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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.
| Chandra and XMM-Newton Observations of a Sample of Low-Redshift FR I and FR II Radio Galaxy Nuclei
We present spectral results from Chandra and XMM-Newton observations ofa sample of 22 low-redshift (z<0.1) radio galaxies and considerwhether the core emission originates from the base of a relativisticjet, or an accretion flow, or contains contributions from both. We findcorrelations between the unabsorbed X-ray, radio, and optical fluxes andluminosities of FR I-type radio-galaxy cores, implying a common originin the form of a jet. On the other hand, we find that the X-ray spectraof FR II-type radio galaxy cores are dominated by absorbed emission,with NH>~1023 atoms cm-2, which islikely to originate in an accretion flow. We discuss several models thatmay account for the different nuclear properties of FR I- and FR II-typecores and also demonstrate that both heavily obscured, accretion-relatedand unobscured, jet-related components may be present in all radiogalaxy nuclei. Any absorbed, accretion-related components in FR I-typegalaxies have low radiative efficiencies.
| 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.
| Gamma-ray emissions of AGN and cosmological standard candles
In this work, we compile a sample which contains 71 GeV Gamma-ray-loudActive Galactic Nuclei (AGNs) (14 BL Lacs and 57 FSRQs), 53 FR I radiogalaxies and 63 FR II radio galaxies. We make a nonlinear least-squarefit to this sample, and find that the best fit value of the Hubbleconstant is H0=71.5±3.8 kms-1Mpc-1 with a reduced χ ~= 2.46 by assumingMv = -23.0 and accepting q0 = 1.0, and thecorresponding regression line has a correlation index R ~= 0.78. Thebest fit value of H0 = 71.5±3.8 kms-1Mpc-1 is in well agreement with H0 =72±8 km s-1 obtained by the Hubble Space TelescopeKey Project. Our results show that the GeV Gamma-ray emissions of AGNscan be used as cosmological standard candles indeed.
| A relativistic model of the radio jets in NGC 315
We apply our intrinsically symmetrical, decelerating relativistic jetmodel to deep Very Large Array imaging of the inner +/-70arcsec of thegiant low-luminosity radio galaxy NGC315. An optimized model accuratelyfits the data in both total intensity and linear polarization. We inferthat the velocity, emissivity and field structure in NGC315 are verysimilar to those of the other low-luminosity sources we have modelled,but that all of the physical scales are larger by a factor of about 5.We derive an inclination to the line of sight of 38°+/- 2° forthe jets. Where they first brighten, their on-axis velocity isβ=v/c~ 0.9. They decelerate to β~ 0.4 between 8 and 18kpc fromthe nucleus and the velocity thereafter remains constant. The speed atthe edge of the jet is ~0.6 of the on-axis value where it is bestconstrained, but the transverse velocity profile may deviatesystematically from the Gaussian form we assume. The proper emissivityprofile is split into three power-law regions separated by shortertransition zones. In the first of these, at ~3kpc (the flaring point)the jets expand rapidly at constant emissivity, leading to a largeincrease in the observed brightness on the approaching side. At ~10kpc,the emissivity drops abruptly by a factor of 2. Where the jets are wellresolved, their rest-frame emission is centre brightened. The magneticfield is modelled as random on small scales but anisotropic and we ruleout a globally ordered helical configuration. To a first approximation,the field evolves from a mixture of longitudinal and toroidal componentsto predominantly toroidal, but it also shows variations in structurealong and across the jets, with a significant radial component inplaces. Simple adiabatic models fail to fit the emissivity variations.
| Chandra and XMM-Newton observations of NGC 6251
We present new X-ray observations of the nucleus, jet and extendedemission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-Scamera, together with a reanalysis of archival Chandra/ACIS-I andXMM-Newton/EPIC data. We find that the nuclear X-ray spectrum is wellfitted with an absorbed power law, and that there is tentative, but nothighly significant, evidence for Fe Kα emission. We argue that theobserved nuclear X-ray emission is likely to originate in a relativisticjet, based on the double-peaked nature, and our synchrotron self-Comptonmodelling, of the radio to X-ray spectral energy distribution. However,we cannot rule out a contribution from an accretion flow. We resolveX-ray jet emission in three distinct regions, and argue in favour of asynchrotron origin for all three; inverse Compton emission models arepossible but require extreme parameters. We detect thermal emission onboth galaxy and group scales, and demonstrate that hot gas can confinethe jet, particularly if relativistic beaming is important. We showevidence that the radio lobe has evacuated a cavity in theX-ray-emitting gas, and suggest that the lobe is close to the plane ofthe sky, with the jet entering the lobe close to the surface nearest tothe observer.
