Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 1218



Upload your image

DSS Images   Other Images

Related articles

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.

When Less Is More: Are Radio Galaxies below the Fanaroff-Riley Break More Polarized on Parsec Scales?
We present images showing the first detections of polarization on parsecscales in the nuclei of four Fanaroff-Riley type I (low-luminosity)radio galaxies. Observations with VLBI at λ=3.6 cm reveal thepresence of ordered magnetic fields within ~1650 Schwarzschild radii ofthe putative central supermassive black hole. The relatively highfractional polarization in the parsec-scale jets of these galaxies isconsistent with the standard scheme unifying low-luminosity radiogalaxies with BL Lac objects. This result also suggests that these radiogalaxies lack the obscuring tori that apparently depolarize the nuclearemission in the more powerful FR II type radio galaxies, and that theirsupermassive black holes are poorly fed and/or inefficient radiators.

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.

An X-Ray View of Weak-Line Radio Galaxies/LINERs
We present X-ray observations of nine weak-line radio galaxies (WLRGs),optically classified as confirmed or possible LINERs. The data weretaken from the Chandra, XMM-Newton, and BeppoSAX archives. The Chandraimages typically show complex X-ray morphologies, with hard (2-10 keV)point sources embedded in diffuse soft (0.3-2.0 keV) emission in allcases except those of 1246-41 (NGC 4696), in which only diffuse emissionis detected on the scale of the cluster, and 0334-01 (3C 15), in whichonly a point source is detected. The nuclear X-ray spectra are wellfitted at hard energies by an absorbed power law, with a wide range ofphoton indices, Γ=1.5-2.7. Excess absorption over the Galacticvalue is detected in six of the nine sources, with column densitiesNH~1021-1022 cm-2. A thermalcomponent is required at softer energies, in agreement with the resultsof the spatial analysis. We find that there is no correlation betweenthe core X-ray luminosity and the radio core dominance parameter,suggesting that the bulk of the core X-ray emission is not beamed butrather is isotropic and thus likely related to the accretion flow. In anattempt to constrain the nature of the accretion flow, we calculate theratios of bolometric to Eddington luminosities,Lbol/LEdd, and the radiative efficiency ηbased on the Bondi accretion rates. We find thatLbol/LEdd~10-4-10-6 andη~10-2-10-6 for all the objects in our sample,suggesting radiatively inefficient accretion flows.

The HST view of the nuclear emission line region in low luminosity radio-galaxies
We study the properties of the emission line regions in two samples oflow luminosity radio-galaxies, while focusing on the Compact EmissionLine Region (CELR) revealed to be a characteristic feature of theseobjects by HST narrow-band imaging. We find a strong correlation betweenline and optical continuum nuclear emission, which suggests that theoptical cores (most likely of non-thermal origin) can be directlyassociated to the source of ionizing photons, i.e. that we are seeing ajet-ionized narrow line region. A photon budget argument indicates thatthe optical nuclear sources produce sufficient photon flux provided thatthe covering factor of the circum-nuclear gas is rather large, onaverage 0.3. Analysis of HST images and spectra suggests that the CELRmay take the form of a pc-scale, high filling factor structure, possiblyan optically thin torus. Estimates of the CELR mass lead to values assmall as 10{-}10^3 Mȯ, and photon counting sets a limitto the Broad Line Region mass of M_BLR < 10-2Mȯ. When considered together with the low accretion rateand the tenuous torus structure, a general paucity of gas in theinnermost regions of low luminosity radio-galaxies emerges as the maincharacterizing difference from more powerful Active Galactic Nuclei.

