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A Multiwavelength Study of the Jets in FR-I Radio Galaxies: I. Data and Analysis
We compile a sample of 11 Fanaroff-Riley type I Radio Galaxies (FR-IRGs) with multi-wavelength observations to address the dynamic behaviorof jets in these objects. Optical images acquired by the Hubble SpaceTelescope (HST) are carefully analyzed. The method and reductionprocedure are described in detail. Unresolved optical cores emerge afterhaving properly removed starlight from the host galaxies in eight of theFR-I RGs, of which five are new identifications. Broad band spectralproperties of these newly identified compact cores are compared withthat previously found in FR-I RGs, as well as the low-energy-peaked BLLac objects. The similarity between them argues for the same non-thermalsynchrotron origin. Well-resolved optical jets with knotty morphologiesare found in three FR-I RGs in our sample, namely 3C 15, 3C 66B and B20755+37. The optical counterparts to the inner radio/X-ray jets areidentified and a clear one-to-one correspondence between the optical,radio and X-ray knots is found. The structure and information on theoptical jets are discussed. Physical parameters such as the knotsposition, flux and size are also presented. Detailed comparison betweenthe multi-wavelength data and radiative and dynamic models of jet willbe made in a forthcoming paper.
| Kiloparsec-Scale Jets in FR I Radio Galaxies and the γ-Ray Background
We discuss the contribution of kiloparsec-scale jets in FR I radiogalaxies to the diffuse γ-ray background radiation. The analyzedγ-ray emission comes from inverse-Compton scattering of starlightphoton fields by the ultrarelativistic electrons whose synchrotronradiation is detected from such sources at radio, optical, and X-rayenergies. We find that these objects, under the minimum-power hypothesis(corresponding to a magnetic field of 300 μG in the brightest knotsof these jets), can contribute about one percent to the extragalacticγ-ray background measured by EGRET. We point out that this resultalready indicates that the magnetic fields in kiloparsec-scale jets oflow-power radio galaxies are not likely to be smaller than 10 μG onaverage, as otherwise the extragalactic γ-ray background would beoverproduced.
| The Chandra view of the 3C/FR I sample of low luminosity radio-galaxies
We present results from Chandra observations of the 3C/FR I sample oflow luminosity radio-galaxies. We detected a power-law nuclear componentin 12 objects out of the 18 with available data. In 4 galaxies wedetected nuclear X-ray absorption at a level of NH (0.2{-}6) × 1022 cm-2. X-ray absorbedsources are associated with the presence of highly inclined dusty disks(or dust filaments projected onto the nuclei) seen in the HST images.This suggests the existence of a flattened X-ray absorber, but of muchlower optical depth than in classical obscuring tori. We thus have anunobstructed view toward most FR I nuclei, while absorption plays only amarginal role in the remaining objects. Three pieces of evidence supporta jet origin for the X-ray cores: i) the presence of strong correlationsbetween the nuclear luminosities in the radio, optical, and X-ray bands,extending over 4 orders of magnitude and having a much smallerdispersion ( 0.3 dex) when compared to similar trends found for otherclasses of AGNs, all of which points to a common origin for the emissionin the three bands; ii) the close similarity of the broad-band spectralindices with the sub-class of BL Lac objects sharing the same range ofextended radio-luminosity, in accord with the FR I/BL Lacs unifiedmodel; iii) the presence of a common luminosity evolution of spectralindices in both FR I and BL Lacs. The low luminosities of the X-raynuclei, regardless of their origin, strengthens the interpretation oflow efficiency accretion in low luminosity radio-galaxies.
| The host galaxy/AGN connection in nearby early-type galaxies. Is there a miniature radio-galaxy in every "core" galaxy?
