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Precession of the super-massive black hole in NGC 1275 (3C 84)?
The X-ray holes at the centre of the Perseus cluster of galaxies are notall at the same position angle with respect to the centre of thecluster. This configuration would result if the jet inflating thebubbles is precessing, or moving around, and the bubbles detach atdifferent times. The orientations which best fit the observed traveldirections are an inclination of the precession axis to the line ofsight of 120° and an opening angle of 50°. From the time-scalesfor the bubbles seen in the cluster, the precession time-scale,τprec, is around 3.3 × 107yr. Thebubbles rising up through different parts of the cluster may haveinteracted with the central cool gas, forming the whorl of cool gasobserved in the temperature structure of the cluster. The dynamics ofbubbles rising in fluids is discussed. The conditions present in thecluster are such that oscillatory motion, observed for bubbles rising influids on Earth, should take place. However, the time-scale for thismotion is longer than that taken for the bubbles to evolve intospherical-cap bubbles, which do not undergo a path instability, so suchmotion is not expected to occur.
| Orientation and size of the `Z' in X-shaped radio galaxies
Some X-shaped radio galaxies show a Z-symmetric morphology in the lessluminous secondary lobes. Within the scenario of a merger between twogalaxies, each hosting a supermassive black hole in its centre, thisstructure has been explained before. As the smaller galaxy spiralstowards the common centre, it releases gas to the interstellar medium ofthe larger active galaxy. The ram pressure of this streaming gas willbend the lobes of the pre-merger jet into a Z-shape. After the blackholes have merged, the jet propagates in a new direction that is alignedwith the angular momentum of the binary black hole. In this paper wedeproject the pre- and post-merger jets. Taking into account theexpected angles between the jet pairs and with the assumption that theirdirections are uncorrelated, we show that one of three possibleorientations of the jets with respect to the line of sight is morelikely than the others. This actually depends on the distance where thebending occurs. Another result of our deprojection is that the streaminggas bends the jet into a Z-shape in a range between about 30 and 100 kpcdistance to the centre of the primary galaxy. We confirm this finding bycomparing our predictions for the properties of the rotational velocityfield and its radius with observations and numerical simulations ofmerging galaxies. Thus, our results support the merger scenario asexplanation for X- and Z-shaped radio galaxies with the jet pointingalong the former axis of orbital angular momentum of the binary.
| Observational Evidence of Jet Precession in Galactic Nuclei Caused by Accretion Disks
We show that the observational data of extragalactic radio sources tendto support the theoretical relationship between the jet precessionperiod and the optical luminosity of the sources, as predicted by themodel in which an accretion disk causes the central black hole toprecess.
| Canonical Particle Acceleration in FR I Radio Galaxies
Matched-resolution multifrequency VLA observations of four radiogalaxies are used to derive the asymptotic low-energy slope of therelativistic electron distribution. When available, low-energy slopesare also determined for other sources in the literature. They provideinformation on the acceleration physics independent of radiative andother losses, which confuse measurements of the synchrotron spectra inmost radio, optical, and X-ray studies. We find a narrow range ofinferred low-energy electron energy slopes n(E)~E-2.1 for thecurrently small sample of lower luminosity sources classified as FR I(not classical doubles). This distribution is close to, but apparentlyinconsistent with, the test particle limit of n(E)~E-2.0expected from strong diffusive shock acceleration in the nonrelativisticlimit. Relativistic shocks or those modified by the back-pressure ofefficiently accelerated cosmic rays are two alternatives to producesomewhat steeper spectra. We note for further study the possibility ofacceleration through shocks, turbulence, or shear in the flaring andbrightening regions in FR I jets as they move away from the nucleus.Jets on parsec scales and the collimated jets and hot spots of FR II(classical double) sources would be governed by different accelerationsites and mechanisms; they appear to show a much wider range of spectrathan those for FR I sources.
