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TeV Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters
Galaxy clusters might be sources of TeV gamma rays emitted byhigh-energy protons and electrons accelerated by large-scale structureformation shocks, galactic winds, or active galactic nuclei.Furthermore, gamma rays may be produced in dark matter particleannihilation processes at the cluster cores. We report on observationsof the galaxy clusters Perseus and A2029 using the 10 m Whipple Cerenkovtelescope during the 2003-2004 and 2004-2005 observing seasons. We applya two-dimensional analysis technique to scrutinize the clusters for TeVemission. In this paper we first determine flux upper limits on TeVgamma-ray emission from point sources within the clusters. Second, wederive upper limits on the extended cluster emission. We subsequentlycompare the flux upper limits with EGRET upper limits at 100 MeV andtheoretical models. Assuming that the gamma-ray surface brightnessprofile mimics that of the thermal X-ray emission and that the spectrumof cluster cosmic rays extends all the way from thermal energies tomulti-TeV energies with a differential spectral index of -2.1, ourresults imply that the cosmic-ray proton energy density is less than7.9% of the thermal energy density for the Perseus Cluster.
| Stopping Cooling Flows with Jets
We describe two-dimensional gasdynamical models of jets that carry massas well as energy to the hot gas in galaxy clusters. These flows havemany attractive attributes for solving the galaxy cluster cooling flowproblem: why the hot gas temperature and density profiles resemblecooling flows but show no spectral evidence of cooling to lowtemperatures. Using an approximate model for the cluster A1795, we showthat mass-carrying jets can reduce the overall cooling rate to or belowthe low values implied by X-ray spectra. Biconical subrelativistic jets,described by several ad hoc parameters, are assumed to be activated whengas flows toward or cools near a central supermassive black hole. As thejets proceed out from the center, they entrain more and more ambientgas. The jets lose internal pressure by expansion and are compressed bythe ambient cluster gas, becoming rather difficult to observe. For awide variety of initial jet parameters and several feedback scenarios,the global cooling can be suppressed for many gigayears whilemaintaining cluster temperature profiles similar to those observed. Theintermittency of the feedback generates multiple generations of X-raycavities similar to those observed in the Perseus Cluster and elsewhere.
| 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.
| RM-synthesis of the Perseus cluster
We present low frequency radio polarimetric observations of the Perseuscluster. The data were taken with the Westerbork Synthesis RadioTelescope (WSRT) between 315-360 MHz. We have discovered faint,extended, highly polarized emission that we associate with the Perseuscluster \citep{DeBruynBrentjens2005}. We propose that at least one ofthese structures is associated with large-scale structure formation gasinflow from the Perseus-Pisces supercluster. The extragalactic emissionis seen through a polarized Galactic synchrotron foreground screen. Thediscovery was made possible by a novel rotation measure analysistechnique, called Faraday Rotation Measure Synthesis, or RM-synthesisfor short \citep{BrentjensDeBruyn2005}.
| 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 Small X-Ray Corona of the Narrow-Angle Tail Radio Galaxy NGC 1265 Soaring through the Perseus Cluster
A deep Chandra observation of NGC 1265 (3C 83.1B), the prototype for thenarrow-angle tail (NAT) radio galaxy, reveals a small cool X-ray thermalcorona (~0.6 keV) embedded in the hot ICM of the Perseus cluster (~6.7keV). The corona is asymmetric with a sharp edge (~2.2", or 0.8 kpc fromthe nucleus) to the south and an extension to the north (at least~8'' from the nucleus), which are interpreted as the resultof ram pressure, as it cannot be explained solely by the static ICMconfinement. We estimate that the corona is moving with a velocity of~2.4-4.2 times the local sound speed to the south. The presence of thesharp edge on this small corona indicates that the transport processesare largely suppressed by the magnetic field there. The magnetic fieldaround the corona also suppresses heat conduction by at least a factorof ~60 across the corona boundary. We conclude that it is unrealistic tostudy the interaction between the small X-ray coronae and the hot ICMwithout considering the roles played by the magnetic field, a factor notincluded in current simulations. An absorbed(NH=1.5-3×1022 cm-2) nucleus isalso detected, which is not usual for FR I radio galaxies. Weak X-rayemission from three inner radio knots in the jets is also detected.Indentations at the east and west of the corona indicate interactionbetween the jets and the X-ray corona. Narrow jets carry great amountsof energy out of the central AGN and release the energy outside thecorona, preserving the tiny and vulnerable corona. This case revealsthat the inner kiloparsec core of the corona of massive galaxies cansurvive both high-speed stripping and powerful AGN feedback. Thus, thecooling of the X-ray coronae potentially provides fuel to the centralSMBH in rich environments in which the amount of galactic cold gas is ata minimum.