| A Chandra observation of the X-ray environment and jet of 3C 296
We have observed the twin-jet radio galaxy 3C 296 with Chandra. X-rayemission is detected from the nucleus, from the inner parts of the radiojet and from a small-scale thermal environment around the jetdeceleration region. As we have found in previous observations of othertwin-jet radio galaxies, the X-ray jet and a steep pressure gradient inthe external thermal environment are associated with the region wherestrong bulk deceleration of the jet material is suggested by radioobservations. Our observations provide additional evidence that theinner jets of twin-jet objects are always associated with a relativelycool, dense central X-ray emitting component with a short cooling time.
| The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?
We study a complete, distance-limited sample of 25 LINERs, 21 of whichhave been imaged with the Hubble Space Telescope. In nine objects wedetect an unresolved nucleus. To study their physical properties, wecompare the radio and optical properties of the nuclei of LINERs withthose of other samples of local active galactic nuclei (AGNs), namely,Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our resultsshow that the LINER population is not homogeneous, as there are twosubclasses: (1) the first class is similar to the LLRG class, as itextends the population of radio-loud nuclei to lower luminosities; (2)the second is similar to Seyfert galaxies and extends the properties ofradio-quiet nuclei toward the lowest luminosities. The objects areoptimally discriminated in the plane formed by the black hole massversus nuclear radio loudness: all radio-loud LINERs haveMBH>~108Msolar, while Seyfertgalaxies and radio-quiet LINERs haveMBH<~108Msolar. The different natureof the various classes of local AGNs are best understood when thefraction of the Eddington luminosity they irradiate,Lo/LEdd, is plotted against the nuclearradio-loudness parameter: Seyfert galaxies are associated withrelatively high radiative efficienciesLo/LEdd>~10-4 (and high accretionrates onto low-mass black holes); LLRGs are associated with lowradiative efficiencies (and low accretion rates onto high-mass blackholes); all LINERs have low radiative efficiency (and accretion rates)and can be radio-loud or radio-quiet, depending on their black holemass.Based on observations obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| 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.
| A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies
We examine the properties of central dust in nearby quiescent and activeearly-type galaxies. The active galaxies are low-power radio galaxieswith Fanaroff & Riley type I or I/II radio jets. We focus on (a) thecomparison of the dust distributions in the active and quiescent galaxysamples; and (b) the relation between the radio jet and dustorientations. Our main observational conclusions are: (i) in line withprevious studies, the dust detection rate is higher in radio-jetgalaxies than in non radio-jet galaxies; (ii) radio galaxies contain ahigher fraction of regular dust “ellipses” compared toquiescent galaxies which contain more often irregular dustdistributions; (iii) the morphology, size and orientation of dustellipses and lanes in quiescent early-types and active early-types withkpc-scale radio jets is very similar; (iv) dust ellipses are alignedwith the major axis of the galaxy, dust lanes do not show a preferredalignment except for large (>kpc) dust lanes which are aligned withthe minor axis of the galaxy; and (v) as projected on the sky, jets donot show a preferred orientation relative to the galaxy major axis (andhence dust ellipses), but jets are preferentially perpendicular to dustlanes. We show that the dust ellipses are consistent with being nearlycircular thin disks viewed at random viewing angles. The lanes arelikely warped dust structures, which may be in the process of settlingdown to become regular disks or are being perturbed by anon-gravitational force. We use the observed dust-jet orientations toconstrain the three-dimensional angle θDJ between jetand dust. For dust-lane galaxies, the jet is approximately perpendicularto the dust structure, while for dust-ellipse galaxies there is a muchwider distribution of θDJ. We discuss two scenariosthat could explain the dust/jet/galaxy orientation dichotomy. If lanesare indeed settling, then the jet orientation apparently is roughlyaligned with the angular momentum of the dust before it settles. Iflanes are perturbed by a jet-related force, it appears that it causesthe dust to move out of its equilibrium plane in the galaxy into a planewhich is perpendicular to the jet.