| A Chandra Study of the Multicomponent X-Ray Emission from the X-shaped Radio Galaxy 3C 403
We present results from a 49.4 ks Chandra ACIS-S observation of thenearby (z=0.059) X-shaped FR type II (FR II) radio galaxy 3C 403. Thisis the first Chandra observation of an X-shaped radio galaxy, and one ofthe goals of this pioneering study is to determine the relationshipbetween the X-ray-emitting gas and the X-shaped radio morphology. Wefind that the X-ray isophotes of the hot gas within ~3.5" of the centralgalaxy are highly elliptical (eccentricity ~0.57) and coaligned with theelliptical optical isophotes. This supports the hypothesis that X-shapedradio sources are created by propagation of jets through asymmetricdensity distributions. Within large uncertainties, there is no evidencethat the lobes or wings are overpressurized relative to the interstellarmedium (ISM), supporting the scenario in which the wings are the resultof strong backflows of material from the jet head and subsequent buoyantevolution. We have detected X-ray emission from several of the radioknots to the east of the active nucleus and diffuse emission from theradio lobe to the west. The X-ray emission from the eastern knots cannotbe explained by an inverse Compton model unless they are far fromequipartition. Using archival Hubble Space Telescope (HST) data, opticalemission is detected from two knots, and the radio/optical/X-ray spectraare well fitted by simple synchrotron models. This is one of thestrongest examples to date of X-ray synchrotron emission from multipleknots in the jet of an FR II radio galaxy. X-ray emission is alsodetected from the radio wings at a flux consistent with inverse Comptonscattering of cosmic microwave background (CMB) photons fromrelativistic electrons if the wings are near equipartition. The nuclearspectrum is well described by a multicomponent model that includes aheavily absorbed power law (NH~4×1023cm-2) and a bright (EW~244 eV), broadened Fe line. A second,less absorbed, power-law component, likely representing unresolvedemission from a parsec-scale jet, is also required.
| The Bologna Complete Sample of Nearby Radio Sources
We present a new, complete sample of 95 radio sources selected from theB2 Catolog of Radio Sources and the Third Cambridge Revised Catalog(3CR), with z<0.1. Since no selection effect on the core radio power,jet velocity, or source orientation is present, this sample is wellsuited for statistical studies. In this first paper we present theobservational status of all sources on the parsec (milliarcsecond) andkiloparsec (arcsecond) scale; we give new parsec-scale data for 28sources and discuss their parsec-scale properties. By combining thesedata with those in the literature, information on the parsec-scalemorphology is available for a total of 53 radio sources with differentradio power and kiloparsec-scale morphologies. We investigate theirproperties. We find a dramatically higher fraction of two-sided sourcesin comparison with that of previous flux-limited VLBI surveys.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| A transition in the accretion properties of radio-loud active nuclei
We present evidence for the presence of a transition in the accretionproperties of radio-loud sources. For a sample of radio galaxies andradio-loud quasars, selected based on their extended radio properties,the accretion rate is estimated from the black hole mass and nuclearluminosity. The inferred distribution is bimodal, with a paucity ofsources at accretion rates, in Eddington units, of the order of~10-2- assuming a radiative efficiency of 10 per cent - andpossibly spanning 1-2 orders of magnitude. Selection biases are unlikelyto be responsible for such behaviour. We discuss possible physicalexplanations, including a fast transition to low accretion rates, achange in the accretion mode/actual accretion rate/radiative efficiency,the lack of stable disc solutions at intermediate accretion rates or theinefficiency of the jet formation processes in geometrically thin flows.This transition might be analogous to spectral states (and jet)transitions in black hole binary systems.
