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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.
| Understanding the Nuclear Gas Dispersion in Early-Type Galaxies in the Context of Black Hole Demographics
The majority of nearby early-type galaxies contain detectable amounts ofemission-line gas at their centers. The nuclear gas kinematics form avaluable diagnostic of the central black hole (BH) mass. Here we analyzeand model Hubble Space Telescope STIS observations of a sample of 27galaxies; 16 Fanaroff-Riley Type I radio galaxies and 11 (more) normalearly-type galaxies. We focus here on what can be learned from thenuclear velocity dispersion (line width) of the gas as a complement tothe many studies dealing with gas rotation velocities. We find that thedispersion in a STIS aperture of ~0.1"-0.2" generally exceeds thelarge-scale stellar velocity dispersion of the galaxy. This isqualitatively consistent with the presence of central BHs but raises thequestions of whether the excess gas dispersion is of gravitational ornongravitational origin and whether the implied BH masses are consistentwith our current understanding of BH demography (as predicted by theM-σ relation between BH mass and stellar velocity dispersion). Toaddress this we construct purely gravitational axisymmetric dynamicalmodels for the gas, both thin-disk models and models with more generalaxis ratios and velocity anisotropies. For the normal galaxies thenuclear gas dispersions are adequately reproduced assuming disks aroundthe BHs with masses that follow the M-σ relation. In contrast, thegas dispersions observed for the radio galaxies generally exceed thosepredicted by any of the models. We attribute this to the presence ofnongravitational motions in the gas that are similar to or larger thanthe gravitational motions. The nongravitational motions are presumablydriven by the active galactic nucleus (AGN), but we do not find arelation between the radiative output of the AGN and thenongravitational dispersion. Given the uncertainties about the dynamicalstate of the gas, it is not possible to uniquely determine the BH massfor each galaxy from its nuclear gas dispersion. However, for the sampleas a whole the observed dispersions do not provide evidence forsignificant deviations from the M-σ relation.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| The Active Galaxy NGC 3862 in a Compact Group in the Cluster A1367
We study a compact group of 18 galaxies in the cluster A1367 withredshifts z = 0.0208 0.025. The group’s center of activity in theradio is the galaxy NGC 3862, whose radio flux is an order of magnitudestronger than for the other members of the group. We present coordinatesderived from the Palomar plate archive together with recessionalvelocities, and analyze other characteristics of the group’sgalaxies. The results of 1400 MHz observations of NGC 3862 with theRATAN-600 radio telescope are presented. These observations indicatethat the galaxy’s radio emission is variable.
| When Less Is More: Are Radio Galaxies below the Fanaroff-Riley Break More Polarized on Parsec Scales?
We present images showing the first detections of polarization on parsecscales in the nuclei of four Fanaroff-Riley type I (low-luminosity)radio galaxies. Observations with VLBI at λ=3.6 cm reveal thepresence of ordered magnetic fields within ~1650 Schwarzschild radii ofthe putative central supermassive black hole. The relatively highfractional polarization in the parsec-scale jets of these galaxies isconsistent with the standard scheme unifying low-luminosity radiogalaxies with BL Lac objects. This result also suggests that these radiogalaxies lack the obscuring tori that apparently depolarize the nuclearemission in the more powerful FR II type radio galaxies, and that theirsupermassive black holes are poorly fed and/or inefficient radiators.
| 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.
| On the Magnetic Field in the Kiloparsec-Scale Jet of Radio Galaxy M87
Several low-power kiloparsec-scale jets in nearby radio galaxies areknown for their synchrotron radiation extending up to optical and X-rayphoton energies. Here we comment on high-energy γ-ray emission ofone particular object of this kind, i.e., the kiloparsec-scale jet ofthe M87 radio galaxy, resulting from Comptonization of the starlightphoton field of the host galaxy by the synchrotron-emitting jetelectrons. In our analysis, we include the relativistic bulk velocity ofthe jet, as well as the Klein-Nishina effects. We show that upper limitsto the kiloparsec-scale jet inverse Compton radiation imposed by theHESS and HEGRA Cerenkov Telescopes-which detected a variable source ofvery high energy γ-ray emission within 0.1d (~30 kpc) of the M87central region-give us an important constraint on the magnetic fieldstrength in this object, namely, that the magnetic field cannot besmaller than the equipartition value (referring solely to the radiatingelectrons) in the brightest knot of the jet, and most likely, is evenstronger. In this context, we point out a need for the amplification ofthe magnetic energy flux along the M87 jet from the subparsec tokiloparsec scales, suggesting the turbulent dynamo as a plausibleprocess responsible for the aforementioned amplification.