| 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.
| No evidence for a different accretion mode for all 3CR FR I radio galaxies
We have analysed the optical and radio properties of a sample of 3CR FRI radio galaxies which have Hubble Space Telescope (HST) imaging capableof detecting optical cores. The jet powers of the FR I radio galaxiesare estimated from their low-frequency radio luminosities, and theoptical core luminosity is taken as an upper limit on the emission fromany unobscured accretion disc. We argue that if the accretion discs inthese sources are assumed to be advection-dominated accretion flows(ADAFs), or adiabatic inflow-outflow solution (ADIOS) flows, then theBlandford-Znajek mechanism provides insufficient power to explain thehigh radio luminosities of at least a third, and perhaps all, of thesample. We suggest instead that a significant fraction (the`high-jet-power' third), and perhaps most, of the 3CR FR I radiogalaxies have normal accretion discs, but that their optical cores canbe hidden, with any HST-detected optical synchrotron emission comingfrom jets on scales larger than the obscuring material. A normalaccretion disc hypothesis, at least for the high-jet-power third of the3CR FR Is, explains why narrow-line luminosity correlates with radioluminosity. It also explains why one object in the sample (3C 386) hasan observed broad-line nucleus. We conclude that there is no evidence tosuggest that there is a difference in accretion mode between FR I and FRII radio galaxies.
| Implications for unified schemes from the quasar fraction and emission-line luminosities in radio-selected samples
We use a principal components analysis of radio-selected (3CRR, 6CE and7CRS) active galactic nuclei (AGN) data sets to define two parametersrelated to low-frequency (151-MHz) radio luminosity L151 and[OIII] luminosity L[OIII]: a parameter α encoding theL151-L[OIII] correlation and a parameter βencoding scatter about this correlation. We describe methods forconstructing generalized luminosity functions (GLFs) based on α,β, redshift and schemes for unifying quasars and radio galaxies.These GLFs can be used to generate radio luminosity functions (RLFs)which improve on those of Willott et al. (2001a), mostly because theyincorporate scatter and are therefore much smoother.Luminosity-dependent unified schemes (e.g. a receding-torus scheme) havebeen invoked to explain the low quasar-to-radio galaxy fraction at lowα and the differences in emission-line luminosities of radioquasars and radio galaxies. With the constraints of the 3CRR, 6CE and7CRS data sets and radio source counts, our GLF approach was used todetermine whether a receding-torus-like scheme is required if there aretwo populations of radio sources: one at low α, consisting of`starved AGN' the other at high α, consisting of `Eddington-tunedAGN'. Because of the overlap between these two populations and theeffects of the β parameter, schemes with or without a recedingtorus can produce a low quasar fraction at low α and differencesin [OIII] luminosity between radio galaxies and quasars. The recedingtorus may be a physical process important in one or more populations ofradio sources, but this is not yet proved either by the quasar fractionor the emission-line properties of radio-selected samples.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| Dust in 3CR radio galaxies: On the FR 1 - FR 2 difference
We compare three 3CR samples of 11 FR 1 galaxies, 17 FR 2 galaxies and18 lobe-dominated quasars contained in the ISO Data Archive. In contrastto the powerful FR 2 galaxies with edge-brightened lobes, the low radiopower FR 1 galaxies in our sample do not exhibit any high MIR or FIRdust luminosity, which is typical for a buried, intrinsically moreluminous AGN. This consolidates the fact already inferred from opticalstudies that their AGNs have only a relatively low luminosity. Also theFR 1 galaxies show a high FIR/MIR luminosity ratio, compared to quasars,suggesting that their FIR luminosity is substantially powered by theinterstellar radiation field (ISRF) of the giant elliptical hosts.Finally, we discuss the FR 1 - FR 2 morphological dichotomy. FR 1galaxies do not have more interstellar matter (ISM) than FR 2s as traced- on the large scale - by the cool FIR emitting dust and - in thenuclear region - by the warm MIR emitting dust. Due to the lack ofcentral gas we suggest that the black holes of our FR 1 galaxies are fedat a lower accretion rate than those of the FR 2 galaxies.Based on observations with the Infrared Space Observatory ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the UK) and with theparticipation of ISAS and NASA.
