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J0041+3224: a new double-double radio galaxy
We report the discovery of a double-double radio galaxy (DDRG),J0041+3224, with the Giant Metrewave Radio Telescope (GMRT) andsubsequent high-frequency observations with the Very Large Array (VLA).The inner and outer doubles are aligned within ~4° and arereasonably collinear with the parent optical galaxy. The outer doublehas a steeper radio spectrum compared to the inner one. Using anestimated redshift of 0.45, the projected linear sizes of the outer andinner doubles are 969 and 171 kpc, respectively. The time-scale ofinterruption of jet activity has been estimated to be ~20 Myr, similarto other known DDRGs. We have compiled a sample of known DDRGs, and havere-examined the inverse correlation between the ratio of theluminosities of the outer to the inner double and the size of the innerdouble, lin. Unlike the other DDRGs with lin>~50 kpc, the inner double of J0041+3224 is marginally more luminous thanthe outer one. The two DDRGs with lin<~ few kpc have amore luminous inner double than the outer one, possibly due to a higherefficiency of conversion of beam energy as the jets propagate throughthe dense interstellar medium. We have examined the symmetry parametersand found that the inner doubles appear to be more asymmetric in bothits armlength and its flux density ratios compared to the outer doubles,although they appear marginally more collinear with the core than theouter double. We discuss briefly the possible implications of thesetrends.
| The low-power nucleus of PKS 1246-410 in the Centaurus cluster
We present Chandra, Very Large Array (VLA) and Very Long Baseline Array(VLBA) observations of the nucleus of NGC 4696, a giant elliptical inthe Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS1246-410 in the Centaurus cluster is a nearby example of a radio galaxyin a dense cluster environment. In analysing the new X-ray data, we havefound a compact X-ray feature coincident with the optical and radiocore. While nuclear emission from the X-ray source is expected, itsluminosity is low, <1040ergs-1. We estimate theBondi accretion radius to be 30pc and the accretion rate to be0.01Msolaryr-1, which under the canonicalradiative efficiency of 10 per cent would overproduce by 3.5 orders ofmagnitude the radiative luminosity. Much of this energy can be directedinto the kinetic energy of the jet, which over time inflates theobserved cavities seen in the thermal gas. The VLBA observations reveala weak nucleus and a broad, one-sided jet extending over 25pc inposition angle -150°. This jet is deflected on the kiloparsec-scaleto a more east-west orientation (position angle of -80°).
| 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.
| 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.
| The SCUBA Local Universe Galaxy Survey - III. Dust along the Hubble sequence
We present new results from the Submillimetre Common-User BolometerArray (SCUBA) Local Universe Galaxy Survey (SLUGS), the first largesystematic submillimetre (submm) survey of the local Universe. Since ourinitial survey of a sample of 104 IRAS-selected galaxies we have nowcompleted a survey of a sample of 81 optically selected galaxies,observed with the SCUBA camera on the James Clerk Maxwell Telescope.Since SCUBA is sensitive to the 90 per cent of dust too cold to radiatesignificantly in the IRAS bands our new sample represents the firstunbiased SCUBA survey of dust in galaxies along the whole length of theHubble sequence.We find little change in the properties of dust in galaxies along theHubble sequence, except a marginally significant trend for early-typegalaxies to be less-luminous submm sources than late types. Wenevertheless detected six out of 11 elliptical galaxies, although someof the emission may possibly be synchrotron rather than dust emission.As in our earlier work on IRAS galaxies we find that the IRAS and submmfluxes are well fitted by a two-component dust model with dustemissivity index β= 2. The major difference from our earlier workis that we find the ratio of the mass of cold dust to the mass of warmdust is much higher for our optically selected galaxies and can reachvalues of ~1000. Comparison of the results for the IRAS and opticallyselected samples shows that there is a population of galaxies containinga large proportion of cold dust that is unrepresented in the IRASsample.We derive local submm luminosity and dust mass functions, both directlyfrom our optically selected SLUGS sample, and by extrapolation from theIRAS Point Source Catalogue Redshift Survey (PSCz) survey using themethod of Serjeant and Harrison (by extrapolating the spectral energydistributions of the IRAS PSCz survey galaxies out to 850μm we probea wider range of luminosities than probed directly by the SLUGSsamples), and find excellent agreement between the two. We find them tobe well fitted by Schechter functions except at the highestluminosities. We find that as a consequence of the omission of coldgalaxies from the IRAS sample the luminosity function presented in ourearlier work is too low by a factor of 2, reducing the amount of cosmicevolution required between the low-z and high-z Universe.
