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Multifrequency observations of the jets in the radio galaxy NGC315
We present images of the jets in the nearby radio galaxy NGC315 madewith the Very Large Array at five frequencies between 1.365 and 5GHzwith resolutions between 1.5 and 45arcsec. Within 15arcsec of thenucleus, the spectral index of the jets is α= 0.61. Further fromthe nucleus, the spectrum is flatter, with significant transversestructure. Between 15 and 70arcsec from the nucleus, the spectral indexvaries from ~0.55 on-axis to ~0.44 at the edge. This spectral structuresuggests a change of dominant particle acceleration mechanism withdistance from the nucleus and the transverse gradient may be associatedwith shear in the jet velocity field. Further from the nucleus, thespectral index has a constant value of 0.47. We derive the distributionof Faraday rotation over the inner +/-400arcsec of the radio source andshow that it has three components: a constant term, a linear gradient(both probably due to our Galaxy) and residual fluctuations at the levelof 1-2radm-2. These residual fluctuations are smaller in thebrighter (approaching) jet, consistent with the idea that they areproduced by magnetic fields in a halo of hot plasma that surrounds theradio source. We model this halo, deriving a core radius of ~225arcsecand constraining its central density and magnetic field strength. Wealso image the apparent magnetic field structure over the first+/-200arcsec from the nucleus.
| 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.
| Deceleration from Entrainment in the Jet of the Quasar 1136-135?
By modeling the multiwavelength emission of successive regions in thejet of the quasar PKS 1136-135, we find indications that the jet suffersdeceleration near its end on a (deprojected) scale of ~400 kpc. We adopta continuous flow approximation, and we discuss the possibility that theinferred deceleration from a Lorentz factor of Γ=6.5 to 2.5 isinduced by entrainment of external gas. Some consequences of thisscenario are discussed.
| 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.
| Magnetic turbulence in cool cores of galaxy clusters
The analysis of Faraday rotation measurements (RM) gives insight intothe properties of cluster magnetic fields. Recently, Kolmogorov-likemagnetic turbulence was reported for the cool core of the Hydra A galaxycluster which was derived using a maximum likelihood analysis of an RMmap of this cluster (Vogt & Enßlin 2005). Here, we present ourunderstanding of this power spectrum by kinetic energy injection throughactive galaxies which drive a turbulent non-helical magnetic dynamo. Wedevelop an analytical model of the turbulence in cool cores and showthat it fits well to Faraday rotation measurements for a number of coolcore clusters. Moreover, our model allows predictions for magneticfields in clusters in which the appropriate observational information isstill missing, and allows predictions for yet unobserved quantities likethe hydrodynamical turbulence velocity and characteristic length scale,and suggests some level of magnetic intermittency and large-scalehydrodynamic viscosity.
| Faraday rotation variations along radio jets: the magnetic field in galaxy and group halos
Our modelling of FR I radio jets as decelerating, relativistic flowsallows us to derive their orientations accurately. We present images ofFaraday rotation for two of these these objects (3C 31 and NGC 315) andshow that the fluctuations of rotation measure (RM) are larger in thefainter (receding) jets, as expected if the rotation occurs in the hotgalaxy/group halos. The gas density is much lower in NGC 315 and the RMfluctuations are only just detectable.
| 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.
| Magnetic turbulence in cool cores of galaxy clusters
We argue that the recently reported Kolmogorov-like magnetic turbulencespectrum in the cool core of the Hydra A galaxy cluster can beunderstood by kinetic energy injection by active galaxies that drives aturbulent non-helical magnetic dynamo into its saturated state. Althoughdramatic differences exist between small-scale dynamo scenarios, theirsaturated state is expected to be similar, as we show for threescenarios: the flux rope dynamo, the fluctuation dynamo, and theexplosive dynamo. Based on those scenarios, we develop an analyticalmodel of the hydrodynamic and magnetic turbulence in cool cores. Themodel implies magnetic field strengths that fit well with Faradayrotation measurements and minimum energy estimates for the sample ofcool core clusters having such data available. Predictions for magneticfields in clusters for which the appropriate observational informationis still missing, and for yet unobserved quantities like thehydrodynamical turbulence velocity and characteristic length-scale areprovided. The underlying dynamo models suggest magnetic intermittencyand possibly a large-scale hydrodynamic viscosity. We conclude that thesuccess of the model to explain the field strength in cool core clustersindicates that in general cluster magnetic fields directly reflecthydrodynamical turbulence, also in clusters without cool cores.