A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies
We examine the properties of central dust in nearby quiescent and activeearly-type galaxies. The active galaxies are low-power radio galaxieswith Fanaroff & Riley type I or I/II radio jets. We focus on (a) thecomparison of the dust distributions in the active and quiescent galaxysamples; and (b) the relation between the radio jet and dustorientations. Our main observational conclusions are: (i) in line withprevious studies, the dust detection rate is higher in radio-jetgalaxies than in non radio-jet galaxies; (ii) radio galaxies contain ahigher fraction of regular dust “ellipses” compared toquiescent galaxies which contain more often irregular dustdistributions; (iii) the morphology, size and orientation of dustellipses and lanes in quiescent early-types and active early-types withkpc-scale radio jets is very similar; (iv) dust ellipses are alignedwith the major axis of the galaxy, dust lanes do not show a preferredalignment except for large (>kpc) dust lanes which are aligned withthe minor axis of the galaxy; and (v) as projected on the sky, jets donot show a preferred orientation relative to the galaxy major axis (andhence dust ellipses), but jets are preferentially perpendicular to dustlanes. We show that the dust ellipses are consistent with being nearlycircular thin disks viewed at random viewing angles. The lanes arelikely warped dust structures, which may be in the process of settlingdown to become regular disks or are being perturbed by anon-gravitational force. We use the observed dust-jet orientations toconstrain the three-dimensional angle θDJ between jetand dust. For dust-lane galaxies, the jet is approximately perpendicularto the dust structure, while for dust-ellipse galaxies there is a muchwider distribution of θDJ. We discuss two scenariosthat could explain the dust/jet/galaxy orientation dichotomy. If lanesare indeed settling, then the jet orientation apparently is roughlyaligned with the angular momentum of the dust before it settles. Iflanes are perturbed by a jet-related force, it appears that it causesthe dust to move out of its equilibrium plane in the galaxy into a planewhich is perpendicular to the jet.

A transition in the accretion properties of radio-loud active nuclei
We present evidence for the presence of a transition in the accretionproperties of radio-loud sources. For a sample of radio galaxies andradio-loud quasars, selected based on their extended radio properties,the accretion rate is estimated from the black hole mass and nuclearluminosity. The inferred distribution is bimodal, with a paucity ofsources at accretion rates, in Eddington units, of the order of~10-2- assuming a radiative efficiency of 10 per cent - andpossibly spanning 1-2 orders of magnitude. Selection biases are unlikelyto be responsible for such behaviour. We discuss possible physicalexplanations, including a fast transition to low accretion rates, achange in the accretion mode/actual accretion rate/radiative efficiency,the lack of stable disc solutions at intermediate accretion rates or theinefficiency of the jet formation processes in geometrically thin flows.This transition might be analogous to spectral states (and jet)transitions in black hole binary systems.

No evidence for a different accretion mode for all 3CR FR I radio galaxies
We have analysed the optical and radio properties of a sample of 3CR FRI radio galaxies which have Hubble Space Telescope (HST) imaging capableof detecting optical cores. The jet powers of the FR I radio galaxiesare estimated from their low-frequency radio luminosities, and theoptical core luminosity is taken as an upper limit on the emission fromany unobscured accretion disc. We argue that if the accretion discs inthese sources are assumed to be advection-dominated accretion flows(ADAFs), or adiabatic inflow-outflow solution (ADIOS) flows, then theBlandford-Znajek mechanism provides insufficient power to explain thehigh radio luminosities of at least a third, and perhaps all, of thesample. We suggest instead that a significant fraction (the`high-jet-power' third), and perhaps most, of the 3CR FR I radiogalaxies have normal accretion discs, but that their optical cores canbe hidden, with any HST-detected optical synchrotron emission comingfrom jets on scales larger than the obscuring material. A normalaccretion disc hypothesis, at least for the high-jet-power third of the3CR FR Is, explains why narrow-line luminosity correlates with radioluminosity. It also explains why one object in the sample (3C 386) hasan observed broad-line nucleus. We conclude that there is no evidence tosuggest that there is a difference in accretion mode between FR I and FRII radio galaxies.

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.

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

A scheme to unify low-power accreting black holes. Jet-dominated accretion flows and the radio/X-ray correlation
We explore the evolution in power of black holes of all masses, andtheir associated jets, within the scheme of an accretion rate-dependentstate transition. Below a critical value of the accretion rate allsystems are assumed to undergo a transition to a state where thedominant accretion mode is optically thin and radiatively inefficient.In these significantly sub-Eddington systems, the spectral energydistribution is predicted to be dominated by non-thermal emission from arelativistic jet whereas near-Eddington black holes will be dominatedinstead by emission from the accretion disk. Reasonable candidates forsuch a sub-Eddington state include X-ray binaries in the hard andquiescent states, the Galactic Center (Sgr A*), LINERs, FR I radiogalaxies, and a large fraction of BL Lac objects. Standard jet physicspredicts non-linear scaling between the optically thick (radio) andoptically thin (optical or X-ray) emission of these systems, which hasbeen confirmed recently inX-ray binaries. We show that this scaling relation is also a function ofblack hole mass and only slightly of the relativistic Doppler factor.Taking the scaling into account we show that indeed hard and quiescentstate X-ray binaries, LINERs, FR I radio galaxies, and BL Lacs can beunified and fall on a common radio/X-ray correlation. This suggests thatjet domination is an important stage in the luminosity evolution ofaccreting black hole systems.

