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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.
| Precession of the super-massive black hole in NGC 1275 (3C 84)?
The X-ray holes at the centre of the Perseus cluster of galaxies are notall at the same position angle with respect to the centre of thecluster. This configuration would result if the jet inflating thebubbles is precessing, or moving around, and the bubbles detach atdifferent times. The orientations which best fit the observed traveldirections are an inclination of the precession axis to the line ofsight of 120° and an opening angle of 50°. From the time-scalesfor the bubbles seen in the cluster, the precession time-scale,τprec, is around 3.3 × 107yr. Thebubbles rising up through different parts of the cluster may haveinteracted with the central cool gas, forming the whorl of cool gasobserved in the temperature structure of the cluster. The dynamics ofbubbles rising in fluids is discussed. The conditions present in thecluster are such that oscillatory motion, observed for bubbles rising influids on Earth, should take place. However, the time-scale for thismotion is longer than that taken for the bubbles to evolve intospherical-cap bubbles, which do not undergo a path instability, so suchmotion is not expected to occur.
| 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.
| 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.
| 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.
| Magnetic fields in jets: ordered or disordered?
The question of the degree of order in the magnetic fields ofrelativistic jets is important to any understanding of their production.Both vector-ordered (e.g. helical) and disordered, but anisotropicfields can produce the high observed degrees of polarization. We outlineour models of jets in FR I radio galaxies as decelerating relativisticflows. We then present theoretical calculations of the synchrotronemission from different field configurations and compare them withobserved emission from FR I jets. We show that large-scale helicalfields (with significant poloidal and toroidal components) areinconsistent with observations. The combination of an ordered toroidaland disordered poloidal component is consistent with our data, as is anentirely disordered field. Jets must also contain small, but significantamounts of radial field.
| Merger origin of radio galaxies investigated with H I observations
We present results of an H I study of a complete sample of nearby radiogalaxies. Our goal is to investigate whether merger or interactionevents could be at the origin of the radio-AGN activity. Around five ofour radio galaxies, hosted mainly by early-type galaxies, we detectextended H I in emission. In most cases this H I is distributed in large(up to 190 kpc) and massive (up to M_HI 1010Mȯ) disk- or ring-like structures, that show fairlyregular rotation around the host galaxy. This suggests that in thesesystems a major merger likely occurred, but at least several Gyr ago.For the H I-rich radio galaxy B2 0648+27 we confirm such a merger originthrough the detection of a post-starburst stellar population thatdominates the visible light throughout this system. The timescale of thecurrent episode of radio-AGN activity in our H I- rich radio galaxies isseveral orders of magnitude smaller than the merger timescales.Therefore the radio-AGN activity either started late in the lifetime ofthe merger event, or is not directly related to the merger event at all.Another intriguing result is that the H I- rich (> 109Mȯ) radio galaxies in our sample all have compact radiosources, while none of the extended radio sources contain these amountsof extended H I. This strongly suggests that there is a relation betweenthe size of the radio jet and the presence of large amounts of neutralgas associated with the host galaxy.
| 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.
| 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.
| Unification in the low radio luminosity regime: evidence from optical line emission
We address the question of whether or not the properties of alllow-luminosity flat spectrum radio sources, not just the obvious BL Lacobjects, are consistent with them being the relativistically beamedcounterparts of the low radio luminosity radio galaxies (theFanaroff-Riley type 1, FR I). We have accumulated data on a well-definedsample of low redshift, core-dominated, radio sources all of which haveone-sided core-jet structures seen with very long baselineinterferometry, just like most BL Lac objects. We first compare theemission-line luminosities of the sample of core-dominated radio sourceswith a matched sample of FR I radio galaxies. The emission lines in thecore-dominated objects are on average significantly more luminous thanthose in the comparison sample, inconsistent with the simplest unifiedmodels in which there is no orientation dependence of the line emission.We then compare the properties of our core-dominated sample with thoseof a sample of radio-emitting UGC galaxies selected without bias to corestrength. The core-dominated objects fit well on the UGC correlationbetween line emission and radio core strength found by Verdoes Kleijn etal. The results are not consistent with all the objects participating ina simple unified model in which the observed line emission isorientation independent, though they could fit a single, unified modelprovided that some FR I radio galaxies have emission line regions thatbecome more visible when viewed along the jet axis. However, they areequally consistent with a scenario in which, for the majority ofobjects, beaming has minimal effect on the observed core luminosities ofa large fraction of the FR I population and that intrinsically strongercores simply give rise to stronger emission lines. We conclude that FR Iunification is much more complex than usually portrayed, and modelscombining beaming with an intrinsic relationship between core andemission line strengths need to be explored.