| 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.
| 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.
| Diffuse polarized emission associated with the Perseus cluster
We report on full-polarization radio observations of the Perseus cluster(Abell 426) using the Westerbork Synthesis RadioTelescope (WSRT) at wavelengths from 81-95 cm. We detect faint, veryextended polarized emission throughout the cluster region. We haveemployed a novel technique, Rotation Measure synthesis (Brentjens &de Bruyn, 2005, A&A, 441, 1217) to unravel the polarizationproperties of the emission across the full field of view. We detectpolarized emission over a wide range of RM from about 0 to 90 radm-2. Low RM emission (RM < 15 rad m-2) isattributed to the local Galactic foreground. It has a chaotic structurewith smooth changes in polarization angle on scales of the order of10´-30´, not unlike those seen by Haverkorn et al. (2003a,A&A, 403, 1045) at the same frequencies. Emission at values of RM> 30 rad m-2 on the other hand, shows organized structureson scales up to a degree and displays rapidly fluctuating polarizationangles on scales of the synthesized beam. A Galactic foregroundinterpretation for the high RM emission can not be ruled out, butappears extremely implausible. WSRT observations at 21 cm of the RM of adozen discrete sources surrounding the Perseus cluster indicate a smoothlarge-scale gradient in the Galactic foreground RM. The diffusestructures have a clear excess RM of about 40 rad m-2relative to these distant radio galaxies. This excess Faraday depth, thegenerally good spatial association with the cluster and the differentmorphology of the high RM emission, compared to the genuine Galacticforeground emission, all point to an association of the high RM emissionwith the Perseus cluster. The polarized emission reaches typical surfacebrightness levels of 0.5-1 mJy per 2 arcmin×3 arcmin beam and mustbe rather highly polarized (20%). Due to dynamic range limitationsand lack of sensitivity to large-scale structure we have not yetdetected the corresponding total intensity. Most of the polarizedemission, located at distances of about 1° from the cluster centre,appears too bright, by about 1-2 orders of magnitude, to be explainableas Thomson scattered emission of the central radio source off thethermal electrons in the cluster. However, this remains a viableexplanation for the highly polarized 21 cm emission from the inner10´-20´and part of the 81-95 cm emission. The bulk of theemission associated with the Perseus cluster may instead be related tobuoyant bubbles of relativistic plasma, probably relics from stillactive or now dormant AGN within the cluster. A lenticular shapedstructure, referred to as the lens, and measuring 0.5-1 Mpc isstrikingly similar to the structures predicted b Enßlin et al.(1998, A&A, 332, 395). At the western edge of the cluster, we detectvery long, linear structures that may be related to shocks caused byinfall of gas into the Perseus cluster along the Perseus-Piscesfilamentary structure of the cosmic web.
| 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.
| Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
| 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.
| 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.
| Jet Deflection via Crosswinds: Laboratory Astrophysical Studies
We present new data from high energy density laboratory experimentsdesigned to explore the interaction of a heavy hypersonic radiative jetwith a crosswind. The jets are generated with the MAGPIE pulsed powermachine, where converging conical plasma flows are produced from acylindrically symmetric array of inclined wires. Radiative hypersonicjets emerge from the convergence point. The crosswind is generated byablation of a plastic foil via soft X-rays from the plasma convergenceregion. Our experiments show that the jets are deflected by the actionof the crosswind, with the angle of deflection dependent on theproximity of the foil. Shocks within the jet beam are apparent in thedata. Analysis of the data shows that the interaction of the jet andcrosswind is collisional and therefore in the hydrodynamic regime. MHDplasma code simulations of the experiments are able to recover thedeflection behavior seen in the experiments. We consider theastrophysical relevance of these experiments, applying published modelsof jet deflection developed for active galactic nuclei and young stellarobjects. Fitting the observed jet deflections to quadratic trajectoriespredicted by these models allows us to recover a set of plasmaparameters consistent with the data. We also present results ofthree-dimensional numerical simulations of jet deflection using a newastrophysical adaptive mesh refinement code. These simulations showhighly structured shocks occurring within the beam similar to what wasobserved in the experiments.
| 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.
| 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.
| 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
| Decametric survey of discrete sources in the Northern sky. XIIIb. Spectra of discrete sources in the declination zone from +300 to +400
New data obtained in the course of the decametric survey of the Northernsky with the UTR-2 radiotelescope in the declination zone from+300 to +400 were used together with data fromhigh-frequency radio surveys to construct spectra for 62 radio sourcesin the frequency range from 12.6 to 50000 MHz. Spectra plots andparameters for these radio sources are given together with statisticalanalysis.