| 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.
| The HST view of the nuclear emission line region in low luminosity radio-galaxies
We study the properties of the emission line regions in two samples oflow luminosity radio-galaxies, while focusing on the Compact EmissionLine Region (CELR) revealed to be a characteristic feature of theseobjects by HST narrow-band imaging. We find a strong correlation betweenline and optical continuum nuclear emission, which suggests that theoptical cores (most likely of non-thermal origin) can be directlyassociated to the source of ionizing photons, i.e. that we are seeing ajet-ionized narrow line region. A photon budget argument indicates thatthe optical nuclear sources produce sufficient photon flux provided thatthe covering factor of the circum-nuclear gas is rather large, onaverage 0.3. Analysis of HST images and spectra suggests that the CELRmay take the form of a pc-scale, high filling factor structure, possiblyan optically thin torus. Estimates of the CELR mass lead to values assmall as 10{-}10^3 Mȯ, and photon counting sets a limitto the Broad Line Region mass of M_BLR < 10-2Mȯ. When considered together with the low accretion rateand the tenuous torus structure, a general paucity of gas in theinnermost regions of low luminosity radio-galaxies emerges as the maincharacterizing difference from more powerful Active Galactic Nuclei.
| 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.
| 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.
| Adiabatic relativistic models for the jets in the radio galaxy 3C 31
We present a general approach to the modelling of the brightness andpolarization structures of adiabatic, decelerating relativistic jets,based on the formalism of Matthews and Scheuer. We compare thepredictions of adiabatic jet models with deep, high-resolutionobservations of the radio jets in the FR I radio galaxy 3C 31. Adiabaticmodels require coupling between the variations of velocity, magneticfield and particle density. They are therefore more tightly constrainedthan the models we have previously presented for 3C 31. We show thatadiabatic models provide a poorer description of the data in two crucialrespects: they cannot reproduce the observed magnetic-field structuresin detail, and they also predict too steep a brightness decline alongthe jets for plausible variations of the jet velocity. We find that theinnermost regions of the jets show the strongest evidence fornon-adiabatic behaviour, and that the adiabatic models provideprogressively better descriptions of the jet emission at largerdistances from the galactic nucleus. We briefly discuss physicalprocesses which might contribute to this non-adiabatic behaviour. Inparticular, we develop a parametrized description of distributedparticle injection, which we fit to the observed total intensities. Weshow that particles are preferentially injected where bright X-rayemission is observed, and where we infer that the jets areoverpressured.
| 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.
| A Sample of Low-Redshift BL Lacertae Objects. I. The Radio Data
We present a new sample of 30 nearby (z<0.2) BL Lac objects, selectedto study the nuclear as well as the large-scale properties of low-powerradio sources. In this first paper, we show and discuss new radio datataken with the Very Large Array (19 objects at 1.4 GHz, either in A or Cconfiguration, or both) as well as with the Very Long Baseline Array (15sources at 5 GHz). On the kiloparsec scale, all objects exhibit acompact core and a variety of radio morphologies (jets, halos, andsecondary compact components). On the parsec scale, we find weak coresand a few short, one-sided jets. From the jet/counterjet ratio, coredominance, and synchrotron self-Compton model, we estimate the intrinsicorientation and velocity of the jets. The resulting properties of BL Lacobjects are similar to those of a parent population composed of FR Iradio galaxies.