This is the second of a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. We selected two sampleswith 5 GHz VLA radio flux measurements down to 1 mJy, reaching levels ofradio luminosity as low as 1036 erg s-1. In PaperI we presented a study of the surface brightness profiles for the 65objects with available archival HST images out of the 116 radio-detectedgalaxies. We classified early-type galaxies into "core" and "power-law"galaxies, discriminating on the basis of the slope of their nuclearbrightness profiles, following the Nukers scheme. Here we focus on the29 core galaxies (hereafter CoreG). We used HST and Chandra data toisolate their optical and X-ray nuclear emission. The CoreG invariablyhost radio-loud nuclei, with an average radio-loudness parameter of LogR = L5 {GHz} / LB 3.6. The optical and X-raynuclear luminosities correlate with the radio-core power, smoothlyextending the analogous correlations already found for low luminosityradio-galaxies (LLRG) toward even lower power, by a factor of 1000, covering a combined range of 6 orders of magnitude. This supportsthe interpretation of a common non-thermal origin of the nuclearemission also for CoreG. The luminosities of the nuclear sources, mostlikely dominated by jet emission, set firm upper limits, as low asL/L_Edd 10-9 in both the optical and X-ray band, on anyemission from the accretion process. The similarity of CoreG and LLRGwhen considering the distributions host galaxies luminosities and blackhole masses, as well as of the surface brightness profiles, indicatesthat they are drawn from the same population of early-type galaxies.LLRG represent only the tip of the iceberg associated with (relatively)high activity levels, with CoreG forming the bulk of the population. Wedo not find any relationship between radio-power and black hole mass. Aminimum black hole mass of M_BH = 108 Mȯ isapparently associated with the radio-loud nuclei in both CoreG and LLRG,but this effect must be tested on a sample of less luminous galaxies,likely to host smaller black holes. In the unifying model for BL Lacsand radio-galaxies, CoreG likely represent the counterparts of the largepopulation of low luminosity BL Lac now emerging from the surveys at lowradio flux limits. This suggests the presence of relativistic jets alsoin these quasi-quiescent early-type "core" galaxies.
| X-Ray Emission Properties of Large-Scale Jets, Hot Spots, and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet knots, hot spots, and radiolobes recently observed with Chandra and ASCA. This report discusses theorigin of their X-ray emissions and investigates the dynamics of thejets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1keV) frequencies for more than 40 radio galaxies. We examine threemodels for the X-ray production: synchrotron (SYN), synchrotronself-Compton (SSC), and external Compton (EC) on cosmic microwavebackground (CMB) photons. For the SYN sources-mostly jet knots in nearbylow-luminosity radio galaxies-X-ray photons are produced byultrarelativistic electrons with energies 10-100 TeV that must beaccelerated in situ. For the other objects, conservatively classified asSSC or EC sources, a simple formulation of calculating the ``expected''X-ray fluxes under an equipartition hypothesis is presented. We confirmthat the observed X-ray fluxes are close to the expected ones fornonrelativistic emitting plasma velocities in the case of radio lobesand the majority of hot spots, whereas a considerable fraction of jetknots are too bright in X-rays to be explained in this way. We examinetwo possibilities to account for the discrepancy in a framework of theinverse Compton model: (1) the magnetic field is much smaller than theequipartition value, and (2) the jets are highly relativistic onkiloparsec and megaparsec scales. We conclude that if the inverseCompton model is the case, the X-ray-bright jet knots are most likelyfar from the minimum-power condition. We also briefly discuss the otherpossibility, namely, that the observed X-ray emission from all the jetknots is synchrotron in origin.
| 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.
| Relativistic models of two low-luminosity radio jets: B2 0326+39and B2 1553+24
We apply the intrinsically symmetrical, decelerating relativistic jetmodel developed by Laing & Bridle for 3C 31 to deep, full-synthesis8.4-GHz VLA imaging of the two low-luminosity radio galaxies B2 0326+39and B2 1553+24. After some modifications to the functional forms used todescribe the geometry, velocity, emissivity and magnetic-fieldstructure, these models can accurately fit our data in both totalintensity and linear polarization. We conclude that the jets in B20326+39 and B2 1553+24 are at angles of 64°+/- 5° and to theline of sight, respectively. In both objects, we find that the jetsdecelerate from 0.7-0.8c to < 0.2c over a distance of approximately10 kpc, although in B2 1553+24 this transition occurs much further fromthe nucleus than in B2 0326+39 or 3C 31. The longitudinal emissivityprofiles can be divided into sections, each fitted accurately by a powerlaw; the indices of these power laws decrease with distance from thenucleus. B2 0326+39 also requires a discontinuity in emissivity to inorder to fit a region with several bright knots of emission. In B21553+24, the sudden brightening of the jet can be explained by acombination of rapid expansion of the jet and a continuous variation ofemissivity. The magnetic fields in both objects are dominated by thelongitudinal component in the high-velocity regions close to the nucleusand by the toroidal component further out, but B2 0326+39 also has asignificant radial component at large distances, whereas B2 1553+24 doesnot. Simple adiabatic models fail to fit the emissivity variations inthe regions of high velocity but provide good descriptions of theemissivity after the jets have decelerated. Given the small angle to theline of sight inferred for B2 1553+24, there should be a significantpopulation of similar sources at less extreme orientations. Such objectsshould have long (>~200 kpc), straight, faint jets and we show thattheir true sizes are likely to have been underestimated in existingimages.
