Contents
Images
Upload your image
DSS Images Other Images
Related articles
Dust tori in radio galaxies
Aims: We investigate the quasar - radio galaxy unificationscenario and detect dust tori within radio galaxies of various types. Methods: Using VISIR on the VLT, we acquired sub-arcsecond (~0.40arcsec) resolution N-band images, at a wavelength of 11.85 ?m, of thenuclei of a sample of 27 radio galaxies of four types in the redshiftrange z=0.006-0.156. The sample consists of 8 edge-darkened, low-powerFanaroff-Riley class I (FR-I) radio galaxies, 6 edge-brightened, classII (FR-II) radio galaxies displaying low-excitation optical emission, 7FR-IIs displaying high-excitation optical emission, and 6 FR-II broademission line radio galaxies. Out of the sample of 27 objects, 10 nucleiare detected and several have constraining non-detections atsensitivities of 7 mJy, the limiting flux a point source has whendetected with a signal-to-noise ratio of 10 in one hour of sourceintegration. Results: On the basis of the core spectral energydistributions of this sample we find clear indications that many FR-Iand several low-excitation FR-II radio galaxies do not contain warm dusttori. At least 57±19 percent of the high-excitation FR-IIs andalmost all of the broad line radio galaxies exhibit excess infraredemission, which must be attributed to warm dust reradiating accretionactivity. The FR-I and low-excitation FR-II galaxies are all of lowefficiency, which is calculated as the ratio of bolometric to Eddingtonluminosity Lbol/LEdd<10-3. Thissuggests that thick tori are absent at low accretion rates and/or lowefficiencies. The high-excitation FR-II galaxies are a mixed populationwith three types of nuclei: 1) low efficiency with dust torus; 2) lowefficiency with weakly emitting dust torus; and 3) high efficiency withweak dust torus. We argue that the unification viewing angle range 0-45degrees of quasars should be increased to ~60 degrees, at least at lowerluminosities.
| An optical spectroscopic survey of the 3CR sample of radio galaxies with z < 0.3 . II. Spectroscopic classes and accretion modes in radio-loud AGN
In a previous paper we presented a homogeneous and 92% complete opticalspectral dataset of the 3CR radio sources with redshift <0.3. Here weuse the emission line measurements to explore the spectroscopicproperties of the sample. The 3CR sources show a bimodal distribution ofexcitation index, a new spectroscopic indicator that measures therelative intensity of low and high excitation lines. This unveils thepresence of two main sub-populations of radio-loud AGN to which we referto, following previous studies, as high and low excitation galaxies (HEGand LEG, respectively). In addition to the two main classes, we find onesource with a spectrum typical of star forming galaxies, and 3 objectsof extremely low level of excitation. All broad-line objects are HEGfrom the point of view of their narrow emission line ratios and all HEGare FR II radio-galaxies with log L178 [erg s-1]⪆ 32.8. Conversely LEG cover the whole range of radio powerencompassed by this 3CR subsample (30.7 ⪉ log L178 ⪉35.4) and they are of both FR I and FR II type. The brightest LEG areall FR II. HEG and LEG obey to two (quasi) linear correlations betweenthe optical line and extended radio luminosities, with HEG beingbrighter than LEG in the [O III] line by a factor of ~10. HEG and LEGare offset also in a plane that compares the black hole mass and theionizing nuclear luminosity. However, although HEG are associated withhigher nuclear luminosities, we find LEG among the brightest radiosources of the sample and with a clear FR II morphology,indistinguishable from those seen in HEG. This suggests that LEG are notsimply objects with a lower level of accretion. We speculate that thedifferences between LEG and HEG are related to a different mode ofaccretion: LEG are powered by hot gas, while HEG require the presence ofcold accreting material. The high temperature of the accreting gas inLEG accounts for the lack of “cold” structures (i.e.molecular torus and broad line region), for the reduced radiative outputof the accretion disk, and for the lower gas excitation.Based on observations made with the Italian Telescopio Nazionale Galileooperated on the island of La Palma by the Centro Galileo Galilei of INAF(Istituto Nazionale di Astrofisica) at the Spanish Observatorio delRoque del los Muchachos of the Instituto de Astrofísica deCanarias.