| 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
| ISOCAM survey and dust models of 3CR radio galaxies and quasars
We present a survey of all 3CR sources imaged with ISOCAM onboard theInfrared Space Observatory (ISO). The sample consists mostly ofradio-loud active galactic nuclei (AGN). For each source, we presentspatially integrated mid-infrared (MIR, 5-18 μm) fluxes measured fromnewly calibrated ISOCAM images. In total, we detected 68 objects of the3CR catalogue, at redshifts z ≤2.5, and obtained upper limits for 17objects. In addition, we detected 10 galaxies not listed in the 3CRcatalogue. The one with the highest redshift is 4C+72.26 at z = 3.53.ISOCAM data are combined with other photometric measurements toconstruct the spectral energy distribution (SED) from optical to radiowavelengths. The MIR emission may include synchrotron radiation of theAGN, stars of the host galaxy or dust. Extrapolation of radio corefluxes to the MIR show that the synchrotron contribution is in mostcases negligible. In order to describe dust emission we apply newradiative transfer models. In the models the dust is heated by a centralsource which emits photons up to energies of 1 keV. By varying threeparameters, luminosity, effective size and extinction, we obtain a fitto the SED for our objects. Our models contain also dust at large(several kpc) distances from the AGN. Such a cold dust component wasneglected in previous computations which therefore underestimated theAGN contribution to the far infrared (FIR). In 53 cases ( 75% ofour detected 3CR sources), the MIR emission can be attributed to dust.The hot dust component is mainly due to small grains and PAHs. Themodelling demonstrates that AGN heating suffices to explain the ISObroad band data, starburst activity is not necessary. In the models, atype 1 AGN is represented by a compact dust distribution, the dust istherefore very warm and emission of PAHs is weak because ofphoto-destruction. In AGNs of type 2, the dust is relatively colder butPAH bands are strong.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) with the participation of ISAS and NASA.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
| A scheme to unify low-power accreting black holes. Jet-dominated accretion flows and the radio/X-ray correlation
We explore the evolution in power of black holes of all masses, andtheir associated jets, within the scheme of an accretion rate-dependentstate transition. Below a critical value of the accretion rate allsystems are assumed to undergo a transition to a state where thedominant accretion mode is optically thin and radiatively inefficient.In these significantly sub-Eddington systems, the spectral energydistribution is predicted to be dominated by non-thermal emission from arelativistic jet whereas near-Eddington black holes will be dominatedinstead by emission from the accretion disk. Reasonable candidates forsuch a sub-Eddington state include X-ray binaries in the hard andquiescent states, the Galactic Center (Sgr A*), LINERs, FR I radiogalaxies, and a large fraction of BL Lac objects. Standard jet physicspredicts non-linear scaling between the optically thick (radio) andoptically thin (optical or X-ray) emission of these systems, which hasbeen confirmed recently inX-ray binaries. We show that this scaling relation is also a function ofblack hole mass and only slightly of the relativistic Doppler factor.Taking the scaling into account we show that indeed hard and quiescentstate X-ray binaries, LINERs, FR I radio galaxies, and BL Lacs can beunified and fall on a common radio/X-ray correlation. This suggests thatjet domination is an important stage in the luminosity evolution ofaccreting black hole systems.
| Spectra, optical identifications, and statistics of a complete sample of radio sources at declinations 10° 12°30′
The results of 0.97, 2.3, 3.9, 7.7, 11.1, and 21.7 GHz observations of acomplete sample of radio sources obtained on the RATAN-600 radiotelescope are presented. The sample is comprised of sources from the4.85-GHz MGB survey, and contains all sources at declinations 10°12°30′ (J2000) with Galactic latitudes |b|>15° and fluxdensities S 4.85>200 mJy. Optical identifications have been obtainedfor about 86% of the radio sources with flat spectra and 59% of thosewith steep spectra. The spectra of the flat-spectrum sources have beendecomposed into extended and compact components.