| 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 galaxy cluster Abell 3581 as seen by Chandra
We present results from an analysis of a Chandra observation of thecluster of galaxies Abell 3581. We discover the presence of apoint-source in the central dominant galaxy that is coincident with thecore of the radio source PKS 1404-267. The emission from theintracluster medium is analysed, both as seen in projection on the sky,and after correcting for projection effects, to determine the spatialdistribution of gas temperature, density and metallicity. We find thatthe cluster, despite hosting a moderately powerful radio source, shows atemperature decline to around 0.4 Tmax within the central 5kpc. The cluster is notable for the low entropy within its core. We testand validate the XSPEC PROJCT model for determining the intrinsiccluster gas properties.
| 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.
| 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.
| Resolving the Steep-Spectrum Emission in the Central Radio Source in ZwCl 0735.7+7421
It has long been know that an extremely steep spectrum radio source liesat the center of ZwCl 0735.7+7421, a galaxy cluster with high X-rayluminosity and a cooling core. In this Letter, we present VLAobservations of this radio source at both 1425 and 325 MHz. With aresolution below 21" (75 kpc) for both 1425 and 325 MHz, we show themorphology of the central source and find that it is most likely a large(400 kpc) radio galaxy rather than diffuse cluster emission. We estimatea steep spectral index of α1400325=-1.54 forthe core (although it may be contaminated by lobe emission), while theouter lobes are extremely steep objects, both withα1400325<-3.1. We also find evidence forrestarted core activity in the form of a set of inner lobes oriented ata somewhat different angle from the outer lobes. A spectral analysisextending the frequency range down to 74 MHz appears to show a turnoverat very low frequencies. Comparison of the minimum-energy radiopressures with the average thermal pressure surrounding the radiocavities from McNamara et al. shows that the radio lobes appear to beroughly in pressure balance with the thermal gas.
| 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.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| A 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.
| 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.
| 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.
| Revised Rates of Stellar Disruption in Galactic Nuclei
We compute rates of tidal disruption of stars by supermassive blackholes in galactic nuclei, using downwardly revised black hole massesfrom the MBH-σ relation. In galaxies with steep nucleardensity profiles, which dominate the overall event rate, the disruptionfrequency varies inversely with assumed black hole mass. We compute atotal rate for nondwarf galaxies of ~10-5 yr-1Mpc-3, about a factor of 10 higher than in earlier studies.Disruption rates are predicted to be highest in nucleated dwarfgalaxies, assuming that such galaxies contain black holes. Monitoring ofa rich galaxy cluster for a few years could rule out the existence ofintermediate-mass black holes in dwarf galaxies.
| Dust and Ionized Gas in Nine Nearby Early-Type Galaxies Imaged with the Hubble Space Telescope Advanced Camera for Surveys
We present V and I continuum images and Hα+[N II] maps of nineearly-type galaxies observed with the Wide Field Channel of the AdvancedCamera for Surveys on the Hubble Space Telescope. Dust and ionized gasare detected in all galaxies. The optical nebulae are primarilyconcentrated on the nuclei and extend out to radii of a few hundredparsecs in compact clumps, filaments, or disks. Two galaxies, NGC 6166and NGC 6338, also possess diffuse, ionized filaments on kiloparsecscales. The ionized gas is entirely contained within the nuclear disksof ESO 208-G021, NGC 3078, and NGC 7720. In the radio-loud galaxy NGC6166, emission-line filaments are detected along the radio lobes,possibly as a result of shock ionization. A wide range of ionized gasmasses, Mg~7×102-3×106Msolar, are calculated from the observed fluxes. Even in thissmall sample, the orientation of the ionized material correlates wellwith the major or minor axis of the galaxies, consistent with anexternal origin for the dust and gas.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
| The Globular Cluster System of NGC 1399. II. Kinematics of a Large Sample of Globular Clusters
We study the kinematics and dynamics of the globular cluster system ofNGC 1399, the brightest elliptical galaxy near the center of the Fornaxcluster of galaxies. The observational data consists ofmedium-resolution spectra, obtained at the Very Large Telescope withFORS2 and the Mask Exchange Unit (MXU). Our sample comprises 468 radialvelocities in the magnitude range 201.6) and blue clusters (C-R<1.6), and find velocitydispersions for these groups of 255+/-13 and 291+/-14 km s-1,respectively, again radially constant. Any possible rotation of eitherof these cluster populations is below the detection limit, with theexception of a weak signature of rotation for the blue clusters moredistant than 6'. Spherical models point to a circular velocity of419+/-30 km s-1, assuming isotropy for the red clusters. Thisvalue is constant out to 40 kpc. The inferred dark halo potential can bewell represented by a logarithmic potential. A halo of the NFW type alsoprovides a good fit to the observations. The orbital structure of theclusters can only be weakly constrained. It is consistent with isotropyfor the red clusters and a slight tangential bias for the blue clusters.Some mass profiles derived from X-ray analyses do not agree with aconstant circular velocity within our radial range, irrespective of itsexact value. Interpreting the extreme low radial velocities as spacevelocities of bound clusters near their pericentric distances wouldrequire an extension of the cluster system of at least 200 kpc.Implications for formation scenarios of the cluster system are brieflycommented on.Based on observations collected at the European Southern Observatory,Cerro Paranal, Chile; ESO program 66.B-0393.