| The Spitzer View of FR I Radio Galaxies: On the Origin of the Nuclear Mid-Infrared Continuum
We present Spitzer mid-infrared (MIR) spectra of 25 FR I radio galaxiesand investigate the nature of their MIR continuum emission. MIR spectraof star-forming galaxies and quiescent elliptical galaxies are used toidentify host galaxy contributions while radio/optical core data areused to isolate the nuclear nonthermal emission. Out of the 15 sourceswith detected optical compact cores, four sources are dominated byemission related to the host galaxy. Another four sources show signs ofwarm, nuclear dust emission: 3C15, 3C84, 3C270, and NGC 6251. It islikely that these warm dust sources result from hidden active galacticnuclei of optical spectral type 1. The MIR spectra of seven sources aredominated by synchrotron emission, with no significant component ofnuclear dust emission. In parabolic spectral energy distribution fits ofthe nonthermal cores FR Is tend to have lower peak frequencies andstronger curvature than blazars. This is roughly consistent with thecommon picture in which the core emission in FR Is is less stronglybeamed than in blazars.
| The stellar content of low redshift radio galaxies from near-infrared spectroscopy
Context: We present medium spectral resolution near-infrared (NIR)HK-band spectra for eight low redshift (z < 0.06) radio galaxies tostudy the NIR stellar properties of their host galaxies. The sample wasselected from a radio galaxy sample imaged previously in the B- andR-band. They were found to be bluer than inactive elliptical galaxies,possibly indicating a recent star formation episode. As a homogeneouscomparison sample, we used nine inactive elliptical galaxies that wereobserved with the same telescope and detector with similar resolutionand wavelength range. Aims: The aim of the study is, by using theadvantage of NIR absorption features, to compare the NIR spectralproperties of radio galaxies to those of inactive early-type galaxiesand, furthermore, produce the first NIR HK-band spectra for low redshiftradio galaxies. Methods: For the radio galaxy and inactive ellipticalsamples, spectral indices of several diagnostic absorption features,namely SiI(1.589 μm), CO(1.619 μm) in the H-band and NaI(2.207μm), CaI(2.263 μm), CO(> 2.29 μm) in the K-band, weremeasured. The strength of absorption lines depends on the luminosityand/or temperature of stars and, therefore, spectral indices can be usedto trace the stellar population of galaxies. To characterize the age ofthe populations, the measured EWs of the absorption features were fittedwith the corresponding theoretical evolutionary curves of the EWscalculated by the stellar synthesis model. Results: On average, EW(CO2.29) of radio galaxies is somewhat greater than that of inactiveellipticals. Most likely, EW(CO 2.29) is not significantly affected bydilution, thus indicating that elliptical galaxies containing AGN are ina different stage in their evolution than inactive ellipticals. This isalso supported by comparing other NIR absorption line features, such asCaI and NaI, with each other. Based on the theoretical evolutionarycurves of EWs, absorption features are consistent with the intermediateage stellar population, suggesting that host galaxies contain both oldand intermediate age components. An intermediate age population is alsoconsistent with previous optical spectroscopy studies, which have shownevidence of an intermediate age (~2 Gyr) stellar population in radiogalaxies, and also in some of the early-type galaxies. Conclusions: Anintermediate stellar population component indicates that radio galaxieshave experienced a star formation epoch relatively recently. Theexistence of an intermediate stellar population is a link between thestar formation episode, possibly induced by an interaction or mergingevent, and triggering of nuclear activity.