| X-Ray Emission from the Hot Interstellar Medium and Southwest Radio Lobe of the Nearby Radio Galaxy Centaurus A
We present results from two Chandra/ACIS-I observations and oneXMM-Newton observation of X-ray emission from the interstellar medium(ISM) and the inner radio lobes of the nearby radio galaxy Centaurus A.The ISM has an average radial surface brightness profile that is welldescribed by a β-model profile with index β=0.40+/-0.04 and atemperature of kBTISM~0.29 keV beyond 2 kpc fromthe nucleus. We find that diffuse X-ray emission is coincident with theouter half of the southwest radio lobe, and a bright X-ray enhancementis detected along the edge of the lobe. On the basis of energetic andlifetime arguments, we reject a nonthermal explanation for thisemission. We model this emission as a thin, hot shell or cap ofX-ray-emitting plasma surrounding the radio lobe that was created by thesupersonic inflation of the lobe. This plasma shell is both hotter than(kBTSH~2.9 keV) and greatly overpressurizedrelative to the ambient ISM, indicating supersonic expansion. Weestimate that the lobe is expanding into the ISM at approximately Mach8.5, or 2400 km s-1. We are not directly observing the bowshock, but rather the cooler, denser material that is accumulating aheadof the contact discontinuity. The thermal energy in the shell is asignificant fraction of the thermal energy of the hot ISM, demonstratingthe possibility that the hot ISM of early galaxies can be reenergized byoutflows from nuclear activity. Interestingly, no similarly bright X-rayemission is detected in or along the edge of the northeast lobe,implying that there are differences in the dynamics and evolution of thekiloparsec-scale radio components.
| 870 Micron Observations of Nearby 3CRR Radio Galaxies
We present submillimeter continuum observations at 870 μm of thecores of low-redshift 3CRR radio galaxies, observed at the HeinrichHertz Submillimeter Telescope. The cores are nearly flat-spectrumbetween the radio and submillimeter, which implies that thesubmillimeter continuum is likely to be synchrotron emission and notthermal emission from dust. The emitted power from nuclei detected atoptical wavelengths and in the X-rays is similar in the submillimeter,optical, and X-rays. The submillimeter-to-optical and X-ray power ratiossuggest that most of these sources resemble misdirected BL Lac-typeobjects with synchrotron emission peaking at low energies. However, wefind three exceptions, the FR I galaxy 3C 264 and the FR II galaxies 3C390.3 and 3C 338 with high X-ray-to-submillimeter luminosity ratios.These three objects are candidate misdirected high- orintermediate-energy peaked BL Lac-type objects. With additional infraredobservations and from archival data, we compile spectral energydistributions for a subset of these objects. The steep dips observednear the optical wavelengths in many of these objects suggest thatextinction inhibits the detection and reduces the flux of opticalcontinuum core counterparts. High-resolution near- or mid-infraredimaging may provide better measurements of the underlying synchrotronemission peak.
| The Ultraviolet Continuum Emission of FR I and FR II Radio Galaxies and a Proposal for a Unified AGN Model for FR I sources
This paper is the second in a series of two on the UV continuum emission(in the range from 1400 to 3700 Å) of radio galaxies that wereextracted from the Hubble Space Telescope (HST) Archives. The sampleconsists of 31 3C and Parkes radio galaxies that have redshifts below0.2 (the majority have redshifts of ~0.03) and radio powers of~1025-1027 W Hz-1 (usingH0=50 km-1 s-1 Mpc andq0=0.0). Paper I describes the sample selection and theproperties of individual sources; this paper deals with the analysis. Wefind that only about half the radio galaxies display any UV flux atwavelengths shorter than 2300 Å. More specifically, those galaxiesthat are dominated by a nuclear UV component are either BL Lac objectsor radio galaxies with broad emission lines. We detect a nuclear and anextended UV component only among half the radio galaxies with narrowemission lines. Although we do not find a correlation of the UVluminosity with emission-line luminosity or radio power, there doesappear to be a dependence on radio morphology. While (narrow line) FR IIsources do not show a nuclear UV component, FR I's do, however, only ifthey also have an optical jet (this is the case for seven of 21 FR I's).These results are broadly consistent with orientation-dependentunification models. In radio galaxies in which the torus does notobscure our view of the engine, the observed UV radiation appears to bepoint source-like. This is the case for broad-line radio galaxies and BLLac objects. In other radio galaxies that are oriented at an angle tous, the torus presumably blocks the nuclear UV component. Thenarrow-line FR I galaxies with optical jets can then be interpreted asobjects at a critical angle at which some, but