| The Cross-Wavelet Transform and Analysis of Quasi-periodic Behavior in the Pearson-Readhead VLBI Survey Sources
We introduce an algorithm for applying a cross-wavelet transform toanalysis of quasi-periodic variations in a time series and introducesignificance tests for the technique. We apply a continuous wavelettransform and the cross-wavelet algorithm to the Pearson-Readhead VLBIsurvey sources using data obtained from the University of Michigan 26 mparaboloid at observing frequencies of 14.5, 8.0, and 4.8 GHz. Thirty ofthe 62 sources were chosen to have sufficient data for analysis, havingat least 100 data points for a given time series. Of these 30 sources, alittle more than half exhibited evidence for quasi-periodic behavior inat least one observing frequency, with a mean characteristic period of2.4 yr and standard deviation of 1.3 yr. We find that out of the 30sources, there were about four timescales for every 10 time series, andabout half of those sources showing quasi-periodic behavior repeated thebehavior in at least one other observing frequency.
| Pearson-Readhead Survey Sources. II. The Long-Term Centimeter-Band Total Flux and Linear Polarization Properties of a Complete Radio Sample
Using UMRAO centimeter-band total flux density and linear polarizationmonitoring observations of the complete Pearson-Readhead extragalacticsource sample obtained between 1984 August and 2001 March, we identifythe range of variability in extragalactic objects as functions ofoptical and radio morphological classification and relate total fluxdensity variations to structural changes in published coeval VLBI mapsin selected objects. As expected, variability is common in flat- orinverted-spectrum (α<=0.5) core-dominated QSOs and BL Lac objects.Unexpectedly, we find flux variations in several steep-spectrum samplemembers, including the commonly adopted flux standard 3C 147. Suchvariations are characteristically several-year rises or declines orinfrequent outbursts, requiring long-term observations for detection: weattribute them to the brightening of weak core components, a change thatis suppressed by contributions from extended structure in all but thestrongest events, and identify a wavelength dependence for the amplitudeof this variability consistent with the presence of opacity in someportions of the jet flow. One morphological class of steep-spectrumobjects, the compact symmetric objects (CSOs), characteristically showsonly low-level variability. We examine the statistical relation betweenfractional polarization and radio class based on the data at 14.5 and4.8 GHz. The blazars typically exhibit flat-to-inverted polarizationspectra, a behavior attributed to opacity effects. Among thesteep-spectrum objects, the lobe-dominated FR I galaxies have steepfractional polarization spectra, while the FR II galaxies exhibitfractional polarization spectra ranging from inverted to steep, with noidentifiable common property that accounts for the range in behavior.For the CSO/gigahertz-peaked spectrum sources, we verify that thefractional polarizations at 4.8 GHz are only of the order of a fewtenths of a percent, but at 14.5 GHz we find significantly higherpolarizations, ranging from 1% to 3%; this frequency dependence supportsa scenario invoking Faraday depolarization by a circumnuclear torus. Wehave identified preferred orientations of the electric vector of thepolarized emission (EVPA) at 14.5 and 4.8 GHz in roughly half of theobjects and compared these with orientations of the flow directionindicated by VLBI morphology. When comparing the distributions of theorientation offsets for the BL Lac objects and the QSOs, we finddifferences in both range and mean value, in support of intrinsic classdifferences. In the shock-in-jet scenario, we attribute this to theallowed range of obliquities of shocks developing in the flow relativeto the flow direction: in the BL Lac objects the shocks are nearlytransverse to the flow direction, while in the QSOs they include abroader range of obliquities and can be at large angles to it. The factthat we find long-term stability in EVPA over many events implies that adominant magnetic field orientation persists; in the core-dominatedobjects, with small contribution from the underlying quiescent jet, thisplausibly suggests that the magnetic field has a long-term memory, withsubsequent shock events exhibiting similar EVPA orientation, or,alternatively, the presence of a standing shock in the core. We havelooked for systematic, monotonic changes in EVPA, which might beexpected in the emission from a precessing jet, a model currentlyinvoked for some AGNs; none were identified. Further, we carried out aScargle periodogram analysis of the total flux density observations, butfound no strong evidence for periodicity in any of the sample sources.The only well-established case in support of both jet precession andperiodic variability remains the non-sample member OJ 287.
| 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 problems of ageing
I present a brief overview of observational, modeling and theoreticalissues related to ageing calculations based on spectral steepening.These include problems such as inhomogeneous magnetic fields, diffusionof relativistic particles, confusion from multiple particle populationsand particle acceleration. Although some of the effects are only oforder unity, others call into question the entire ageing paradigm. Irefer to and show some data illustrating these problems and make a fewrecommendations about how we should proceed given these uncertainties.
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