| An optical spectroscopic survey of the 3CR sample of radio galaxies with z < 0.3. I. Presentation of the data
We present a homogeneous and 92% complete dataset of optical nuclearspectra for the 113 3CR radio sources with redshifts <0.3, obtainedwith the Telescopio Nazionale Galileo. For these sources we could obtainuniform and uninterrupted coverage of the key spectroscopic opticaldiagnostics. The observed sample, including powerful classical FR IIradio-galaxies and FR I, together spanning four orders of magnitude inradio-luminosity, provides a broad representation of the spectroscopicproperties of radio galaxies. In this first paper we present an atlas ofthe spectra obtained, provide measurements of the diagnostic emissionline ratios, and identify active nuclei with broad line emission. Thesedata will be used in follow-up papers to address the connection betweenthe optical spectral characteristics and the multiwavelength propertiesof the sample.Based on observations made with the Italian Telescopio Nazionale Galileooperated on the island of La Palma by the Centro Galileo Galilei of INAF(Istituto Nazionale di Astrofisica) at the Spanish Observatorio delRoque del los Muchachos of the Instituto de Astrofisica de Canarias.
| Blind and non-blind source detection in WMAP 5-yr maps
We have analysed the efficiency in source detection and flux densityestimation of blind and non-blind detection techniques exploiting theMHW2 filter applied to the Wilkinson Microwave Anisotropy Probe (WMAP)5-yr maps. A comparison with the AT20G bright source sample, with acompleteness limit of 0.5 Jy and accurate flux measurements at 20 GHz,close to the lowest frequency of WMAP maps, has allowed us to assess thecompleteness and the reliability of the samples detected with the twoapproaches, as well as the accuracy of flux and error estimates, andtheir variations across the sky. The uncertainties on flux estimatesgiven by our procedure turned out to be about a factor of 2 lower thanthe rms differences with AT20G measurements, consistent with thesmoothing of the fluctuation field yielded by map filtering. Fluxestimates were found to be essentially unbiased except that, close tothe detection limit, a substantial fraction of fluxes are found to beinflated by the contribution of underlying positive fluctuations. Thisis consistent with expectations for the Eddington bias associated to thetrue errors on flux density estimates. The blind and non-blindapproaches are found to be complementary: each of them allows thedetection of sources missed by the other. Combining results of the twomethods on the WMAP 5-yr maps, we have expanded the non-blindlygenerated New Extragalactic WMAP Point Source (NEWPS) catalogue that wasbased on WMAP 3-yr maps. After having removed the probably spuriousobjects not identified with known radio sources, the new version of theNEWPS catalogue, NEWPS_5yr comprises 484 sources detected with asignal-to-noise ratio SNR >= 5.
| Recent star formation in nearby 3CR radio-galaxies from UV HST observations
We analyzed HST images of 31 nearby (z ≲ 0.1) 3CR radio-galaxies.We compared their UV and optical images to detect evidence of recentstar formation. Six objects were excluded because they are highlynucleated or had very low UV count rates. After subtracting the emissionfrom their nuclei and/or jets, 12 of the remaining 25 objects,presenting an UV/optical colors NUV-r < 5.4, are potentialstar-forming candidates. Considering the contamination from otherAGN-related processes (UV emission lines, nebular continuum, andscattered nuclear light), there are 6 remaining star-forming“blue” galaxies. We then divide the radio galaxies, on thebasis of the radio morphology, radio power, and diagnostic optical lineratios, into low and high excitation galaxies, LEG and HEG. While thereis no correlation between the FR type (or radio power) and color, the FRtype is clearly related to the spectroscopic type. In fact, all HEG(with one possible exception) show morphological evidence of recent starformation in UV compact knots, extended over 5-20 kpc. Conversely, thereis only 1 “blue” LEG out of 19, including in this class alsoFR I galaxies. The picture that emerges, considering color, UV, optical,and dust morphology, is that only in HEG recent star formation isassociated with these relatively powerful AGN, which are most likelytriggered by a recent, major, wet merger. Conversely, in LEG galaxiesthe fraction of actively star-forming objects is not enhanced withrespect to quiescent galaxies. The AGN activity in these sources can beprobably self-sustained by their hot interstellar medium.Based on observations obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Incorporated, under NASA contract NAS 5-26555.