| Optical nuclei of radio-loud AGN and the Fanaroff-Riley divide
We investigate the nature of the point-like optical nuclei that havebeen found in the centres of the host galaxies of a majority of radiogalaxies by the Hubble Space Telescope. We examine the evidence thatthese optical nuclei are relativistically beamed, and look fordifferences in the behaviour of the nuclei found in radio galaxies ofthe two Fanaroff-Riley types. We also attempt to relate this behaviourto the properties of the optical nuclei in their highly beamedcounterparts (the BL Lac objects and radio-loud quasars) as hypothesizedby the simple Unified Scheme. Simple model-fitting of the data suggeststhat the emission may be coming from a non-thermal relativistic jet. Itis also suggestive that the contribution from an accretion disk is notsignificant for the FRI objects and for the narrow-line radio galaxiesof FRII type, while it may be significant for the Broad-line objects,and consistent with the idea that the FRII optical nuclei seem to sufferfrom extinction due to an obscuring torus while the FRI optical nucleido not. These results are broadly in agreement with the Unified Schemefor radio-loud AGNs.Appendix C is only available in electronic form athttp://www.edpsciences.org
| Unifying B2 radio galaxies with BL Lacertae objects
In an earlier paper we presented nuclear X-ray flux densities, measuredwith ROSAT, for the B2 bright sample of nearby low-luminosity radiogalaxies. In this paper we construct a nuclear X-ray luminosity functionfor the B2 radio galaxies, and discuss the consequences of our resultsfor models in which such radio galaxies are the parent population of BLLacertae (BL Lac) objects. Based on our observations of the B2 sample,we use Monte Carlo techniques to simulate samples of beamed radiogalaxies, and use the selection criteria of existing samples of BL Lacobjects to compare our simulated results to what is observed. We findthat previous analytical results are not applicable since the BL Lacsamples are selected on beamed flux density. A simple model in which BLLacs are the moderately beamed (γ~ 3) counterparts of radiogalaxies, with some random dispersion (~0.4 decades) in the intrinsicradio-X-ray relationship, can reproduce many of the features of theradio-selected and X-ray-selected BL Lac samples, including their radioand X-ray luminosity functions and the distributions of theirradio-to-X-ray spectral indices. In contrast, models in which the X-rayand radio emission have systematically different beaming parameterscannot reproduce important features of the radio-galaxy and BL Lacpopulations, and recently proposed models in which the radio-to-X-rayspectral index is a function of source luminosity cannot in themselvesaccount for the differences in the slopes of the radio- andX-ray-selected BL Lac luminosity functions. The redshift distributionand number counts of the X-ray-selected Einstein Medium SensitivitySurvey (EMSS) sample are well reproduced by our best models, supportinga picture in which these objects are beamed Fanaroff-Riley type I radiogalaxies with intrinsic luminosities similar to those of the B2 sample.However, we cannot match the redshift distribution of the radio-selected1-Jy sample, and it is likely that a population of Fanaroff-Riley typeII radio galaxies is responsible for the high-redshift objects in thissample, in agreement with previously reported results on the sample'sradio and optical emission-line properties.
| An X-Ray Atlas of Groups of Galaxies
A search was conducted for a hot intragroup medium in 109 low-redshiftgalaxy groups observed with the ROSAT PSPC. Evidence for diffuse,extended X-ray emission is found in at least 61 groups. Approximatelyone-third of these detections have not been previously reported in theliterature. Most of the groups are detected out to less than half of thevirial radius with ROSAT. Although some spiral-rich groups do contain anintragroup medium, diffuse emission is restricted to groups that containat least one early-type galaxy.
| The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes
Existing radio images of a few X-shaped radio galaxies revealZ-symmetric morphologies in their weaker secondary lobes that cannot benaturally explained by either the galactic merger or radio-lobe backflowscenarios, the two dominant models for these X-shaped radio sources. Weshow that the merger picture can explain these morphologies provided onetakes into account that, prior to the coalescence of their supermassiveblack holes, the smaller galaxy releases significant amounts of gas intothe interstellar medium of the dominant active galaxy. This rotatinggas, whose angular momentum axis will typically not be aligned with theoriginal jets, is likely to provide sufficient ram pressure at adistance ~10 kpc from the nucleus to bend the extant jets emerging fromthe central engine, thus producing a Z-symmetry in the pair of radiolobes. Once the two black holes have coalesced some 107 yrlater, a rapid reorientation of the jets along a direction close to thatof the orbital angular momentum of the swallowed galaxy relative to theprimary galaxy would create the younger primary lobes of the X-shapedradio galaxy. This picture naturally explains why such sources typicallyhave powers close to the FR I/II break. We suggest that purelyZ-symmetric radio sources are often en route to coalescence and theconcomitant emission of substantial gravitational radiation, whileX-shaped ones have already merged and radiated.