| Construction of a Celestial Coordinate Reference Frame from VLBI Data
A large number (2 million) of VLBI observations have been reducedin order to refine the measured coordinates of the observed radiosources. The data reduction was carried out in the OCCAM package usingthe least squares colocation method. Corrections to the coordinates of642 objects were derived. The accuracy of the catalog is no worse than0.2 milliseconds of arc for stable sources.
| Obscuration and Origin of Nuclear X-Ray Emission in FR I Radio Galaxies
We present X-ray observations of the nuclear region of 25 Fanaroff-Rileytype I (FR I) radio galaxies from the 3CRR and B2 catalogs, using datafrom the Chandra and XMM-Newton archives. We find the presence of aX-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources thedetection of a CCCX is uncertain, while in the remaining 9/25 sources noCCCX is found. All the sources are embedded in a diffuse soft X-raycomponent, generally on kiloparsec scales, which is in agreement withthe halo of the host galaxy and/or with the intracluster medium. TheX-ray spectra of the cores are described by a power law with photonindices Γ=1.1-2.6. In eight sources excess absorption over theGalactic value is detected, with rest-frame column densitiesNzH~1020-1021cm-2 thus, we confirm the previous claim, based on opticaldata, that most FR I radio galaxies lack a standard optically thicktorus. We find significant correlations between the X-ray coreluminosity and the radio and optical luminosities, suggesting that atleast a fraction of the X-ray emission originates in a jet; however, theorigin of the X-rays remains ambiguous. If the X-ray emission isentirely attributed to an isotropic, accretion-related component, wefind very small Eddington ratios,Lbol/LEdd~10-3to10-8, and wecalculate the radiative efficiency to beη~10-2to10-6 on the basis of the Bondiaccretion rates from the spatial analysis. This suggests thatradiatively inefficient accretion flows are present in the cores oflow-power radio galaxies.
| The VSOP 5 GHz Active Galactic Nucleus Survey. IV. The Angular Size/Brightness Temperature Distribution
The VLBI Space Observatory Programme (VSOP) mission is a Japanese-ledproject to study radio sources with submilliarcsecond angularresolution, using an orbiting 8 m telescope on board the satellite HALCAwith a global Earth-based array of telescopes. A major program is the 5GHz VSOP Survey Program, which we supplement here with Very LongBaseline Array observations to produce a complete and fluxdensity-limited sample. Using statistical methods of analysis of theobserved visibility amplitude versus projected (u, v) spacing, we havedetermined the angular size and brightness temperature distribution ofbright radio emission from active galactic nuclei. On average, the coreshave a diameter (full width, half-power) of 0.20 mas, which containsabout 20% of the total source emission, and 14%+/-6% of the cores areless than 0.04 mas in size. About 20%+/-5% of the radio cores have asource frame brightness temperatureTb>1.0×1013 K, and 3%+/-2% haveTb>1.0×1014 K. A model of the highbrightness temperature tail suggests that the radio cores havebrightness temperatures ~1×1012 K and are beamed towardthe observer with an average bulk motion of β=0.993+/-0.004.
| 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
| On the Lx-σv relation of groups of galaxies
We analyse the Lx-σv relation for the newMulchaey et al. group Atlas. We find that once we take into account thepossible statistical bias introduced by the cutoff in luminosity, werecover a relation that is consistent with that of clusters, i.e.,Lx ∝ σ4.1. The larger scatter of thisrelation for groups of galaxies could be attributed to an orientationeffect, due to which the radial velocity dispersion of groups orientedclose to orthogonal to the line of sight, would be underestimated. Thiseffect could also contribute to flattening the slope of the groupLx- σv relation.