| Nonthermal Hard X-Ray Emission in Galaxy Clusters Observed with the BeppoSAX PDS
We study the X-ray emission in a sample of galaxy clusters using theBeppoSAX PDS instrument in the 20-80 keV energy band. We estimate thenonthermal hard X-ray (HXR) cluster emission by modeling the thermalcontribution from the cluster gas and the nonthermal contamination fromthe unobscured active galactic nuclei (AGNs) in the clusters. We alsoevaluate the systematic uncertainties due to the backgroundfluctuations. Assuming negligible contamination from the obscured AGNs,the resulting nonthermal component is detected at a 2 σ level in~50% of the nonsignificantly AGN-contaminated clusters: A2142, A2199,A2256, A3376, Coma, Ophiuchus, and Virgo. The data are consistent with ascenario whereby relaxed clusters have no hard X-ray component ofnonthermal origin, whereas merger clusters do, with a 20-80 keVluminosity of ~1043-1044h-250 ergs s-1. The co-added spectrumof the above clusters indicates a power-law spectrum for the HXRemission with a photon index of 2.8+0.3-0.4 in the12-115 keV band, and we find indication that it has extendeddistribution. These indications argue against significant contaminationfrom obscured AGNs, which have harder spectra and a centrallyconcentrated distribution. These results are supportive of theassumption of the merger shock acceleration of electrons in clusters,which has been proposed as a possible origin of the nonthermal hardX-ray emission models. Assuming that the cosmic microwave backgroundphotons experience inverse Compton scattering from themerger-accelerated relativistic electrons and thus produce the observedHXR, the measured hard X-ray slope corresponds to a differentialmomentum spectra of the relativistic electrons with a slope ofμ=3.8-5.0. In presence of cluster magnetic fields this relativisticelectron population produces synchrotron emission with a spectral indexof 1.4-2.1, consistent with radio halo observations of merger clusters.Thus both hard X-ray and radio observations of merger clusters areconsistent with the inverse Compton model. The observed slope of the HXRemission is also consistent with that predicted by the nonthermalbremsstrahlung, which thus cannot be ruled out by the fit to the currentdata, even though this model requires an extreme, untenable clusterenergetics. Assuming a centrally concentrated distribution of HXRemission, the data require a harder slope for the HXR spectrum, which isconsistent with secondary electron models, but this model yields a worsefit to the PDS data and thus seems to be disfavored over the primaryelectron inverse Compton model.
| 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.
| Radio and Far-Infrared Emission as Tracers of Star Formation and Active Galactic Nuclei in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114galaxies in the seven nearest clusters (<100 Mpc) with prominentX-ray emission to investigate the impact of the cluster environment onthe star formation and active galactic nucleus (AGN) activity in themember galaxies. The X-ray selection criterion is adopted to focus onthe most massive and dynamically relaxed clusters. A large majority ofcluster galaxies show an excess in radio emission over that predictedfrom the radio-FIR correlation, the fraction of sources with radioexcess increases toward cluster cores, and the radial gradient in theFIR/radio flux ratio is a result of radio enhancement. Of theradio-excess sources, 70% are early-type galaxies, and the same fractionhost an AGN. The galaxy density drops by a factor of 10 from thecomposite cluster center out to 1.5 Mpc, yet galaxies show no change inFIR properties over this region and show no indication of masssegregation. We have examined in detail the physical mechanisms thatmight impact the FIR and radio emission of cluster galaxies. Whilecollisional heating of dust may be important for galaxies in clustercenters, it appears to have a negligible effect on the observed FIRemission for our sample galaxies. The correlations between radio and FIRluminosity and radius could be explained by magnetic compression fromthermal intracluster medium pressure. We also find that simple delayedharassment cannot fully account for the observed radio, FIR, and mid-IRproperties of cluster galaxies.
| Parsec-Scale Properties of Markarian 501
We present the results of a high angular resolution study of the BL Lacobject Markarian 501 in the radio band. We consider data taken at 14different epochs, ranging between 1.6 and 22 GHz in frequency, andincluding new Space VLBI observations obtained on 2001 March 5 and 6 at1.6 and 5 GHz. We study the kinematics of the parsec-scale jet andestimate its bulk velocity and orientation with respect to the line ofsight. Limb-brightened structure in the jet is clearly visible in ourdata, and we discuss its possible origin in terms of velocity gradientsin the jet. Quasi-simultaneous, multiwavelength observations allow us tomap the spectral index distribution and to compare it to the jetmorphology. Finally, we estimate the physical parameters of theparsec-scale jet.
| 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
| Multiple merging in the Abell cluster 1367
We present a dynamical analysis of the central 1.3 square degreesof the cluster of galaxies Abell 1367, based on 273 redshiftmeasurements (of which 119 are news). From the analysis of the 146confirmed cluster members we derive a significantly non-Gaussianvelocity distribution, with a mean location CBI =6484±81 km s-1 and a scale SBI =891±58 km s-1. The cluster appears elongated from theNorth-West to the South-East with two main density peaks associated withtwo substructures. The North-West subcluster is probably in the earlyphase of merging into the South-East substructure (0.2 Gyr beforecore crossing). A dynamical study of the two subclouds points out theexistence of a group of star-forming galaxies infalling into the core ofthe South-East subcloud and suggests that two other groups are infallinginto the NW and SE subclusters respectively. These three subgroupscontain a higher fraction of star-forming galaxies than the clustercore, as expected during merging events. Abell 1367 appears as a youngcluster currently forming at the intersection of two filaments.Based on observations obtained with the William Herschel Telescopeoperated on the island of La Palma (Spain) by the Isaac Newton Group,with the Loiano telescope belonging to the University of Bologna (Italy)and with the G.Haro telescope of the INAOE (Mexico).Table 7 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/425/429
| The inner kiloparsec of the jet in 3C 264.