| Double-peaked Low-Ionization Emission Lines in Active Galactic Nuclei
We present a new sample of 116 double-peaked Balmer line active galacticnuclei (AGNs) selected from the Sloan Digital Sky Survey. Double-peakedemission lines are believed to originate in the accretion disks of AGNs,a few hundred gravitational radii (RG) from the supermassiveblack hole. We investigate the properties of the candidate disk emitterswith respect to the full sample of AGNs over the same redshifts,focusing on optical, radio, and X-ray flux, broad-line shapes andnarrow-line equivalent widths, and line flux ratios. We find that thedisk emitters have medium luminosities (~1044 ergss-1) and FWHM on average 6 times broader than the AGNs in theparent sample. The double-peaked AGNs are 1.6 times more likely to beradio sources and are predominantly (76%) radio-quiet, with about 12% ofthe objects classified as LINERs. Statistical comparison of the observeddouble-peaked line profiles with those produced by axisymmetric andnonaxisymmetric accretion disk models allows us to impose constraints onaccretion disk parameters. The observed Hα line profiles areconsistent with accretion disks with inclinations smaller than 50°,surface emissivity slopes of 1.0-2.5, outer radii larger than~2000RG, inner radii of (200-800)RG, and localturbulent broadening of 780-1800 km s-1. The comparisonsuggests that 60% of accretion disks require some form of asymmetry(e.g., elliptical disks, warps, spiral shocks, or hot spots).
| The gaseous environments of radio galaxies
X-ray emission traces the gaseous environments of radio sources. Themedium must be present for jet confinement, but what are its influenceon jet fueling, dynamics, propagation, and disruption? The observationalsituation is both complicated and enriched by radio sources beingmulti-component X-ray emitters, with several possible regions ofnon-thermal emission. Recent work, primarily based on sensitive ROSATpointings, is used to contrast the X-ray emission and environments ofradio sources with: (a) low power, (b) high power at high redshift, (c)high power at lower redshift, and (d) GHz peaked spectrum emission. Thetrends in external gas density and pressure near extended radiostructures are reviewed. Imminently-available X-ray measurements withvastly improved resolution and sensitivity have great potential forresolving many open issues.
| Jet reorientation in active galactic nuclei: two winged radio galaxies
Winged, or X-shaped, radio sources form a small class of morphologicallypeculiar extragalactic sources. We present multifrequency radioobservations of two such sources. We derive maximum ages since anyre-injection of fresh particles of 34 and 17Myr for the wings of 3C223.1and 3C403 respectively, based on the lack of synchrotron and inverseCompton losses. On morphological grounds we favour an explanation interms of a fast realignment of the jet axis which occurred within a fewMyr. There is no evidence for merger activity, and the host galaxies arefound to reside in no more than poor cluster environments. A number ofpuzzling questions remain about those sources: in particular, althoughthe black hole could realign on sufficiently short time-scales, theorigin of the realignment is unknown.
| HST images of B2 radio galaxies: A link between circum-nuclear dust and radio properties?
Almost 60% of the B2 low luminosity radio galaxies have been observedwith the Hubble Space Telescope. We present an analysis of the dustfeatures, which are often present in the form of circum-nuclear disks orlanes, and show that there are correlations between radio source anddust properties. It is found that nearby radio sources in which a jethas been detected tend to have dust more often than sources withoutjets; the dust is often in the form of disks or lanes. Moreover theradio jets are close to perpendicular to the disk or lane in the weakerradio sources (with P < 1024 WHz-1). Instronger sources the orientation effect appears to be weak or evenabsent. Also the dust masses found in the weaker radio sources aresmaller than in the stronger ones (log M/Msun ~ 3 against 5respectively). More generally it appears that there is a correlationbetween dust mass and total radio power (for those sources in which dusthas been detected); we show that this correlation is not induced byredshift. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA, Inc., under NASA contract NAS 5-26555 and by STScIgrant GO-3594.01-91A.