| 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.
| Evidence for radio-source heating of groups
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
| Molecular Gas and Nuclear Activity in Radio Galaxies Detected by IRAS
This paper reports the latest results from a millimeter-wave (CO)spectroscopic survey of IRAS-detected radio galaxies withL1.4GHz~1023-1028 W Hz-1 inthe redshift range z~0.02-0.15. The IRAS flux-limited sample contains 33radio galaxies with different radio morphologies and a broad range ofinfrared luminosities (LIR=109-1012Lsolar), allowing for an investigation of (1) whether low-zradio-selected active galactic nuclei (AGNs) reside in moleculargas-rich host galaxies and (2) whether the CO properties are correlatedwith the properties of the host galaxy or the AGN. All of the radiogalaxies in Mazzarella et al. and Mirabel et al. have been reobserved.Three new CO detections have been made, raising the total number of COdetections to nine and setting the survey detection rate at ~25%. Manyof the CO lines have double-peaked profiles, and the CO line widths arebroad (average ΔvFWHM~500+/-130 km s-1),exceeding the average CO widths of both ultraluminous infrared galaxies(300+/-90 km s-1) and Palomar-Green QSOs (260+/-160 kms-1), and thus being indicative of massive host galaxies. TheCO luminosities translate into molecular gas masses of~(0.4-7)×109 Msolar, however, the 3 σCO upper limits for nondetections do not rule out a molecular gas massas high as that of the Milky Way (~3×109Msolar). Optical images of eight out of nine moleculargas-rich radio galaxies show evidence of close companions and/or tidalfeatures. Finally, there is no obvious correlation between radio powerand molecular gas mass. However, it is notable that only one F-R IIgalaxy out of 12 is detected in this CO survey; the remaining detectionsare of galaxies hosting F-R I and compact radio jets.
| Molecular Disks in the Elliptical Galaxies NGC 83 and NGC 2320
The molecular gas in (some) early-type galaxies holds important clues tothe history and the future of these galaxies. In pursuit of these clues,we have used the BIMA millimeter array to map CO emission in the giantelliptical galaxies NGC 83 and NGC 2320 and to search for CO emissionfrom the S0 galaxy NGC 5838. We also present V and R images of NGC 83and NGC 2320 that trace their dust distributions and enable a search fordisky stellar structures. The molecular gas in NGC 83 is well relaxed,but both CO and dust in NGC 2320 show asymmetric structures that may belinked to a recent acquisition of the gas. However, the specific angularmomentum distribution of molecular gas in NGC 2320 is consistent withthat of the stars. Internal origin of the gas (stellar mass loss)cannot, therefore, be ruled out on angular momentum grounds alone. Wealso consider the evidence for star formation activity and disk growthin these two elliptical galaxies. Radio continuum and FIR fluxes of NGC83 suggest star formation activity. NGC 2320 has bright [O III]emission, but its large radio-FIR flux ratio and the mismatch betweenthe kinematics of CO and [O III] suggest that the ionized gas should notbe attributed to star formation. The origin and future of these twoCO-rich early-type galaxies are thus complex, multifaceted stories.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| BIMA Millimeter-Wave Observations of the Core-Jet and Molecular Gas in the FR I Radio Galaxy NGC 3801
We present BIMA 3 mm observations of the radio continuum source and themolecular gas disk in the radio-loud Fanaroff and Riley type I (FR I)galaxy NGC 3801. We have detected a continuum source in the nucleus anddetermined that it has a flat millimeter-wave spectrum, suggesting thatthe emission is nonthermal and due to an AGN; the radio core is notevident in existing VLA observations. We also map the extended 3 mmemission from the previously known radio jets. In addition, we detect CO(1-0) emission associated with the dust disk observed in previous HSTimages. A velocity gradient is observed, indicating a 2 kpc radiusrotating gas ring or disk oriented roughly perpendicular to the radiojets. The inferred molecular gas mass of the disk isM(H2)=3×108 Msolar, about 1% ofthe dynamical mass. We also find a ~108 Msolarmolecular gas clump not associated with the gas disk. There is evidencethat this gas is associated with a merger and is infalling. Thissuggests that FR I activity is related to merger activity, as is thoughtto be the case for FR II radio galaxies. We also find indications thatone of the radio jets is entraining gas from the infalling moleculargas.