| Radio spectra of the low-luminosity active galactic nucleus NGC 266 at centimetre-to-submillimetre wavelengths
We report multi-frequency and multi-epoch radio continuum observationswith multi-spatial resolution for the low-luminosity active galacticnucleus (LLAGN) NGC 266. In the centimetre regime, we find diffusecomponents with Very Large Array (VLA) observations, and a variablecompact core with a rising spectrum with Very Long Baseline Array (VLBA)observations. Although the spectral index of the rising spectrum isconsistent with the prediction of the simple advection-dominatedaccretion flow (ADAF) model, the observed radio power is slightly highcompared with that of the model prediction. A spectral break atcentimetre-to-millimetre wavelengths is inferred from the upper limitsof flux densities from Nobeyama Millimetre Array (NMA) and James ClerkMaxwell Telescope (JCMT) data at millimetre and submillimetrewavelengths, respectively. More complicated considerations are requiredfor the theoretical model to interpret such observed radio properties.
| 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.
| Group, field and isolated early-type galaxies - II. Global trends from nuclear data
We have derived ages, metallicities and enhanced-element ratios[α/Fe] for a sample of 83 early-type galaxies essentially ingroups, the field or isolated objects. The stellar-population propertiesderived for each galaxy correspond to the nuclear re/8aperture extraction. The median age found for Es is 5.8+/-0.6 Gyr andthe average metallicity is +0.37+/-0.03 dex. For S0s, the median age is3.0+/-0.6 Gyr and [Z/H]= 0.53+/-0.04 dex. We compare the distribution ofour galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Ourelliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster.We find that the galaxies lie in a plane defined by [Z/H]= 0.99logσ0- 0.46 log(age) - 1.60, or in linear terms Z~σ0× (age) -0.5. More massive (largerσ0) and older galaxies present, on average, large[α/Fe] values, and therefore must have undergone shorterstar-formation time-scales. Comparing group against field/isolatedgalaxies, it is not clear that environment plays an important role indetermining their stellar-population history. In particular, ourisolated galaxies show ages differing by more than 8 Gyr. Finally weexplore our large spectral coverage to derive log(O/H) metallicity fromthe Hα and NIIλ6584 and compare it with model-dependent[Z/H]. We find that the O/H abundances are similar for all galaxies, andwe can interpret it as if most chemical evolution has already finishedin these galaxies.