| On the origin of X-shaped radio-sources: New insights from the properties of their host galaxies
A significant fraction of extended radio sources presents a peculiarX-shaped radio morphology: in addition to the classical double lobedstructure, radio emission is also observed along a second axis ofsymmetry in the form of diffuse wings or tails. We re-examine the originof these extensions relating the radio morphology to the properties oftheir host galaxies. The orientation of the wings shows a strikingconnection with the structure of the host galaxy as they arepreferentially aligned with its minor axis. Furthermore, wings are onlyobserved in galaxies of high projected ellipticity. Hydrodynamicalsimulations of the radio-source evolution show that X-shapedradio-sources naturally form in this geometrical situation: as a jetpropagates in a non-spherical gas distribution, the cocoon surroundingthe radio-jets expands laterally at a high rate producing wings of radioemission, in a way that is reminiscent of the twin-exhaust model forradio-sources. Based on observations obtained at the Italian TelescopioNazionale Galileo (TNG) operated on the island of La Palma by the CentroGalileo Galilei of the CNAA (Consorzio Nazionale per l'Astronomia el'Astrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| The HST survey of the B2 sample of radio-galaxies: Optical nuclei and the FR I/BL Lac unified scheme
We examine the optical properties of the nuclei of low luminosityradio-galaxies using snapshot HST images of the B2 sample. In agreementwith the results obtained from the analysis of the brighter 3C/FR Isample, we find a correlation between fluxes (and luminosities) of theoptical and radio cores. This provides further support for theinterpretation that the optical nuclear emission in FR I is dominated bysynchrotron emission and that accretion in these sources takes place ina low efficiency radiative regime. In the framework of the FR I/BL Lacsunified scheme, we find that the luminosity difference between FR I andBL Lac nuclei can be reproduced with a common beaming factor in both theradio and the optical band, independent of the extended radioluminosity, thus supporting such a scenario. The corresponding bulkLorentz factor is significantly smaller than is expected fromobservational and theoretical considerations in BL Lacs: this can beinterpreted as due to a velocity structure in the jet, with a fast spinesurrounded by a slower layer. Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by AURA, Inc., under NASA contract NAS5-26555 and by STScI grant GO-3594.01-91A.
| A new list of extra-galactic radio jets
A catalogue of extra-galactic jets is very useful both in observationaland theoretical studies of active galaxies. With the use of new powerfulradio instruments, the detailed structures of very compact or weak radiosources are investigated observationally and many new radio jets aredetected. In this paper, we give a list of 661 radio sources withdetected radio jets known to us prior to the end of December 2000. Allreferences are collected for the observations of jets in radio, IR,optical, UV and X-ray wave-bands. Table 1 and references to Table 1 areonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/757
| A multi-frequency study of the radio galaxy NGC 326. I. The data
We present the results of a multi-frequency study of the inversionsymmetric radio galaxy NGC 326 based on Very Large Array observations at1.4, 1.6, 4.8, 8.5 and 14.9 GHz. The morphological, spectral andpolarization properties of this peculiar object are studied at differentlevels of spatial resolutions. The interpretation of the data will bediscussed in forthcoming papers.
| X-Ray-emitting Atmospheres of B2 Radio Galaxies
We report ROSAT PSPC spatial and spectral analyses of the eight B2 radiogalaxies NGC 315, NGC 326, 4C 35.03, B2 0326+39, NGC 2484, B2 1040+31,B2 1855+37, and 3C 449, expected to be representative of the class oflow-power radio galaxies. Multiple X-ray components are present in each,and the gas components have a wide range of linear sizes and follow anextrapolation of the cluster X-ray luminosity/temperature correlation,implying that there is no relationship between the presence of a radiogalaxy and the gas fraction of the environment. No large-scale coolingflows are found. There is no correlation of radio-galaxy size with thescale or density of the X-ray atmosphere. This suggests that processeson scales smaller than those of the overall gaseous environments are themajor influence on radio-source dynamics. The intergalactic medium isusually sufficient to confine the outer parts of the radio structures,in some cases even to within 5 kpc of the core. In the case of NGC 315,an extrapolation suggests that the pressure of the atmosphere may matchthe minimum pressure in the radio source over a factor of ~40 in linearsize (a factor of ~1600 in pressure).