We present new multi-frequency EVN, MERLIN and VLA observations of theradio source 3C 264, sensitive to linear scales ranging from parsec toseveral kiloparsecs. The observations confirm the existence of regionswith different properties in the first kiloparsec of the jet. The mostremarkable feature is the transition between a well collimated narrowjet at distances from the core below 80 pc, to a conical-shaped widejet, with an opening angle of 20°. Another change ofproperties, consisting of an apparent deflection of the jet ridge lineand a diminution of the surface brightness, occurs at a distance of300 pc from the core, coincident with the radius of a ring observedat optical wavelengths. Our observations add new pieces of informationon the spectrum of the radio-optical jet of 3C 264, with resultsconsistent with a synchrotron emission mechanism and a spectrum breakfrequency in the infrared. Brightness profiles taken perpendicularly tothe jet of 3C 264 are consistent with a spine brightened jet atdistances below 100 pc from the core, and an edge-brightened jet beyond,which can be interpreted as evidence of a transverse jet velocitystructure. Our observations do not allow us to distinguish between thepresence of a face-on dust and gas disk at the center of the host galaxyof 3C 264, or rather an evacuated bubble. However, the properties of thejet structure, the changes in the polarization angle, and the plausiblejet orientation can be naturally brought into agreement in the bubblescenario.
| 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.
| The radio jet velocities at high resolution
Different methods to derive jet velocity and orientation on parsecscales are presented. From these methods, I will discuss the velocitydistribution of parsec-scale jets and the possible presence ofacceleration or deceleration. Moreover, evidence of jet velocitystructures will be reported. Finally, I will present new data on thesuperluminal giant radio source 1144+35, to discuss, in detail, theproperties of pc-scale jets.
| Space Telescope Imaging Spectrograph Spectroscopy of the Emission-Line Gas in the Nuclei of Nearby FR-I Galaxies
We present the results of the analysis of a set of medium-resolutionspectra, obtained by the Space Telescope Imaging Spectrograph on boardthe Hubble Space Telescope, of the emission-line gas present in thenuclei of a complete sample of 21 nearby, early-type galaxies with radiojets (the UGC FR-I Sample). For each galaxy nucleus we presentspectroscopic data in the region of Hα and the derived kinematics.We find that in 67% of the nuclei the gas appears to be rotating and,with one exception, the cases where rotation is not seen are eitherface-on or have complex central morphologies. We find that in 62% of thenuclei the fit to the central spectrum is improved by the inclusion of abroad component. The broad components have a mean velocity dispersion of1349+/-345 km s-1 and are redshifted from the narrow linecomponents (assuming an origin in Hα) by 486+/-443 kms-1.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Radio-selected Galaxies in Very Rich Clusters at z <= 0.25. I. Multiwavelength Observations and Data Reduction Techniques
Radio observations were used to detect the ``active'' galaxy populationwithin rich clusters of galaxies in a nonbiased manner that is notplagued by dust extinction or the K-correction. We present wide-fieldradio, optical (imaging and spectroscopy), and ROSAT All-Sky Survey(RASS) X-ray data for a sample of 30 very rich Abell (R>=2) clusterswith z<=0.25. The VLA radio data samples the ultrafaint radio(L1.4>=2×1022 W Hz-1) galaxypopulation within these extremely rich clusters for galaxies withMR<=-21. This is the largest sample of low-luminosity 20cm radio galaxies within rich Abell clusters collected to date.The radio-selected galaxy sample represents the starburst (starformation rate >=5 Msolar yr-1) and activegalactic nuclei populations contained within each cluster. Archival andnewly acquired redshifts were used to verify cluster membership for most(~95%) of the optical identifications. Thus, we can identify all thestarbursting galaxies within these clusters, regardless of the level ofdust obscuration that would affect these galaxies being identified fromtheir optical signature. Cluster sample selection, observations, anddata reduction techniques for all wavelengths are discussed.
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