| Group, field and isolated early-type galaxies - I. Observations and nuclear data
This is the first paper of a series on the investigation of stellarpopulation properties and galaxy evolution of an observationallyhomogeneous sample of early-type galaxies in groups, field and isolatedgalaxies.Here we present high signal-to-noise ratio (S/N) long-slit spectroscopyof 86 nearby elliptical and S0 galaxies. Eight of them are isolated,selected according to a rigorous criterion, which guarantees a genuinelow-density subsample. The present survey has the advantage of coveringa larger wavelength range than normally found in the literature, whichincludes [OIII]λ5007 and Hα, both lines important foremission correction. Among the 86 galaxies with S/N >= 15 (perresolution element, for re/8 central aperture), 57 have theirHβ-index corrected for emission (the average correction is 0.190Åin Hβ) and 42 galaxies reveal [OIII]λ5007 emission,of which 16 also show obvious Hα emission. Most of the galaxies inthe sample do not show obvious signs of disturbances nor tidal featuresin the morphologies, although 11 belong to the Arp catalogue of peculiargalaxies; only three of them (NGC 750, 751 and 3226) seem to be stronglyinteracting. We present the measurement of 25 central line-strengthindices calibrated to the Lick/IDS system. Kinematic information isobtained for the sample. We analyse the line-strength index versusvelocity dispersion relations for our sample of mainly low-densityenvironment galaxies, and compare the slope of the relations withcluster galaxies from the literature. Our main findings are that theindex-σ0 relations presented for low-density regionsare not significantly different from those of cluster E/S0s. The slopeof the index-σ0 relations does not seem to change forearly-type galaxies of different environmental densities, but thescatter of the relations seems larger for group, field and isolatedgalaxies than for cluster galaxies.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| The recognition of blazars and the blazar spectral sequence
We analyse a group of radio sources, a subset of the 200-mJy sample, allof which have core-jet radio structures measured with very long baselineinterferometry and have flat spectra stretching from the radio to themillimetre/submillimetre band. Thus the objects have most of theproperties expected of blazars. However, they display varied opticalproperties ranging from `Seyfert-like' objects, through BL Lac objects,to `normal' elliptical galaxies. We investigate the distribution ofsynchrotron peak frequencies in their spectral energy distributions(SEDs) and find a broad distribution between 1012 and1016 Hz. Our conclusion is that we should consider virtuallyall objects in the sample as blazars since much of the diversity intheir classification based on traditional optical criteria arises fromdifferences in the frequency at which the non-thermal emission begins todecline. Specifically, an object is only classified as BL Lac when itspeak frequency falls in the near-infrared/optical range. We determinepeak frequencies using the same method for objects from other blazarsamples. An important result is that our objects do not follow theblazar spectral sequence proposed by Fossati et al. and Ghisellini etal. in which, on average, peak frequencies increase as the radioluminosity decreases. Most of our low radio luminosity sources havepeaks in their SEDs at low frequencies, not at the expected highfrequencies. We suggest that at least part of the systematic trend seenby Fossati et al. and Ghisellini et al. results from selection effects.
| Radio-optical scrutiny of the central engine in compact AGN
We combine Very-Long-Baseline Interferometry (VLBI) data for 100active galactic nuclei (AGN) available from the Very Large BaselineArray (VLBA) 2 cm imaging survey and optical spectroscopy to investigatethe relationships in the emission-line region-central engine-radio jetsystem. Here, we present the diversity of spectral types among thebrightest AGN in our sample. We also discuss correlations between themass of the central engine and properties of the parsec-scale radio jetfor 24 AGN selected by the presence of Hbeta broad-emission lines intheir spectra.
| 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.
| A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies
We report on the discovery of a new correlation between globalparameters of the hot interstellar gas in elliptical galaxies. Wereanalyze archival Chandra data for 30 normal early-type systems,removing the contributions of resolved and unresolved point sources toreveal the X-ray morphology of the hot gas. We determine the half-lightradius, RX, and the mean surface brightness, IX,from the gas surface brightness profiles. A spectral analysis determinesthe temperature, TX, of the gas within 3 optical effectiveradii. We find that the galaxies lie on an X-ray gas fundamental plane(XGFP) of the formTX~R0.28XI0.22X.This is close to, but distinct from, a simple luminosity-temperaturerelation. The intrinsic width of the XGFP is only 0.07 dex, nearlyidentical to that of the stellar (optical) fundamental plane (SFP). Thisis surprising since X-ray gas masses are typically ~10-2 ofthe stellar masses. We show that the XGFP is not a simple consequence ofthe virial theorem or hydrostatic equilibrium and that it is essentiallyindependent of the SFP. The XGFP thus represents a genuinely newconstraint on the hydrodynamical evolution of elliptical galaxies.
| 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.