Classifications of the Host Galaxies of Supernovae, Set II
Classifications on the DDO system are given for an additional 231 hostgalaxies of supernovae that have been discovered during the course ofthe Lick Observatory Supernova Search with the Katzman Automatic ImagingTelescope (KAIT). This brings the total number of hosts of supernovae(SNe) discovered (or independently rediscovered) by KAIT, which have sofar been classified on a homogeneous system, to 408. The probabilitythat SNe Ia and SNe II have a different distribution of host-galaxyHubble types is found to be 99.7%. A significant difference is alsofound between the distributions of the host galaxies of SNe Ia and ofSNe Ibc (defined here to include SNe Ib, Ib/c, and Ic). However, nosignificant difference is detected between the frequency distributionsof the host galaxies of SNe II and SNe IIn. This suggests that SNe IInare generally not SNe Ia embedded in circumstellar material that aremasquerading as SNe II. Furthermore, no significant difference is foundbetween the distribution of the Hubble types of the hosts of SNe Ibc andof SNe II. Additionally, SNe II-P and SNe II-L are found to occur amongsimilar stellar populations. The ratio of the number of SNe Ia-pec tonormal SNe Ia appears to be higher in early-type galaxies than it is ingalaxies of later morphological types. This suggests that the ancestorsof SNe Ia-pec may differ systematically in age or composition from theprogenitors of normal SNe Ia. Unexpectedly, five SNe of Types Ib/c, II,and IIn (all of which are thought to have massive progenitors) are foundin host galaxies that are nominally classified as types E and S0.However, in each case the galaxy classification is uncertain, or newlyinspected images show evidence suggesting a later classification. Amongthese five objects, NGC 3720, the host galaxy of SN 2002at, wasapparently misidentified in the Carnegie Atlas of Galaxies.

Emission-Line Diagnostics of the Central Engines of Weak-Line Radio Galaxies
A handful of well-studied weak-line radio galaxies (WLRGs) have beentraditionally classified as low-ionization nuclear emission-line regions(LINERs), suggesting that these two groups of active galactic nuclei(AGNs) might be related. In this paper, we present new opticalemission-line measurements for 20 WLRGs, which we supplement withmeasurements for an additional four from the literature. Classifyingthese objects by their emission-line ratios, we find that 50% of theobjects are robustly classified as LINERs, while an additional 25% arelikely to be LINERs. Photoionization calculations show that the spectralenergy distribution of the well-studied WLRG 3C 270 (NGC 4261) is ableto produce the observed emission-line ratios, but only if the UVemission seen by the narrow emission line gas is significantly higherthan that observed, implying AV=2.5-4.2 mag along our line ofsight to the nucleus. From the photoionization calculations, we findthat the emission-line gas must have an ionization parameter between10-3.5 and 10-4.0 and a wide range in hydrogendensity (102-106 cm-3) to reproduce themeasured emission-line ratios, similar to the properties inferred forthe emission-line gas in LINERs. Thus, we find that properties of theemission-line gas as well as the underlying excitation mechanism areindeed similar in LINERs and WLRGs. By extension, the central engines ofaccretion-powered LINERs and WLRGs, which do host an accreting blackhole, may be qualitatively similar.