| An Attempt to Probe the Radio Jet Collimation Regions in NGC 4278, NGC 4374 (M84), and NGC 6166
NRAO Very Long Baseline Array (VLBA) observations of NGC 4278, NGC 4374(M84), NGC 6166, and M87 (NGC 4486) have been made at 43 GHz in aneffort to image the jet collimation region. This is the first attempt toimage the first three sources at 43 GHz using very long baselineinterferometry (VLBI) techniques. These three sources were chosenbecause their estimated black hole mass and distance implied aSchwarzschild radius with large angular size, giving hope that the jetcollimation regions could be studied. Phase referencing was utilized forthe three sources because of their expected low flux densities. M87 waschosen as the calibrator for NGC 4374 because it satisfied thephase-referencing requirements: near the source and sufficiently strong.Having observed M87 for a long integration time, we have detected itssubparsec jet, allowing us to confirm previous high-resolutionobservations made by Junor, Biretta, & Livio, who have indicatedthat a wide opening angle was seen near the base of the jet. Phasereferencing successfully improved our image sensitivity, yieldingdetections and providing accurate positions for NGC 4278, NGC 4374, andNGC 6166. These sources are point dominated but show suggestions ofextended structure in the direction of the large-scale jets. However,higher sensitivity will be required to study their subparsec jetstructure.
| Dust in 3CR radio galaxies: On the FR 1 - FR 2 difference
We compare three 3CR samples of 11 FR 1 galaxies, 17 FR 2 galaxies and18 lobe-dominated quasars contained in the ISO Data Archive. In contrastto the powerful FR 2 galaxies with edge-brightened lobes, the low radiopower FR 1 galaxies in our sample do not exhibit any high MIR or FIRdust luminosity, which is typical for a buried, intrinsically moreluminous AGN. This consolidates the fact already inferred from opticalstudies that their AGNs have only a relatively low luminosity. Also theFR 1 galaxies show a high FIR/MIR luminosity ratio, compared to quasars,suggesting that their FIR luminosity is substantially powered by theinterstellar radiation field (ISRF) of the giant elliptical hosts.Finally, we discuss the FR 1 - FR 2 morphological dichotomy. FR 1galaxies do not have more interstellar matter (ISM) than FR 2s as traced- on the large scale - by the cool FIR emitting dust and - in thenuclear region - by the warm MIR emitting dust. Due to the lack ofcentral gas we suggest that the black holes of our FR 1 galaxies are fedat a lower accretion rate than those of the FR 2 galaxies.Based on observations with the Infrared Space Observatory ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands and the UK) and with theparticipation of ISAS and NASA.
| Optical nuclei of radio-loud AGN and the Fanaroff-Riley divide
We investigate the nature of the point-like optical nuclei that havebeen found in the centres of the host galaxies of a majority of radiogalaxies by the Hubble Space Telescope. We examine the evidence thatthese optical nuclei are relativistically beamed, and look fordifferences in the behaviour of the nuclei found in radio galaxies ofthe two Fanaroff-Riley types. We also attempt to relate this behaviourto the properties of the optical nuclei in their highly beamedcounterparts (the BL Lac objects and radio-loud quasars) as hypothesizedby the simple Unified Scheme. Simple model-fitting of the data suggeststhat the emission may be coming from a non-thermal relativistic jet. Itis also suggestive that the contribution from an accretion disk is notsignificant for the FRI objects and for the narrow-line radio galaxiesof FRII type, while it may be significant for the Broad-line objects,and consistent with the idea that the FRII optical nuclei seem to sufferfrom extinction due to an obscuring torus while the FRI optical nucleido not. These results are broadly in agreement with the Unified Schemefor radio-loud AGNs.Appendix C is only available in electronic form athttp://www.edpsciences.org
| Radio galaxies in cooling core clusters. Renewed activity in the nucleus of 3C 317?
We present the results of VLBA observations of the radio galaxy 3C 317,associated with the cD galaxy UGC 09799 at the centre of the coolingcore cluster of galaxies A2052. These observations were carried out at1.7 GHz, 4.9 GHz and 8.3 GHz, in polarimetric mode, and allowed us toimage the parsec scale region of the source. Our analysis suggests thatthe nucleus of 3C 317 hosts a very young radio source. The estimatedradiative age for the radio structure within the inner 10 pc is 170yr. Given the existence of extended radio emission on the arcsecondscale, we suggest that 3C 317 is a restarted radio galaxy. Theimplications of this result in the light of the interaction betweenradio plasma and thermal hot gas in clusters of galaxies are brieflydiscussed.
| 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
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