| On the Relation between Circular Velocity and Central Velocity Dispersion in High and Low Surface Brightness Galaxies
In order to investigate the correlation between the circular velocityVc and the central velocity dispersion of the spheroidalcomponent σc, we analyzed these quantities for a sampleof 40 high surface brightness (HSB) disk galaxies, eight giant lowsurface brightness (LSB) spiral galaxies, and 24 elliptical galaxiescharacterized by flat rotation curves. Galaxies have been selected tohave a velocity gradient <=2 km s-1 kpc-1 forR>=0.35R25. We used these data to better define theprevious Vc-σc correlation for spiralgalaxies (which turned out to be HSB) and elliptical galaxies,especially at the lower end of the σc values. We findthat the Vc-σc relation is described by alinear law out to velocity dispersions as low as σc~50km s-1, while in previous works a power law was adopted forgalaxies with σc>80 km s-1. Ellipticalgalaxies with Vc based on dynamical models or directlyderived from the H I rotation curves follow the same relation as the HSBgalaxies in the Vc-σc plane. On the otherhand, the LSB galaxies follow a different relation, since most of themshow either higher Vc or lower σc withrespect to the HSB galaxies. This argues against the relevance of baryoncollapse to the radial density profile of the dark matter halos of LSBgalaxies. Moreover, if the Vc-σc relation isequivalent to one between the mass of the dark matter halo and that ofthe supermassive black hole, then these results suggest that the LSBgalaxies host a supermassive black hole (SMBH) with a smaller masscompared to HSB galaxies with an equal dark matter halo. On the otherhand, if the fundamental correlation of SMBH mass is with the halocircular velocity, then LSB galaxies should have larger black holemasses for a given bulge dispersion. Elliptical galaxies withVc derived from H I data and LSB galaxies were not consideredin previous studies.Based on observations made with European Southern Observatory telescopesat the Paranal Observatory under programs 67.B-0283, 69.B-0573, and70.B-0171.
| Are Quasar Jets Dominated by Poynting Flux?
The formation of relativistic astrophysical jets is presumably mediatedby magnetic fields threading accretion disks and central, rapidlyrotating objects. As it is accelerated by magnetic stresses, the jet'skinetic energy flux grows at the expense of its Poynting flux. However,it is unclear how efficient the conversion from magnetic to kineticenergy is and whether there are any observational signatures of thisprocess. We address this issue in the context of jets in quasars. Usingdata from all spatial scales, we demonstrate that in these objects theconversion from Poynting flux-dominated to matter-dominated jets is verylikely to take place closer to the black hole than in the region wheremost of the Doppler-boosted radiation observed in blazars is produced.We briefly discuss the possibility that blazar activity could be inducedby global MHD instabilities, e.g., via the production of localizedvelocity gradients that lead to dissipative events such as shocks ormagnetic reconnection, in which acceleration of relativistic particlesand production of nonthermal flares is taking place.
| 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.
