two periods of active galactic nucleus activity: one at high redshift during which the black hole forms and another one at an epoch corresponding to z ∼1.5. 相似文献
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M. Bliton E. Rizza J. O. Burns F. N. Owen & M. J. Ledlow 《Monthly notices of the Royal Astronomical Society》1998,301(3):609-625
We examine the ROSAT PSPC X-ray properties of a sample of 15 Abell clusters containing 23 narrow-angle tailed (NAT) radio galaxies. We find that clusters with NATs show a significantly higher level of substructure than a similar sample of radio-quiet clusters, indicating that NAT radio sources are preferentially located in dynamically complex systems. Also, the velocity distribution of the NAT galaxies is similar to that of other cluster members; these velocities are inadequate for producing the ram pressure necessary to bend the radio jets. We therefore propose a new model for NAT formation, in which NATs are associated with dynamically complex clusters undergoing merger events. The U -shaped NAT morphology is produced in part by the merger-induced bulk motion of the ICM bending the jets. 相似文献
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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
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ROSAT X-ray observations of 3CRR radio sources 总被引：1，自引：0，他引：1
M. J. Hardcastle D. M. Worrall 《Monthly notices of the Royal Astronomical Society》1999,309(4):969-990
Over half the 3CRR sample of radio galaxies and quasars has been observed in X-rays with ROSAT pointed observations, and we present results from these observations, discussing many of the sources in detail. The improved spatial resolution of ROSAT over earlier missions allows a better separation of the nuclear and extended components of the X-ray emission. We investigate the relationship between nuclear X-ray and core radio luminosity, and show that our results support a model in which every radio galaxy and quasar has a beamed nuclear soft X-ray component directly related to the radio core. We report evidence for rich cluster environments around several powerful quasars. These X-ray environments are comparable to those of high-redshift radio galaxies. 相似文献
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M. Magliocchetti A. Celotti L. Danese 《Monthly notices of the Royal Astronomical Society》2002,329(2):377-384
This paper presents a detailed analysis of the radio properties for the sample of faint radio sources introduced by Magliocchetti et al. in 2000. The sample comprises mainly intrinsically low-power sources, the majority of which (≳70 per cent) are FR I radio galaxies. These objects show some degree (at 1 σ confidence level) of luminosity evolution, which is also needed to reproduce correctly the total number and shape of the counts distribution at 1.4 GHz. Analysis of the de-evolved local radio luminosity function shows a good agreement between data and model predictions for this class of sources. Particular care has been devoted to the issue of 'lined' galaxies (i.e. objects presenting in their spectra a continuum typical of early-type galaxies plus emission lines of different nature), which appear as an intermediate class of sources between AGN-dominated and starburst galaxies. Different evolutionary behaviour is seen in the two subpopulations of lined and non-lined low-power radio galaxies, the first class indicating a tendency for the radio luminosity to decrease with look-back time, the second one showing positive evolution. We note that different evolutionary properties also seem to characterize BL Lacs selected in different bands, so that one might envisage an association between lined FR I and the subclass of BL Lacs selected in the X-ray band. Lastly, we find evidence for a negligible contribution of starburst galaxies at these low flux levels. 相似文献