| Hubble Space Telescope Near-infrared Snapshot Survey of 3CR Radio Source Counterparts. II. An Atlas and Inventory of the Host Galaxies, Mergers, and Companions
We present the second part of an H-band (1.6 μm) ``atlas'' ofz<0.3 3CR radio galaxies, using the Hubble Space Telescope NearInfrared Camera and Multi-Object Spectrometer (HST NICMOS2). We presentnew imaging for 21 recently acquired sources and host galaxy modelingfor the full sample of 101 (including 11 archival)-an 87% completionrate. Two different modeling techniques are applied, following thoseadopted by the galaxy morphology and the quasar host galaxy communities.Results are compared and found to be in excellent agreement, althoughthe former breaks down in the case of sources with strong activegalactic nuclei (AGNs). Companion sources are tabulated, and thepresence of mergers, tidal features, dust disks, and jets are cataloged.The tables form a catalog for those interested in the structural andmorphological dust-free host galaxy properties of the 3CR sample, andfor comparison with morphological studies of quiescent galaxies andquasar host galaxies. Host galaxy masses are estimated and found totypically lie at around 2×1011 Msolar. Ingeneral, the population is found to be consistent with the localpopulation of quiescent elliptical galaxies, but with a longer tail tolow Sérsic index, mainly consisting of low-redshift (z<0.1)and low-radio-power (FR I) sources. A few unusually disky FR II hostgalaxies are picked out for further discussion. Nearby external sourcesare identified in the majority of our images, many of which we argue arelikely to be companion galaxies or merger remnants. The reduced NICMOSdata are now publicly available from our Web site.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. (AURA),under NASA contract NAS5-26555.
| Radio Frequency Spectra of 388 Bright 74 MHz Sources
As a service to the community, we have compiled radio frequency spectrafrom the literature for all sources within the VLA Low Frequency SkySurvey (VLSS) that are brighter than 15 Jy at 74 MHz. Over 160references were used to maximize the amount of spectral data used in thecompilation of the spectra, while also taking care to determine thecorrections needed to put the flux densities from all reference on thesame absolute flux density scale. With the new VLSS data, we are able tovastly improve on previous efforts to compile spectra of bright radiosources to frequencies below 100 MHz because (1) the VLSS flux densitiesare more reliable than those from some previous low-frequency surveysand (2) the VLSS covers a much larger area of the sky(?>-30deg) than many other low-frequency surveys(e.g., the 8C survey). In this paper, we discuss how the spectra wereconstructed and how parameters quantifying the shapes of the spectrawere derived. Both the spectra and the shape parameters are madeavailable here to assist in the calibration of observations made withcurrent and future low-frequency radio facilities.
| Star formation in the hosts of GHz peaked spectrum and compact steep spectrum radio galaxies
Aims.We are searching for star formation regions in the hosts ofpotentially young radio galaxies (gigahertz peaked spectrum and compactsteep spectrum sources). Methods: We used near-UV imaging with theHubble Space Telescope Advanced Camera for Surveys. Results: We findnear-UV light could be the product of recent star formation in five ofthe nine observed sources, though other explanations are not currentlyruled out. An additional two sources show marginal detections. The UVluminosities of the GPS and CSS sources are similar to those of a sampleof nearby large-scale radio galaxies. Stellar-population synthesismodels are consistent with a burst of recent star formation occurringbefore the formation of the radio source. However, observations at otherwavelengths and colors are needed to definitively establish the natureof the observed UV light. In the CSS source 1443+77,the near-UV light is aligned with and is co-spatial with the radiosource. We suggest that the UV light in this source is produced by starformation triggered and/or enhanced by the radio source.
| Aromatic Features in AGNs: Star-forming Infrared Luminosity Function of AGN Host Galaxies
We describe observations of aromatic features at 7.7 and 11.3 μm inAGNs of three types, including PG, 2MASS, and 3CR objects. The featurehas been demonstrated to originate predominantly from star formation.Based on the aromatic-derived star-forming luminosity, we find that thefar-IR emission of AGNs can be dominated by either star formation ornuclear emission; the average contribution from star formation is around25% at 70 and 160 μm. The star-forming infrared luminosity functionsof the three types of AGNs are flatter than those of field galaxies,implying that nuclear activity and star formation tend to be enhancedtogether. The star-forming luminosity function is also a function of thestrength of nuclear activity from normal galaxies to the bright quasars,with luminosity functions becoming flatter for more intense nuclearactivity. Different types of AGNs show different distributions in thelevel of star formation activity, with 2MASS > PG > 3CR starformation rates.