| The HST snapshot survey of the B2 sample of low luminosity radio-galaxies: a picture gallery
A Hubble Space Telescope snapshot survey of the B2 sample of lowluminosity radio galaxies has, at present, produced V and I images of 41objects. Together with 16 images of B2 sources taken from the HSTarchive, there are now high resolution optical data for ~ 57% of thesample. All host galaxies are luminous ellipticals, except one which isa spiral galaxy, while another one turns out to be a misidentification.We present an album of the images of the B2 radio galaxies observed sofar, and give a brief description of the optical morphology of thegalaxies. Dust features (in the form of disks, lanes or irregularpatches) are seen in most of the galaxies of the sample, ~ 58%. Compactoptical cores are also very common (18/57). A preliminary analysis hasrevealed the presence of an optical jet in three objects, indicatingthey can be detected in a sizeable percentage in these low luminosityradio sources. Brightness profiles of dust-free galaxies are wellrepresented by a Nuker law and all shows the existence of a resolvedshallow cusp. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by AURA, Inc., under NASA contract NAS 5-26555 and by STScIgrant GO-3594.01-91A
| Weak Radio Galaxies. II. Narrow-Band Optical Imaging and Physical Conditions
Not Available
| X-ray observations of low-power radio galaxies from the B2 catalogue
We present an analysis of X-ray data, taken with ROSAT, for awell-defined sample of low-power radio galaxies from the Bologna B2catalogue. Where possible, the HRI has been used in order to takeadvantage of the high spatial resolution provided by this instrument. Avariety of models are fitted to radial profiles in order to separate theresolved and unresolved X-ray emission from the galaxies. We demonstratea strong, approximately linear, correlation between the luminosities ofthe unresolved X-ray components and the 5-GHz luminosities of the radiocores in this sample. This suggests a physical relationship between thesoft X-ray emission of radio galaxies and the jet-generated radio coreemission. We infer a nuclear jet-related origin for at least some of theX-ray emission.
| Redshift and velocity dispersion of the cluster of galaxies around NGC 326
Redshifts of several galaxies thought to be associated with NGC 326 aredetermined. The results confirm the presence of a cluster and find amean redshift of z=0.0477+/-0.0007 and a line-of-sight velocitydispersion sigma_z=599 (+230,-110)kms^-1. The velocity dispersion andpreviously measured X-ray gas temperature of kT~=1.9keV are consistentwith the cluster sigma_zkT relation, and NGC 326 is seen to be a slowlymoving member of the cluster.
| Asymmetries in the jets of weak radio galaxies
We describe a study of the side-to-side asymmetries on kpc scales inthe jets of FR I radio galaxies selected from the B2 sample. The basicdata are jet surface brightnesses and widths determined by fittingtransverse profiles to Very Large Array (VLA) images at a range ofdistances from the core. Differences between the jets at a givendistance from the nucleus are interpreted as effects of Doppler beamingon intrinsically symmetrical flows and are compared with the modelderived for 3C31 by Laing & Bridle and with simpler variants. Thejet/counterjet brightness ratios where the main jet first brightens arecorrelated with core prominence, as expected for a relativistic flow.From the distribution of brightness ratios, we infer that jets have amaximum velocity ~0.9c where they first flare and brighten, but there isalso evidence for additional slower material. Deceleration tosubrelativistic speeds occurs on scales which increase with radio power.Jets in the majority of sources with luminosities <1024WHz-1 at 1.4 GHz become essentially symmetrical (andtherefore subrelativistic) within 2 kpc of the core. In more powerfulsources, jets that flare within the first 2 kpc become symmetrical by 10kpc, but a subset of the most luminous objects has jets which remainasymmetrical to larger distances. The point at which the brighter jetflares appears to correspond to a sudden increase in rest-frameemissivity, but the ratio of distances to the flaring point in main andcounterjets is anticorrelated with brightness ratio, as expected for adecelerating relativistic flow. Brightness and full width athalf-maximum (FWHM) ratios are also anticorrelated, an effect which weinterpret as a result of Doppler beaming for a flow in which thevelocity decreases radially outwards from the jet axis. Jet decelerationby entrainment of external material provides a natural explanation forthese velocity gradients. The jet energy flux is roughly consistent withenergy supply to the lobes over a source lifetime estimated fromspectral index measurements. Our results are qualitatively consistentwith unified models of FR I radio galaxies and BL Lac objects, butrequire some modifications to the standard picture.