The Millennium Arecibo 21 Centimeter Absorption-Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian fit results of Paper I to investigate the propertiesof interstellar H I in the solar neighborhood. The warm and cold neutralmedia (WNM and CNM) are physically distinct components. The CNM spintemperature histogram peaks at about 40 K; its median, weighted bycolumn density, is 70 K. About 60% of all H I is WNM; there is nodiscernible change in this fraction at z=0. At z=0, we derive a volumefilling fraction of about 0.50 for the WNM; this value is very rough.The upper limit WNM temperatures determined from line width range upwardfrom ~500 K; a minimum of about 48% of the WNM lies in the thermallyunstable region 500-5000 K. The WNM is a prominent constituent of theinterstellar medium, and its properties depend on many factors,requiring global models that include all relevant energy sources, ofwhich there are many. We use principal components analysis, togetherwith a form of least-squares fitting that accounts for errors in boththe independent and dependent parameters, to discuss the relationshipsamong the four CNM Gaussian parameters. The spin temperatureTs and column density N(H I) are, approximately, the two mostimportant eigenvectors; as such, they are sufficient, convenient, andphysically meaningful primary parameters for describing CNM clouds. TheMach number of internal macroscopic motions for CNM clouds is typicallyabout 3 so that they are strongly supersonic, but there are widevariations. We discuss the historical τ0-Tsrelationship in some detail and show that it has little physicalmeaning. We discuss CNM morphology using the CNM pressure known from UVstellar absorption lines. Knowing the pressure allows us to show thatCNM structures cannot be isotropic but instead are sheetlike, withlength-to-thickness aspect ratios ranging up to about 280. We presentlarge-scale maps of two regions where CNM lies in very large ``blobbysheets.'' We test the McKee/Ostriker model of the interstellar medium byexplicitly modeling our data with CNM cores contained in WNM envelopes.This modeling scheme works quite well for many sources and also predictsthe WNM filling factor reasonably well. However, it has severaldeficiencies.

870 Micron Observations of Nearby 3CRR Radio Galaxies
We present submillimeter continuum observations at 870 μm of thecores of low-redshift 3CRR radio galaxies, observed at the HeinrichHertz Submillimeter Telescope. The cores are nearly flat-spectrumbetween the radio and submillimeter, which implies that thesubmillimeter continuum is likely to be synchrotron emission and notthermal emission from dust. The emitted power from nuclei detected atoptical wavelengths and in the X-rays is similar in the submillimeter,optical, and X-rays. The submillimeter-to-optical and X-ray power ratiossuggest that most of these sources resemble misdirected BL Lac-typeobjects with synchrotron emission peaking at low energies. However, wefind three exceptions, the FR I galaxy 3C 264 and the FR II galaxies 3C390.3 and 3C 338 with high X-ray-to-submillimeter luminosity ratios.These three objects are candidate misdirected high- orintermediate-energy peaked BL Lac-type objects. With additional infraredobservations and from archival data, we compile spectral energydistributions for a subset of these objects. The steep dips observednear the optical wavelengths in many of these objects suggest thatextinction inhibits the detection and reduces the flux of opticalcontinuum core counterparts. High-resolution near- or mid-infraredimaging may provide better measurements of the underlying synchrotronemission peak.

The Ultraviolet Continuum Emission of FR I and FR II Radio Galaxies and a Proposal for a Unified AGN Model for FR I sources
This paper is the second in a series of two on the UV continuum emission(in the range from 1400 to 3700 Å) of radio galaxies that wereextracted from the Hubble Space Telescope (HST) Archives. The sampleconsists of 31 3C and Parkes radio galaxies that have redshifts below0.2 (the majority have redshifts of ~0.03) and radio powers of~1025-1027 W Hz-1 (usingH0=50 km-1 s-1 Mpc andq0=0.0). Paper I describes the sample selection and theproperties of individual sources; this paper deals with the analysis. Wefind that only about half the radio galaxies display any UV flux atwavelengths shorter than 2300 Å. More specifically, those galaxiesthat are dominated by a nuclear UV component are either BL Lac objectsor radio galaxies with broad emission lines. We detect a nuclear and anextended UV component only among half the radio galaxies with narrowemission lines. Although we do not find a correlation of the UVluminosity with emission-line luminosity or radio power, there doesappear to be a dependence on radio morphology. While (narrow line) FR IIsources do not show a nuclear UV component, FR I's do, however, only ifthey also have an optical jet (this is the case for seven of 21 FR I's).These results are broadly consistent with orientation-dependentunification models. In radio galaxies in which the torus does notobscure our view of the engine, the observed UV radiation appears to bepoint source-like. This is the case for broad-line radio galaxies and BLLac objects. In other radio galaxies that are oriented at an angle tous, the torus presumably blocks the nuclear UV component. Thenarrow-line FR I galaxies with optical jets can then be interpreted asobjects at a critical angle at which some, but not all, nuclear UVemission is blocked. The UVλ luminosities (withλ ranging from 1400 to 3700 Å) and theUVλ-V colors of radio galaxies show a larger scatterthan those of radio-quiet elliptical galaxies. At wavelengths shorterthan 2300 Å, some radio galaxies have on average bluer colors, butbeyond 3000 Å, their colors are on average slightly redder. Thispicture is also consistent with unification models-the galaxies withbluer colors are either BL Lac objects or broad-line radio galaxies. Allother radio galaxies (including the jetted FR I's) have somewhat reddercolors than radio-quiet elliptical galaxies. We suspect that this isprimarily due to reddening by dust, which we know is present in some ofthe radio galaxies in the sample. At longer wavelengths (>3000 Å),all radio galaxies (14 of 14) show an extended component. The morphologyof the extended emission is in most, but not all, cases comparable tothe morphology at optical wavelengths, implying that this is likely tobe starlight. However, the polarization images of two of seven radiogalaxies indicate that scattered light from the active galactic nucleusmay also contribute toward the UV luminosity.