| 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.
| X-Ray Emission Properties of Large-Scale Jets, Hot Spots, and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet knots, hot spots, and radiolobes recently observed with Chandra and ASCA. This report discusses theorigin of their X-ray emissions and investigates the dynamics of thejets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1keV) frequencies for more than 40 radio galaxies. We examine threemodels for the X-ray production: synchrotron (SYN), synchrotronself-Compton (SSC), and external Compton (EC) on cosmic microwavebackground (CMB) photons. For the SYN sources-mostly jet knots in nearbylow-luminosity radio galaxies-X-ray photons are produced byultrarelativistic electrons with energies 10-100 TeV that must beaccelerated in situ. For the other objects, conservatively classified asSSC or EC sources, a simple formulation of calculating the ``expected''X-ray fluxes under an equipartition hypothesis is presented. We confirmthat the observed X-ray fluxes are close to the expected ones fornonrelativistic emitting plasma velocities in the case of radio lobesand the majority of hot spots, whereas a considerable fraction of jetknots are too bright in X-rays to be explained in this way. We examinetwo possibilities to account for the discrepancy in a framework of theinverse Compton model: (1) the magnetic field is much smaller than theequipartition value, and (2) the jets are highly relativistic onkiloparsec and megaparsec scales. We conclude that if the inverseCompton model is the case, the X-ray-bright jet knots are most likelyfar from the minimum-power condition. We also briefly discuss the otherpossibility, namely, that the observed X-ray emission from all the jetknots is synchrotron in origin.
| Rotating Molecular Gas Associated with a Silhouette Disk at the Center of the Radio Galaxy 3C 31
Aperture synthesis observations of 12CO(J=1-0) emissions ofthe radio galaxy 3C 31 (NGC 383) have been made using the NobeyamaMillimeter Array (NMA) and the RAINBOW interferometer (NMA plus theNobeyama 45 m radio telescope). Our high-resolution (1.9"×1.4", or640pc×470pc, at D=70 Mpc) 12CO image shows a circularlyrotating molecular gas disk that closely coincides with a silhouettedisk observed in the Hubble Space Telescope (HST) optical images. Themolecular gas mass (Mgas) of the disk is estimated to be9.7×108Msolar within a radius of 1 kpc, andthe peak gas surface density Σgas is4.0×102Msolar pc-2 at 440 pc fromthe center, if a Galactic I(CO)-to-N(H2) conversion factor(1.8×1020 cm-2 [K kms-1]-1) is applied. The rotation velocity of thedisk is 460 km s-1 at a radius of 1 kpc, giving an enclosedmass (dynamical mass) ofMdyn=5.0×1010Msolar within thisradius. The ratio of gas mass to dynamical mass, Mgas/Mdyn, is less than0.02, indicating that the gas disk at the center of 3C 31 is stableagainst gravitational instabilities, although the total gas mass of thenuclear disk in 3C 31 is fairly large compared with the nuclear gasconcentration observed in late-type spirals.