| Correlation between Eddington Ratios and Broad-Line Luminosities in Flat-Spectrum Radio Quasars, BL Lacertae Objects, and Radio Galaxies
We compiled a sample of 16 flat-spectrum radio quasars (FSRQs), 8 BL Lacobjects, and 15 Fanaroff-Riley (FR) I and FR II radio galaxies, forwhich the intrinsic Eddington ratios and broad-line luminosities areavailable. In the diagram of the intrinsic Eddington ratio-broad-lineluminosity relation, FSRQs are found in the high broad-line luminosity,high intrinsic Eddington ratio region, while BL Lac objects, as well asFR I and FR II radio galaxies, are found in the low broad-lineluminosity, low intrinsic Eddington ratio region. In addition, theintrinsic Eddington ratios are strongly correlated with the broad-lineluminosities and also with the intrinsic bolometric luminosities, butthe correlation with the broad-line luminosities is better. Thus, thebroad-line luminosity, rather than the intrinsic bolometric luminosity,should be used as a fundamental parameter of the unified scheme andevolution of active galactic nuclei. On the other hand, we analyze thetheory proposed by Czerny and coworkers that the formation of thebroad-line region (BLR) is intrinsically connected to the existence of acold accretion disk. Our studies support that BLR formation isintimately connected with the cold disk. Thus, our results provide solidexperimental evidence for the theory that has been proposed by Czernyand coworkers.
| Radio Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications
We investigate how the total radio luminosity of AGN-powered radiosources depends on their accretion luminosity and the central black holemass. Our studies cover about 7 orders of magnitude in accretionluminosity (expressed in Eddington units, i.e., as Eddington ratios) andthe full range of AGN black hole masses. We find that AGNs form twodistinct and well-separated sequences on theradio-loudness-Eddington-ratio plane. The ``upper'' sequence is formedby radio-selected AGNs, and the ``lower'' sequence contains mainlyoptically selected objects. Whereas an apparent ``gap'' between the twosequences may be an artifact of selection effects, the sequencesthemselves mark the real upper bounds of radio loudness of two distinctpopulations of AGNs: those hosted respectively by elliptical and diskgalaxies. Both sequences show the same dependence of the radio loudnesson the Eddington ratio (an increase with decreasing Eddington ratio),which suggests that the normalization of this dependence is determinedby the black hole spin. This implies that central black holes in giantelliptical galaxies have (on average) much larger spins than black holesin spiral/disk galaxies. This galaxy-morphology-related radio dichotomybreaks down at high accretion rates where the dominant fraction ofluminous quasars hosted by elliptical galaxies is radio quiet. This ledto speculations in the literature that formation of powerful jets athigh accretion rates is intermittent and related to switches between twodisk accretion modes, as directly observed in some black hole X-raybinaries. We argue that such intermittency can be reconciled with thespin paradigm, provided that successful formation of relativistic jetsby rotating black holes requires collimation by MHD outflows fromaccretion disks.
| Kinematics of the Local Universe. XIII. 21-cm line measurements of 452 galaxies with the Nançay radiotelescope, JHK Tully-Fisher relation, and preliminary maps of the peculiar velocity field
Aims.This paper presents 452 new 21-cm neutral hydrogen linemeasurements carried out with the FORT receiver of the meridian transitNançay radiotelescope (NRT) in the period April 2003-March 2005. Methods: This observational programme is part of a larger projectaiming at an exhaustive and magnitude-complete HI extragalacticcatalogue for Tully-Fisher applications (the so-called KLUN project, forKinematics of the Local Universe studies, to end in 2008). The wholeon-line HI archive of the NRT today contains reduced HI-profiles for4500 spiral galaxies of declination δ > -40°(http://klun.obs-nancay.fr). Results: As an example of thisapplication, we used the direct Tully-Fisher relation in three (JHK)bands in deriving distances to a large catalogue of 3126 spiral galaxiesdistributed through the whole sky and sampling the radial velocity rangewell between 0 and 8000 km s-1. Thanks to an iterative methodaccounting for selection bias and smoothing effects, we show a detailedand original map of the velocity field in the Local Universe as apreliminary output.Data Tables [see full text], [see full text], and [see full text]and HI-profiles (Fig. [see full text]) are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/465/71
| A catalogue of quasars and active nuclei: 12th edition
Aims.This catalogue is aimed at presenting a compilation of all knownAGN in a compact and convenient form and we hope that it will be usefulto all workers in this field.Methods.Like the eleventh edition, itincludes position and redshift as well as photometry (U, B, V) and 6 cmflux densities when available. We now give 20 cm rather than 11 cm fluxdensities.Results.The present version contains 85 221 quasars,1122 BL Lac objects and 21 737 active galaxies (including 9628 Seyfert1s), almost doubling the number listed in the 11th edition. We also givea list of all known lensed and double quasars.