The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.

A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr ( or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5

A multi-wavelength test of the FR I-BL Lac unifying model
We collect multi-wavelength measurements of the nuclear emission of 20low luminosity FR I radio-galaxies to test the viability of the FR I-BLLac unifying model. Although poorly sampled, the Spectral EnergyDistributions (SED) of FR Is are consistent with the double peaked shapecharacteristic of BL Lacs. Furthermore while the distribution of the FRIs in the broad-band spectral index planes shows essentially no overlapwith the regions where HBL and LBL are located, this can be simply dueto the effects of relativistic beaming. More quantitatively, derivingthe beaming Doppler factor of a given radio-galaxy from its X-rayluminosity ratio with respect to BL Lacs with similar extended radioluminosity, we find that i) the luminosity in all bands, ii) the valueof the spectral indices, iii) the slope of the X-ray spectrum, iv) theoverall SED shape, may be all simultaneously reproduced. However, thecorresponding jet bulk Lorentz factors are significantly smaller thanthose derived for BL Lacs from other observational and theoreticalconsiderations. This suggests to consider a simple variant of theunification scheme that allows for the presence of a velocity structurein the jet.

The black hole mass of low redshift radiogalaxies
We make use of two empirical relations between the black hole mass andthe global properties (bulge luminosity and stellar velocity dispersion)of nearby elliptical galaxies, to infer the mass of the central blackhole (CM MBH) in low redshift radiogalaxies. Using the mostrecent determinations of black hole masses for inactive early typegalaxies we show that the bulge luminosity and the central velocitydispersion are almost equally correlated (similar scatter) with thecentral black-hole mass. Applying these relations to two large andhomogeneous datasets of radiogalaxies we find that they host black-holeswhose mass ranges from ~ 5*E7 to ~ 6*E9CMMsun (average ~ 8.9). CMMBH is found to be proportional to the mass of the bulge (CMMbulge). The distribution of the ratio CM MBH/CMMbulge has a mean value of 8*E-4 and shows ascatter that is consistent with that expected from the associatederrors. At variance with previous claims no significant correlation isinstead found between CM MBH (or CM Mbulge) andthe radio power at 5 GHz.

On the physical conditions in AGN optical jets
The energy budget of all known optical jets is discussed. It is foundthat to power the extended radio lobes of radio galaxies, the jet bulkmotion on kpc scales must be relativistic, on average. Based on variousconstraints, a ``most probable'' region centered aroundΓbulk~7.5 and /θ~20° is found. Because of theconsequent relativistic beaming, the rest frame magnetic field is lowerand electron lifetimes longer. Combining the effects of time dilationand lower emission rate, the electron diffusion length becomes fullyconsistent with the deprojected jet length, without the need forreacceleration.

Optical synchrotron emission from the nuclei of FR I radio galaxies
We studied HST images of a complete sample of 33 FR I radio galaxies.Emission from an unresolved Central Compact Core (CCC) is detected inthe great majority of the objects. CCC optical emission shows a strikingcorrelation with the radio core fluxes, which extends over four decadeswith a slope of /~1. This argues strongly for a non-thermal synchrotronorigin of the CCCs, which would then dominate the nuclear emission bothin the radio and in the optical bands. The high detection rate of CCCssets strong constraints on the existence/geometry of any obscuring,optically thick structure (torus) in FR I. We also find evidence for adifferent behavior of low and high power objects belonging to oursample. CCC fluxes also represent upper limits to any unobscuredthermal/disc emission, which translate into a fraction as small as<~10-7-10-5 of the Eddington luminosity (for a109 Msolar black hole).

Supernova 2000fs in NGC 1218
IAUC 7830 available at Central Bureau for Astronomical Telegrams.