| Two new optical jets in B2 radio galaxies
We present HST observations of previously undetected optical jets in thelow-luminosity radio galaxies B20755+37 and B21553+24. We show thatthere is accurate spatial coincidence between optical and radioemission, implying that the former is likely to be synchrotronradiation. The physical properties of the jets are similar to thoseknown previously: their radio-optical spectral indices are ~0.7, and inB20755+37 the spectrum steepens between optical and X-ray wavelengths.Our results support the hypothesis that optical emission is detectablefrom jets orientated within ~20° of the line of sight for the B2sample.
| The X-ray jet and central structure of the active galaxy NGC 315
We report the Chandra detection of resolved X-ray emission of luminosity3.5 × 1040 erg s-1 (0.4-4.5 keV) andpower-law energy spectral index α= 1.5 +/- 0.7 from a roughly 10arcsec length of the north-west radio jet in NGC 315. The X-ray emissionis brightest at the base of the radio-bright region about 3 arcsec fromthe nucleus, and is consistent with a synchrotron origin. At a projecteddistance of 10 arcsec from the core, the jet is in approximate pressurebalance with an external medium which is also detected through its X-rayemission and which has kT~ 0.6 +/- 0.1 keV, consistent with earlierROSAT results. The high spatial resolution and sensitivity of Chandraseparates nuclear unresolved emission from the extended thermal emissionof the galaxy atmosphere with higher precision than possible withprevious telescopes. We measure an X-ray luminosity of 5.3 ×1041 erg s-1 (0.4-4.5 keV) and a power-law energyindex of α= 0.4 +/- 0.4 for the nuclear component.
| 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.
| Spectral Indices of Core and Extended Components of Extragalactic Radio Sources
We use observed peak and total flux densities at 6 cm and 20 cm todetermine the spectral indices separately for the core and extendedcomponents of QSOs and galaxies, as well as their core-dominanceparameters. Our results indicate that 1) Nine QSOs show both greaterthan 1.0 core-dominance parameters (those objects should be blazars) andgreater than 0.5 spectral indices. The average core spectral index isαCore = 0.85±0.21 for the nine blazars, whichimplies that it is not reliable to use αradio=0.0 forblazars. For the different subclasses, the core andextended spectralindices are as follows: for the blazars, αCore =0.22±0.06 and αExt =0.77±0.12; the galaxies,αCore = 1.01±0.13 and αExt=0.83±0.21, and for the QSOs, αCore = 0.28±0.10and αExt =0.68±0.08. 2) The core spectral index andcore dominance parameter (R) show an anti-correlation,αC = (-1.28±0.26) log R +(0.65±0.11); 3) R isapproximately linearly correlated with redshift (z).}
| 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.
| High-Energy Gamma Rays from FR I Jets
Thanks to the Hubble and Chandra telescopes, some of the large-scalejets in extragalactic radio sources are now being observed at opticaland X-ray frequencies. For the FR I objects the synchrotron nature ofthis emission is surely established, although manyuncertainties-connected for example with the particle accelerationprocesses involved-remain. In this paper we study the production ofhigh-energy γ-rays in FR I kiloparsec-scale jets by inverseCompton emission of the synchrotron-emitting electrons. We considerdifferent origins of seed photons contributing to the inverse Comptonscattering, including nuclear jet radiation as well as ambient, stellar,and circumstellar emission of the host galaxies. We discuss how futuredetections or nondetections of the evaluated γ-ray fluxes canprovide constraints on the unknown large-scale jet parameters, i.e., themagnetic field intensity and the jet Doppler factor. For the nearbysources Centaurus A and M87, we find measurable fluxes of TeV photonsresulting from synchrotron self-Compton process and from Comptonizationof the galactic photon fields, respectively. In the case of Centaurus A,we also find a relatively strong emission component due toComptonization of the nuclear blazar photons, which could be easilyobserved by GLAST at energies ~10 GeV, providing an important test forthe unification of FR I sources with BL Lacertae objects.