| Hubble Space Telescope Near-Infrared Snapshot Survey of 3CR Radio Source Counterparts at Low Redshift
We present newly acquired images of the near-infrared counterpart of 3CRradio sources. All the sources were selected to have a redshift of lessthan 0.3 to allow us to obtain the highest spatial resolution. Theobservations were carried out as a snapshot program using theNear-Infrared Camera and Multiobject Spectrograph (NICMOS) on-board theHubble Space Telescope (HST). In this paper we describe 69 radiogalaxies observed for the first time with NICMOS during HST cycle 13.All the objects presented here are elliptical galaxies. However, each ofthem has unique characteristics such as close companions, dust lanes,unresolved nuclei, arclike features, globular clusters, and jets clearlyvisible from the images or with basic galaxy subtraction.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprogram 10173.
| The scaling relation of early-type galaxies in clusters. II. Spectroscopic data for galaxies in eight nearby clusters
Aims.We present low and intermediate resolution spectroscopic datacollected for 152 early type galaxies in 8 nearby clusters with z ≤0.10. Methods: .We use low resolution data to produce the redshiftand the K-correction for each galaxy, as well as to give their overallspectral energy distribution and some spectral indicators, including the4000 Å break, the Mg2 strength and the NaD equivalent width. Wehave also obtained higher resolution data for early type galaxies inthree of the clusters, to determine their central velocity dispersion. Results: .The effect of the resolution on the measured parametersis discussed. Conclusions: .A new accurate systemic redshift andvelocity dispersion is presented for four of the surveyed clusters, A98,A3125, A3330, and DC2103-39. We have found that the K-correction valuesfor E/S0 bright galaxies in the given nearby clusters are very similar.We also find that the distribution of the line indicators significantlydiffers from cluster to cluster.
| 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.
| The ESO nearby Abell cluster survey. VIII. Morphological and spectral classification of galaxies
We determine the morphological types of 2295 galaxies from the ESONearby Abell Cluster Survey (ENACS) from CCD images obtained with theDutch telescope on La Silla. A comparison with morphological types fromthe literature for 450 of our galaxies shows that the reliability of ourclassification is quite comparable to that of other classifiers. Werecalibrate the ENACS spectral classification with the new morphologicaltypes, and find that early- and late-type galaxies can be distinguishedfrom their spectra with 83% reliability. Ellipticals and S0 galaxies canhardly be distinguished on the basis of their spectra, but late spiralscan be classified from the spectrum alone with more than 70%reliability. We derive pseudo-colors and linestrengths from the ENACSspectra for the galaxies of different morphological types. We considerthe bright (MR ≤ -20) and faint (MR > -20)subsets of the galaxies without emission lines (non-ELG) separately. Wefind a strong and significant correlation between the average color andthe average strength of the metal absorption lines. The averagemetallicity decreases and the average color gets bluer towards laterHubble type. Also, the faint galaxies in each morphological class arebluer and less metal-rich than their brighter counterparts, whichextends the well-established color-magnitude relation of early-typegalaxies to (late) spirals. In view of these very strong global trends,the colors and metallicities of faint S0 galaxies and bright earlyspirals are remarkably similar. The bright early spirals may, onaverage, have somewhat stronger Hδ absorption than the othergalaxies, which could be due to recent starformation. The galaxies withemission lines (ELG) have a bluer spectral continuum than the non-ELG,and the amount of blueing hardly depends on morphological type. Thefraction of ELG depends strongly on morphological type (varying from4±1% for ellipticals to 59±4% for late spirals), but foreach of the morphological types it varies very little with projecteddistance from the cluster center.