Active Galactic Nucleus Black Hole Masses and Bolometric Luminosities
Black hole mass, along with mass accretion rate, is a fundamentalproperty of active galactic nuclei (AGNs). Black hole mass sets anapproximate upper limit to AGN energetics via the Eddington limit. Wecollect and compare all AGN black hole mass estimates from theliterature; these 177 masses are mostly based on the virial assumptionfor the broad emission lines, with the broad-line region size determinedfrom either reverberation mapping or optical luminosity. We introduce200 additional black hole mass estimates based on properties of the hostgalaxy bulges, using either the observed stellar velocity dispersion orthe fundamental plane relation to infer σ these methods assumethat AGN hosts are normal galaxies. We compare 36 cases for which blackhole mass has been generated by different methods and find, forindividual objects, a scatter as high as a couple of orders ofmagnitude. The less direct the method, the larger the discrepancy withother estimates, probably due to the large scatter in the underlyingcorrelations assumed. Using published fluxes, we calculate bolometricluminosities for 234 AGNs and investigate the relation between blackhole mass and luminosity. In contrast to other studies, we find nosignificant correlation of black hole mass with luminosity, other thanthose induced by circular reasoning in the estimation of black holemass. The Eddington limit defines an approximate upper envelope to thedistribution of luminosities, but the lower envelope depends entirely onthe sample of AGNs included. For any given black hole mass, there is arange in Eddington ratio of up to 3 orders of magnitude.

Jet-triggered Type Ia Supernovae in Radio Galaxies?
We report the serendipitous discovery of a supernova (SN) in the nearbyradio galaxy 3C 78. Observations obtained with the Space TelescopeImaging Spectrograph on board the Hubble Space Telescope show, at adistance of 0.54" (300 pc) from the galaxy nucleus, a second brightsource, not present in previous images. As this source was fortuitouslycovered by the spectrograph slit, its spectrum was obtained, and it ischaracteristic of a Type Ia SN. This SN is closely aligned with theradio jet of 3C 78. Analysis of historical records shows that such aclose association between jet and SN occurred in six of the 14 reportedSNe in radio galaxies. The probability that this results from a randomdistribution of SNe in the host galaxy is less than ~0.05%. We thenargue that jets might trigger SN explosions. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555 andSTScI grant GO-3594.01-91A.

Will Jets Identify the Progenitors of Type Ia Supernovae?
We use the fact that a Type Ia supernova has been serendipitouslydiscovered near the jet of the active galaxy 3C 78 to examine thequestion of whether jets can enhance accretion onto white dwarfs. Oneinteresting outcome of such a jet-induced accretion process is anenhanced rate of novae in the vicinity of jets. We present results ofobservations of the jet in M87, which appear to have indeed discovered11 novae in close proximity to the jet. We show that a confirmation ofthe relation between jets and novae and Type Ia supernovae can finallyidentify the elusive progenitors of Type Ia supernovae.

The Nuclei of Radio Galaxies in the Ultraviolet: The Signature of Different Emission Processes
We have studied the nuclei of 28 radio galaxies from the 3CR sample inthe UV band. Unresolved nuclei (central compact cores: CCCs) areobserved in 10 of the 13 FR I galaxies, and in five of the 15 FR IIgalaxies. All sources that do not have a CCC in the optical do not havea CCC in the UV. Two FR I galaxies (3C 270 and 3C 296) have a CCC in theoptical but do not show the UV counterpart. Both of them show largedusty disks observed almost edge-on, possibly implying that they play arole in obscuring the nuclear emission. We have measured optical-UVspectral indices αo,UV between ~0.6 and ~7.0(Fν~ν-α). Broad-line radio galaxieshave the flattest spectra, and their values of αo,UVare also confined to a very narrow range. This is consistent withradiation produced in a geometrically thin, optically thick accretiondisk. On the other hand, FR I nuclei, which are most plausiblyoriginated by synchrotron emission from the inner relativistic jet, showa wide range of αo,UV. There is a clear trend withorientation in that sources observed almost edge-on or with clear signsof dust absorption have the steepest spectra. These observations implythat in FR I galaxies, obscuration can be present, but the obscuringmaterial is not in a ``standard'' geometrically thick torus. The moststriking difference between these absorbing structures and the classicactive galactic nucleus ``tori'' resides in the lower optical depth ofthe FR I galaxy obscuring material. Based on observations obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:03h08m26.10s
Aparent dimensions:1.38′ × 1.096′

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 1218

→ Request more catalogs and designations from VizieR