共查询到20条相似文献，搜索用时 953 毫秒
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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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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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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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Paul A. Jones Ben D. Lloyd W. Bruce McAdam 《Monthly notices of the Royal Astronomical Society》2001,325(2):817-825
Centaurus B (PKS B1343−601) is one of the brightest and closest radio galaxies, with flux density ∼250 Jy at 408 MHz and redshift 0.01215, but it has not been studied much because of its position (i) close to the Galactic plane (it is also known as G309.6+1.7 and Kes 19) and (ii) in the southern sky. It has recently been suggested as the centre of a highly obscured cluster behind the Galactic plane. We present radio observations made with the Australia Telescope Compact Array and Molonglo Observatory Synthesis Telescope to study the jets and lobes. The total intensity and polarization radio images of the FR I jets are used to determine the jet brightness and width variations, magnetic field structure and fractional polarization. The equipartition pressure calculated along the jets declines rapidly over the first 1 arcmin from the galaxy reaching a constant pressure of 10−13 h −4/7 Pa in the lobes blown in the intracluster medium. 相似文献
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A.H.C. Thean T.I. Gillibrand A. Pedlar M.J. Kukula 《Monthly notices of the Royal Astronomical Society》2001,327(2):369-384
We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is , and approximately 25 per cent of the components have a spectral index flatter than . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions. 相似文献
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is , and approximately 25 per cent of the components have a spectral index flatter than . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions. 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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J. Afonso B. Mobasher A. Hopkins A. Georgakakis L. Cram B. Chan 《Astrophysics and Space Science》2003,285(1):149-153
A strong evolution of galaxies is observed for 0<z<1, as evidenced by an increase of almost an order of magnitude in the galaxy star-formation rate density. However, it is known
that dust obscuration has affected our understanding of galaxy evolution over this significant fraction of the age of the
Universe. In order to study galaxy evolution free from dust induced biases, an ultra deep radio survey – the Phoenix Deep
Survey – was initiated. With a detection limit of 60μJy, this homogeneous survey, complemented with multiwavelength (photometric
and spectroscopic) observations, is being used to build a consistent picture of galaxy evolution. The ultra-deep radio source
counts are presented, and interpreted using luminosity function evolutionary models. The discovery of extremely dusty galaxies
from this survey, along with the clustering properties of the sub-mJy radio population, are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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R.J. Beswick A. Pedlar† A.J. Holloway 《Monthly notices of the Royal Astronomical Society》2002,329(3):620-628
We have observed broad H i absorption in the radio galaxy 3C 293 using Multi-Element Radio Linked Interferometric Network (MERLIN) at 0.2-arcsec angular resolution and the Giant Meterwavelength Radio Telescope (GMRT) at arcsec resolution. Extensive H i is found in absorption across the centre of this peculiar radio galaxy, allowing a detailed study of the dynamics of the neutral gas on linear scales down to ∼160 pc. In optical depth position–velocity diagrams across the central few kpc we detect a distinct velocity gradient of 179 km s−1 arcsec−1 associated with the broad absorption. This is interpreted as a ring of neutral gas rotating around the suspected position of the active galactic nucleus (AGN) . The radius of this high velocity gradient ring is found to be >0.74 arcsec (600 pc), implying an upper limit upon the enclosed mass of , assuming a near edge-on disc with an inclination of i . The optical depth of H i is mapped across the entire central region of 3C 293 showing enhancements of a factor of 4 in the areas that are co-spatial with dust lanes seen in Hubble Space Telescope ( HST ) imaging of this galaxy. 相似文献
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StephenSerjeant CarlottaGruppioni SebOliver 《Monthly notices of the Royal Astronomical Society》2002,330(3):621-624
We present a new determination of the local volume-averaged star formation rate from the 1.4-GHz luminosity function of star forming galaxies. Our sample, taken from the B ≤12 Revised Shapley–Ames catalogue (231 normal spiral galaxies over an effective area of 7.1 sr) has ≃100 per cent complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fitting Schechter function has a faint-end slope of −1.27±0.07 in agreement with the local H α luminosity function, characteristic luminosity L ∗=(2.6±0.7)×1022 W Hz−1 and density φ ∗=(4.8±1.1)×10−4 Mpc−3 . The inferred local radio luminosity density of (1.73±0.37±0.03)×1019 W Hz−1 Mpc−3 (Poisson noise, large-scale structure fluctuations) implies a volume-averaged star formation rate ∼2 times larger than the Gallego et al. H α estimate, i.e. ρ 1.4 GHz =(2.10±0.45±0.04)×10−2 M⊙ yr−1 Mpc−3 for a Salpeter initial mass function from 0.1–125 M⊙ and Hubble constant of 50 km s−1 Mpc−1 . We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical–ultraviolet (UV) star formation rates (SFRs) are easily underestimated, particularly at high redshift. 相似文献
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Several radio galaxies are known that show radio morphological signatures that are best interpreted as restarting of nuclear activity after a period of quiescence. The conditions surrounding the phenomenon of nuclear recurrence are not understood. In this paper we have attempted to address this question by examining the nuclear fuelling characteristics in a sample of restarting radio galaxies. We have examined the detection rate for molecular gas in a representative sample of nine restarting radio galaxies, for seven of which we present new upper limits to the molecular gas mass derived from CO line observations we made with the IRAM 30-m telescope. We derive a low CO detection rate for the relatively young restarted radio galaxies suggesting that the cessation of the nuclear activity and its subsequent restarting may be a result of instabilities in the fuelling process rather than a case of depletion of fuel followed by a recent fuel acquisition. It appears that abundant molecular gas content at the level of few 108 –109 M⊙ does not necessarily accompany the nuclear restarting phenomenon. For comparison we also discuss the molecular gas properties of five normal giant radio galaxies, three of which we observed using Swedish-ESO Millimetre Telescope (SEST). Despite obvious signs of interactions and nuclear dust discs none of them has been found to host significant quantities of molecular gas. 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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