| High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars
We discuss our ATCA 18.5 and 22 GHz flux density measurements ofSouthern extragalactic sources in the complete 5 GHz sample of Kühret al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz)spectral indices of steep-spectrum sources for which we have 18.5 GHzdata (66% of the complete sample) are systematically steeper than thelow frequency (2.7-5 GHz) ones, with median α^52.7 =0.76, median α18.55 = 1.18(Sν∝ ν-α), and median steepeningΔα = 0.32, and there is evidence of an anti-correlation ofΔα18.55 with luminosity. Thecompleteness of 18.5 GHz data is much higher (89%) for flat-spectrumsources (mostly quasars), which also exhibit a spectral steepening:median α^52.7=-0.14, medianα18.55=0.16 (Sν∝ν-α), and median Δα = 0.19. Takingadvantage of the almost complete redshift information on flat-spectrumquasars, we have estimated their 5 GHz luminosity function in severalredshift bins. The results confirm that their radio luminosity densitypeaks at z_peak ≃ 2.5 but do not provide evidence for deviationsfrom pure luminosity evolution as hinted at by other data sets. Acomparison of our 22 GHz flux densities with WMAP K-band data forflat-spectrum sources suggests that WMAP flux densities may be low by amedian factor of ≃1.2. The extrapolations of 5 GHz counts andluminosity functions of flat-spectrum radio quasars using the observeddistribution of the 5-18.5 GHz spectral indices match those deriveddirectly from WMAP data, indicating that the high frequency WMAP surveydoes not detect any large population of FSRQs with anomalous spectra.
| 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 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.
| 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.
| 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.
| Does the dichotomy of active galactic nuclei depend only on black hole spins?
A toy model for jet powers and radio loudness of active galactic nuclei(AGNs) is proposed based on the coexistence of the Blandford-Znajek andmagnetic coupling processes in black hole (BH) accretion discs. It turnsout that both the jet powers and radio loudness of AGNs are controlledby more than one physical parameter besides the BH spin. The observeddichotomy between radio-loud and radio-quiet AGNs is well interpreted bythe two parameters, the BH spin and the power-law index of the variationof the magnetic field on the disc. Furthermore, we discuss thecorrelation of jet powers with radio loudness of AGNs in terms of thetwo parameters. It is found that the contours of radio loudness areapproximately in accord with those of jet powers for several 3CR radiosources, implying roughly that the stronger jet powers correspond tostronger radio loudness. In addition we discuss the correlation of thejet powers and radio loudness of AGNs with the position of the inneredge of the accretion disc. These results imply that the `spin paradigm'for radio loudness of AGNs might be modified by a scenario containingmore physical parameters.
| 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
| Magnetic fields and Faraday rotation in clusters of galaxies
We present a numerical approach to investigate the relationship betweenmagnetic fields and Faraday rotation effects in clusters of galaxies. Wecan infer the structure and strength of intra-cluster magnetic fields bycomparing our simulations with the observed polarization properties ofextended cluster radio sources such as radio galaxies and halos. We findthe observations require a magnetic field which fluctuates over a widerange of spatial scales (at least one order of magnitude). If severalpolarized radio sources are located at different projected positions ina galaxy cluster, as is the case for A119, detailed Faraday rotationimages allow us to constrain both the magnetic field strength and theslope of the power spectrum. Our results show that the standard analyticexpressions applied in the literature overestimate the cluster magneticfield strengths by a factor of 2. We investigate the possibleeffects of our models on beam depolarization of radio sources whoseradiation traverses the magnetized intracluster medium. Finally, wepoint out that radio halos may provide important information about thespatial power spectrum of the magnetic field fluctuations on largescales. In particular, different values of the index of the powerspectrum produce very different total intensity and polarizationbrightness distributions.
| 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.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