| The Epochs of Early-Type Galaxy Formation as a Function of Environment
The aim of this paper is to set constraints on the epochs of early-typegalaxy formation through the ``archaeology'' of the stellar populationsin local galaxies. Using our models of absorption-line indices thataccount for variable abundance ratios, we derive ages, totalmetallicities, and element ratios of 124 early-type galaxies in high-and low-density environments. The data are analyzed by comparison withmock galaxy samples created through Monte Carlo simulations taking thetypical average observational errors into account, in order to eliminateartifacts caused by correlated errors. We find that all threeparameters, age, metallicity, and α/Fe ratio, are correlated withvelocity dispersion. We show that these results are robust againstrecent revisions of the local abundance pattern at high metallicities.To recover the observed scatter we need to assume an intrinsic scatterof about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].All low-mass objects withM*<~1010Msolar (σ<~130kms-1) show evidence for the presence of intermediate-agestellar populations with low α/Fe ratios. About 20% of theintermediate-mass objects with1010<~M*/Msolar<~1011[110<~σ/(kms-1)<~230 both elliptical andlenticular galaxies] must have either a young subpopulation or a bluehorizontal branch. On the basis of the above relationships, valid forthe bulk of the sample, we show that the Mg-σ relation is mainlydriven by metallicity, with similar contributions from the α/Feratio (23%) and age (17%). We further find evidence for an influence ofthe environment on the stellar population properties. Massive early-typegalaxies in low-density environments seem on average ~2 Gyr younger andslightly (~0.05-0.1 dex) more metal-rich than their counterparts inhigh-density environments. No offsets in the α/Fe ratios areinstead detected. With the aid of a simple chemical evolution model, wetranslate the derived ages and α/Fe ratios into star formationhistories. We show that most star formation activity in early-typegalaxies is expected to have happened between redshifts ~3 and 5 inhigh-density environments and between redshifts 1 and 2 in low-densityenvironments. We conclude that at least 50% of the total stellar massdensity must have already formed at z~1, in good agreement withobservational estimates of the total stellar mass density as a functionof redshift. Our results suggest that significant mass growth in theearly-type galaxy population below z~1 must be restricted to lessmassive objects, and a significant increase of the stellar mass densitybetween redshifts 1 and 2 should be present, caused mainly by the fieldgalaxy population. The results of this paper further imply the presenceof vigorous star formation episodes in massive objects at z~2-5 andevolved elliptical galaxies around z~1, both observationally identifiedas SCUBA galaxies and extremely red objects, respectively.
| 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.
| Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure
We have examined the compact structure in 250 flat-spectrumextragalactic radio sources using interferometric fringe visibilitiesobtained with the Very Long Baseline Array (VLBA) at 15 GHz. Withprojected baselines out to 440 Mλ, we are able to investigatesource structure on typical angular scales as small as 0.05 mas. Thisscale is similar to the resolution of the VLBI Space ObservatoryProgramme data obtained on longer baselines at a lower frequency andwith somewhat poorer accuracy. For 171 sources in our sample, more thanhalf of the total flux density seen by the VLBA remains unresolved onthe longest baselines. There are 163 sources in our list with a mediancorrelated flux density at 15 GHz in excess of 0.5 Jy on the longestbaselines; these will be useful as fringe finders for short-wavelengthVLBA observations. The total flux densities recovered in the VLBA imagesat 15 GHz are generally close to the values measured around the sameepoch at the same frequency with the RATAN-600 and University ofMichigan Radio Astronomy Observatory telescopes.We have modeled the core of each source with an elliptical Gaussiancomponent. For about 60% of the sources, we have at least oneobservation in which the core component appears unresolved (generallysmaller than 0.05 mas) in one direction, usually transverse to thedirection into which the jet extends. BL Lac objects are on average morecompact than quasars, while active galaxies are on average less compact.Also, in an active galaxy the sub-milliarcsecond core component tends tobe less dominant. Intraday variability (IDV) sources typically have amore compact, more core-dominated structure on sub-milliarcsecond scalesthan non-IDV sources, and sources with a greater amplitude of intradayvariations tend to have a greater unresolved VLBA flux density. Theobjects known to be GeV gamma-ray-loud appear to have a more compactVLBA structure than the other sources in our sample. This suggests thatthe mechanisms for the production of gamma-ray emission and for thegeneration of compact radio synchrotron-emitting features are related.The brightness temperature estimates and lower limits for the cores inour sample typically range between 1011 and 1013K, but they extend up to 5×1013 K, apparently in excessof the equipartition brightness temperature or the inverse Compton limitfor stationary synchrotron sources. The largest component speeds areobserved in radio sources with high observed brightness temperatures, aswould be expected from relativistic beaming. Longer baselines, which canbe obtained by space VLBI observations, will be needed to resolve themost compact high brightness temperature regions in these sources.
| 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.
| 24 year monitoring of extragalactic sources at 22 and 37 GHz
Long term monitoring results from 2001 to mid 2004 of quasarobservations at 22 and 37 GHz done at the Metsähovi radioobservatory are presented. Approximately 10,000 observations arepublished here.
| 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.
| Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.
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