| The Second VLBA Calibrator Survey: VCS2
This paper presents an extension of the Very Long Baseline ArrayCalibrator Survey, called VCS2, containing 276 sources. This surveyfills in regions of the sky that were not completely covered by theprevious VCS1 calibrator survey. The VCS2 survey includes calibratorsources near the Galactic plane,-30deg<δ<-45deg, and VLA calibrators.The positions have been derived from astrometric analysis of the groupdelays measured at 2.3 and 8.4 GHz using the Goddard Space Flight CenterCALC/SOLVE package. From the VLBA snapshot observations, images of thecalibrators are available, and each source is given a quality code foranticipated use. The VCS2 catalog is available from the NRAO Web site.
| A multi-wavelength test of the FR I-BL Lac unifying model
We collect multi-wavelength measurements of the nuclear emission of 20low luminosity FR I radio-galaxies to test the viability of the FR I-BLLac unifying model. Although poorly sampled, the Spectral EnergyDistributions (SED) of FR Is are consistent with the double peaked shapecharacteristic of BL Lacs. Furthermore while the distribution of the FRIs in the broad-band spectral index planes shows essentially no overlapwith the regions where HBL and LBL are located, this can be simply dueto the effects of relativistic beaming. More quantitatively, derivingthe beaming Doppler factor of a given radio-galaxy from its X-rayluminosity ratio with respect to BL Lacs with similar extended radioluminosity, we find that i) the luminosity in all bands, ii) the valueof the spectral indices, iii) the slope of the X-ray spectrum, iv) theoverall SED shape, may be all simultaneously reproduced. However, thecorresponding jet bulk Lorentz factors are significantly smaller thanthose derived for BL Lacs from other observational and theoreticalconsiderations. This suggests to consider a simple variant of theunification scheme that allows for the presence of a velocity structurein the jet.
| The HST survey of the B2 sample of radio galaxies: Detection of two optical jets
We present HST observations of previously undetected optical jets in thelow-luminosity radio galaxies B2 0755+37 and B2 1553+24. We show thatthere is accurate spatial coincidence between optical and radioemission, implying that the former is likely to be synchrotronradiation. The physical properties of the jets are similar to thoseknown previously: their radio-optical spectral indices are ~0.7 and inB2 0755+37, the spectrum steepens between optical and X-ray wavelengths.Our results support the hypothesis that optical emission is detectablefrom jets orientated within ~20o of the line of sight for theB2 sample. 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.
| 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.
| Relativistic models and the jet velocity field in the radio galaxy 3C 31
We compare deep Very Large Array (VLA) imaging of the total intensityand linear polarization of the inner jets in the nearby, low-luminosityradio galaxy 3C 31 with models of the jets as intrinsically symmetrical,decelerating relativistic flows. We show that the principal differencesin appearance of the main and counter-jets within 30 arcsec of thenucleus can result entirely from the effects of relativistic aberrationin two symmetrical, antiparallel, axisymmetric, time-stationaryrelativistic flows. We develop empirical parametrized models of the jetgeometry and the three-dimensional distributions of the velocity,emissivity and magnetic-field structure. We calculate the synchrotronemission by integration through the models, accounting rigorously forrelativistic effects and the anisotropy of emission in the rest frame.The model parameters are optimized by fitting to our 8.4-GHz VLAobservations at resolutions of 0.25 and 0.75 arcsec full width at halfmaximum (FWHM), and the final quality of the fit is extremely good. Thenovel features of our analysis are that we model the two-dimensionalbrightness distributions at large number of independent data pointsrather than using one-dimensional profiles, we allow transverse as wellas longitudinal variations of velocity, field and emissivity and wesimultaneously fit total intensity and linear polarization. We concludethat the jets are ~52° to the line of sight, that they decelerateand that they have transverse velocity gradients. Their magnetic fieldconfiguration has primarily toroidal and longitudinal components. Thejets may be divided into three distinct parts, based not only on thegeometry of their outer isophotes, but also on their kinematics andemissivity distributions: a well-collimated inner region; a flaringregion of rapid expansion followed by recollimation and a conical outerregion. The inner region is poorly resolved, but is best modelled as thesum of fast (0.8-0.9c) and much slower components. The transitionbetween inner and flaring regions marks a discontinuity in the flowwhere the emissivity increases suddenly. The on-axis velocity staysfairly constant at ~0.8c until the end of the flaring region, where itdrops abruptly to ~0.55c, thereafter falling more slowly to ~0.25c atthe end of the modelled region. Throughout the flaring and outerregions, the velocity at the edge of the jet is ~0.7 of its on-axisvalue. The magnetic field in the flaring region is complex, with anessentially isotropic structure at the edge of the jet, but a moreordered toroidal + longitudinal configuration on-axis. In the outerregion, the radial field vanishes and the toroidal component becomesdominant. We show that the emissivity and field structures areinconsistent with simple adiabatic models in the inner and flaringregions. We suggest that the discontinuity between the inner and flaringregions could be associated with a stationary shock structure and thatthe inferred transverse velocity profiles and field structure in theflaring region support the idea that the jets decelerate by entrainingthe external medium. We demonstrate the appearance of our model at otherangles to the line of sight and argue that other low-luminosity radiogalaxies resemble 3C 31 seen at different orientations.