| 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 supermassive binary black hole in the quasar 3C 345
Radio loud active galactic nuclei present a remarkable variety of signsindicating the presence of periodical processes possibly originating inbinary systems of supermassive black holes, in which orbital motion andprecession are ultimately responsible for the observed broad-bandemission variations, as well as for the morphological and kinematicproperties of the radio emission on parsec scales. This scenario,applied to the quasar 3C 345, explains the observed variations of radioand optical emission from the quasar, and reproduces the structuralvariations observed in the parsec-scale jet of this object. The binarysystem in 3C 345 is described by two equal-mass black holes with massesof ≈7.1 × 10^8 {M}ȯ separated by ≈0.33 pcand orbiting with a period 480 yr. The orbital motion induces aprecession of the accretion disk around the primary black hole, with aperiod of ≈2570 yr. The jet plasma is described by a magnetized,relativistic electron-positron beam propagating inside a wider andslower electron-proton jet. The combination of Alfvén waveperturbations of the beam, the orbital motion of the binary system andthe precession of the accretion disk reproduces the variability of theoptical flux and evolution of the radio structure in 3C 345. Thetimescale of quasi-periodic flaring activity in 3C 345 is consistentwith typical disk instability timescales. The present model cannot ruleout a small-mass orbiter crossing the accretion disk and causingquasi-periodic flares.
| 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.
| The GEMS project: X-ray analysis and statistical properties of the group sample
The Group Evolution Multiwavelength Study (GEMS) involves amultiwavelength study of a sample of 60 galaxy groups, chosen to span awide range of group properties. Substantial ROSAT Position SensitiveProportional Counter (PSPC) observations, available for all of thesegroups, are used to characterize the state of the intergalactic mediumin each. We present the results of a uniform analysis of these ROSATdata and a statistical investigation of the relationship between X-rayand optical properties across the sample. Our analysis improves inseveral respects on previous work: (i) we distinguish between systems inwhich the hot gas is a group-scale medium and those in which it appearsto be just a hot halo associated with a central galaxy; (ii) weextrapolate X-ray luminosities to a fixed overdensity radius(r500) using fitted surface brightness models, in order toavoid biases arising from the fact that cooler systems are detectable tosmaller radii, and (iii) optical properties have been rederived in auniform manner from the NASA Extragalactic Database, rather than relyingon the data in the disparate collection of group catalogues from whichour systems are drawn.The steepening of the LX-TX relation in the groupregime reported previously is not seen in our sample, which fits well onto the cluster trend, albeit with large non-statistical scatter. Anumber of biases affect the fitting of regression lines under thesecircumstances, and until the impact of these has been thoroughlyinvestigated it seems best to regard the slope of the groupLX-TX relation as being poorly determined. Asignificant problem in comparing the properties of groups and clustersis the derivation of system radii, to allow different systems to becompared within regions having the same overdensity. We find evidencethat group velocity dispersion (σv) provides a veryunreliable measure of system mass (and hence radius), with a number ofgroups having remarkably low values of σv, given thatthey appear from their X-ray properties to be collapsed systems. Weconfirm that the surface brightness profiles of groups are significantlyflatter than those of clusters - the maximum value of theβfit parameter for our sample is 0.58, lower than thetypical value of 0.67 seen in clusters - however, we find no significanttendency within our sample for cooler groups to show flatter profiles.This result is inconsistent with simple universal pre-heating models.The morphology of the galaxies in the GEMS groups is correlated to theirX-ray properties in a number of ways: we confirm the very strongrelationship between X-ray emission and a dominant early-type centralgalaxy, which has been noted since the early X-ray studies of groups,and also find that spiral fraction is correlated with the temperature ofthe hot gas and hence the depth of the gravitational potential. A classof spiral-rich groups with little or no X-ray emission probablycorresponds to groups that have not yet fully collapsed.
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