not all, nuclear UVemission is blocked. The UVλ luminosities (withλ ranging from 1400 to 3700 Å) and theUVλ-V colors of radio galaxies show a larger scatterthan those of radio-quiet elliptical galaxies. At wavelengths shorterthan 2300 Å, some radio galaxies have on average bluer colors, butbeyond 3000 Å, their colors are on average slightly redder. Thispicture is also consistent with unification models-the galaxies withbluer colors are either BL Lac objects or broad-line radio galaxies. Allother radio galaxies (including the jetted FR I's) have somewhat reddercolors than radio-quiet elliptical galaxies. We suspect that this isprimarily due to reddening by dust, which we know is present in some ofthe radio galaxies in the sample. At longer wavelengths (>3000 Å),all radio galaxies (14 of 14) show an extended component. The morphologyof the extended emission is in most, but not all, cases comparable tothe morphology at optical wavelengths, implying that this is likely tobe starlight. However, the polarization images of two of seven radiogalaxies indicate that scattered light from the active galactic nucleusmay also contribute toward the UV luminosity.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| 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 nuclear dust disks of five nearby 3CR elliptical galaxies
We present broad- and narrow-band WFPC2 images of the nuclear dust disksand rings of five low-/z elliptical galaxies hosting 3C radio sources:NGC 383 (3C 31)/NGC 382, NGC 3862 (3C 264), NGC 4261 (3C 270), UGC 12064(3C 449), and NGC 7720 (3C 465)/NGC 7720A. We detect resolved lineemission in all the disks. In NGC 383, the line emission consists of a``bar'' and spectacular filamentary arms to the north and south while inNGC 7720 and UGC 12064, it is extended along the major axis of thedisks, suggesting a true physical association between the ionizedmaterial and the dust. The color maps clearly reveal that the disks ofNGC 383, NGC 4261, NGC 7720, and possibly NGC 3862 are inclined. Thedisk of NGC 383 is the most disturbed and filamentary and appears toconsist of an ``inner'' (/~0.5 kpc) and ``outer'' (/~2.5 kpc) disk. Wesuggest that the colors of the unresolved nuclei of NGC 383 and NGC 3862may be partially accounted for by optical synchrotron emission (and mayin fact dominate in NGC 3862) while in NGC 4261, the nuclear emission iscompletely dominated by line emission. The disk colors are generallyredder than predicted by a simple ``sandwich'' model, suggesting thatthe disks cannot be simply treated as thin uniform sheets of dust. Wehave begun exploring radiative transfer models with varying dustcomposition, temperature, and distribution and preliminary results arevery promising.
| 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.
| Automated optical identification of a large complete northern hemisphere sample of flat-spectrum radio sources with [formmu1]S6 cm>200 mJy
This paper describes the automated optical APM identification of radiosources from the Jodrell Bank-VLA Astrometric Survey (JVAS), as used forthe search for distant radio-loud quasars. Since JVAS was not intendedto be complete, a new complete sample, JVAS++, has been constructed withselection criteria similar to those of JVAS (S5GHz>200mJy,α1.4-5GHz>-0.5), and with the use of the moreaccurate GB6 and NVSS surveys. Comparison between this sample and JVASindicates that the completeness and reliability of the JVAS survey are~90 and ~70 per cent respectively. The complete sample has been used toinvestigate possible relations between optical and radio properties offlat-spectrum radio sources. From the 915 sources in the sample, 756have an optical APM identification on a red (e) and/or blue (o) plate,resulting in an identification fraction of 83 per cent with acompleteness and reliability of 98 and 99 per cent respectively. About20 per cent are optically identified with extended APM objects on thered plates, e.g., galaxies. However, the distinction between galaxiesand quasars can not be made properly near the magnitude limit of thePOSS-I plates. The identification fraction appears to decrease from>90 per cent for sources with a 5-GHz flux density of >1Jy, to<80 per cent for sources at 0.2Jy. The identification fraction, inparticular that for unresolved quasars, is found to be lower for sourceswith steeper radio spectra. In agreement with previous studies, we findthat the quasars at low radio flux density levels also tend to havefainter optical magnitudes, although there is a large spread. Inaddition, objects with a steep radio-to-optical spectral index are foundto be mainly highly polarized quasars, supporting the idea that in theseobjects the polarized synchrotron component is more prominent. It isshown that the large spread in radio-to-optical spectral index ispossibly caused by source-to-source variations in the Doppler boostingof the synchrotron component.
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