| A Chandra observation of the X-ray environment and jet of 3C 31
We have used a deep Chandra observation of the central regions of thetwin-jet Fanaroff-Riley class I (FRI) radio galaxy 3C 31 to resolve thethermal X-ray emission in the central few kpc of the host galaxy, NGC383, where the jets are thought to be decelerating rapidly. This allowsus to make high-precision measurements of the density, temperature andpressure distributions in this region, and to show that the X-rayemitting gas in the centre of the galaxy has a cooling time of only5×107yr. In a companion paper, these measurements areused to place constraints on models of the jet dynamics. A previouslyunknown one-sided X-ray jet in 3C 31, extending up to 8arcsec from thenucleus, is detected and resolved. Its structure and steep X-rayspectrum are similar to those of X-ray jets known in other FRI sources,and we attribute the radiation to synchrotron emission from ahigh-energy population of electrons. In situ particle acceleration isrequired in the region of the jet where bulk deceleration is takingplace. We also present X-ray spectra and luminosities of the galaxies inthe Arp 331 chain of which NGC 383 is a member. The spectrum and spatialproperties of the nearby bright X-ray source 1E 0104+3153 are used toargue that the soft X-ray emission is mostly due to a foreground groupof galaxies rather than to the background broad absorption-line quasar.
| Radiation from the Relativistic Jet: A Role of the Shear Boundary Layer
Recent radio and optical large-scale jet observations suggest atwo-component jet morphology, consisting of a fast central spinesurrounded by a boundary layer with a velocity shear. We study radiationof electrons accelerated at such boundary layers as an option forstandard approaches involving internal shocks in jets. The accelerationprocess in the boundary layer yields in a natural way a two-componentelectron distribution: a power-law continuum with a bump at the energywhere energy gains equal radiation losses, followed by a cutoff. Forsuch distributions, we derive the observed spectra of synchrotron andinverse Compton radiation, including Comptonization of synchrotron andcosmic microwave background photons. Under simple assumptions of energyequipartition between the relativistic particles and the magnetic field,the relativistic jet velocity at large scales, and a turbulent characterof the shear layer, the considered radiation can substantiallycontribute to the jet radiative output. In the considered conditions thesynchrotron emission is characterized by a spectral index of theradio-to-optical continuum being approximately constant along the jet. Acharacteristic feature of the obtained broadband synchrotron spectrum isan excess at X-ray frequencies, similar to the one observed in someobjects by Chandra. As compared to the uniform jet models, the velocityshear across the radiating boundary region leads to a decrease andfrequency dependence of the observed jet-counterjet radio brightnessasymmetry. We conclude that a careful investigation of the observationaldata looking for the derived effects can allow us to evaluate the roleof the boundary layer acceleration processes and/or impose constraintsfor the physical parameters of such layers in large-scale jets.
| X-Ray Detection of the Inner Jet in the Radio Galaxy 3C 129
During the course of an investigation of the interaction of the radiogalaxy 3C 129 and its ambient cluster gas, we found excess X-rayemission aligned with the northern radio jet. The emission extends fromthe weak X-ray core of the host galaxy ~2.5" to the first resolved radioknot. On a smaller scale, we have also detected a weak radio extensionin the same position angle with the VLBA. Although all the evidencesuggests that Doppler favoritism augments the emission of the northernjet, it is unlikely that the excess X-ray emission is produced byinverse Compton emission. We find many similarities between the 3C 129X-ray jet and recent jet detections from Chandra data of low-luminosityradio galaxies. For most of these current detections, synchrotronemission is the favored explanation for the observed X-rays.
| 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.
| 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.
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