| The peculiar motions of early-type galaxies in two distant regions - II. The spectroscopic data
We present the spectroscopic data for the galaxies studied in the EFARproject, which is designed to measure the properties and peculiarmotions of early-type galaxies in two distant regions. We have obtained1319 spectra of 714 early-type galaxies over 33 observing runs on 10different telescopes. We describe the observations and data reductionsused to measure redshifts, velocity dispersions and the Mgb and Mg_2Lick linestrength indices. Detailed simulations and intercomparison ofthe large number of repeat observations lead to reliable error estimatesfor all quantities. The measurements from different observing runs arecalibrated to a common zero-point or scale before being combined,yielding a total of 706 redshifts, 676 velocity dispersions, 676 Mgblinestrengths and 582 Mg_2 linestrengths. The median estimated errors inthe combined measurements are Delta cz=20 km s^-1, Delta sigma sigma=9.1 per cent, Delta Mgb Mgb=7.2 per cent and Delta Mg_2=0.015 mag.Comparison of our measurements with published data sets shows nosystematic errors in the redshifts or velocity dispersions, and onlysmall zero-point corrections to bring our linestrengths on to thestandard Lick system. We have assigned galaxies to physical clusters byexamining the line-of-sight velocity distributions based on EFAR andZCAT redshifts, together with the projected distributions on the sky. Wederive mean redshifts and velocity dispersions for these clusters, whichwill be used in estimating distances and peculiar velocities and to testfor trends in the galaxy population with cluster mass. The spectroscopicparameters presented here for 706 galaxies combine high-quality data,uniform reduction and measurement procedures, and detailed erroranalysis. They form the largest single set of velocity dispersions andlinestrengths for early-type galaxies published to date.
| The complex radio spectrum of 3C 130
I present four-frequency radio observations of the wide-angle tail radiogalaxy 3C 130. By a technique of simulatedobservations, I assess the systematic errors in the data due to thevarying uv plane coverage in the different observations used. Usingspectral tomography I show that, at least in the southern plume, thesource can be represented by a two-component model consisting of aflat-spectrum core and steep-spectrum sheath, as recently found in otherFRI sources. In addition, there is a strong difference in thehigh-frequency spectra of the northern and southern plumes. I discussthe implications of these observations for the source's jet dynamics andparticle acceleration, and discuss models of the class of WATs as awhole.
| Radiative ages in a representative sample of low luminosity radio galaxies
Two frequency observations, mainly at 1.4 and 5 GHz from the VLA, havebeen used to study spectral variations along the lobes of some nearbylow luminosity radio galaxies that constitute a representative sampleselected from the B2 catalogue. The variations of the spectral indexhave been interpreted as being due to synchrotron and inverse Comptonlosses and characteristic spectral ages are deduced for the relativisticelectrons. The radiative ages are in the range of several 10(7) years.These ages correlate well with the source sizes. They also appear to beconsistent with dynamical ages determined from ram-pressure arguments,if we make reasonable assumptions about the ambient gas density andallow for very moderate deviations from the equipartition conditions.There appears to be a significant difference between the radiative agesof sources in our sample and those of more powerful 3CR radio sources.We briefly discuss the possibility of re-acceleration processes andindicate some objects where these may occur.
| Bilateral symmetry in active galaxies.
Not Available
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