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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.
| Binary Black Hole Accretion Flows in Merged Galactic Nuclei
We consider accretion flows from circumbinary disks onto supermassivebinary black holes on a subparsec scale of the galactic center based ona smoothed particles hydrodynamics (SPH) code. Simulation models arepresented for four cases; that is, a circular binary and an eccentricone, each with equal and unequal masses. We find that thecircumblack-hole disks are formed around each black hole regardless ofthe simulation parameters. There are two-step mechanisms that causeaccretion flow. First, tidally induced elongation of the circumbinarydisk triggers mass inflow toward two closest points on the circumbinarydisk. Then, the gas is increasingly accumulated on these two pointsowing to the gravitational attraction of black holes. Second, when thegas can pass across the maximum loci of the effective binary potential,it starts to overflow via their two points, and freely infalls to eachblack hole. In circular binaries, the gas continues to be supplied fromthe circumbinary disk, (i.e., the gap between the circumbinary disk andthe binary black hole is always closed). In eccentric cases, the masssupply undergoes periodic on/off transitions during one orbital periodbecause of the variation of the periodic potential. The gap starts toclose after the apoastron, and to open again after the next periastronpassage. Due to the gap closing/opening cycles, the mass-capture ratesare eventually strongly phase dependent. This could provide observablediagnosis for the presence of supermassive binary black holes in mergedgalactic nuclei.
| 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 Compact Supermassive Binary Black Hole System
We report on the discovery of a supermassive binary black hole system inthe radio galaxy 0402+379, with a projected separation between the twoblack holes of just 7.3 pc. This is the closest black hole pair yetfound by more than 2 orders of magnitude. These results are based onrecent multifrequency observations using the Very Long Baseline Array(VLBA), which reveal two compact, variable, flat-spectrum, active nucleiwithin the elliptical host galaxy of 0402+379. Multiepoch observationsfrom the VLBA also provide constraints on the total mass and dynamics ofthe system. Low spectral resolution spectroscopy using the Hobby-EberlyTelescope indicates two velocity systems with a combined mass of the twoblack holes of ~1.5×108 Msolar. The twonuclei appear stationary, while the jets emanating from the weaker ofthe two nuclei appear to move out and terminate in bright hot spots. Thediscovery of this system has implications for the number of close binaryblack holes that might be sources of gravitational radiation. Green BankTelescope observations at 22 GHz to search for water masers in thisinteresting system are also presented.
| X-ray detection of the proto supermassive binary black hole at the centre of Abell 400
Context.We report the first X-ray detection of a proto supermassivebinary black hole at the centre of Abell 400. Using the Chandra AdvancedCCD Imaging Spectrometer, we are able to clearly resolve the two activegalactic nuclei in 3C 75, the well known double radio source at thecentre of Abell 400.Aims.Through analysis of the new Chandraobservation of Abell 400 along with 4.5 GHz and 329 MHz Very Large Arrayradio data, we will show new evidence that the active galactic nuclei in3C 75 are a bound system.Methods.Using the high quality X-raydata, we map the temperature, pressure, density and entropy of the innerregions as well as the cluster profile properties out to ˜18arcmin. We compare features in the X-ray and radio images to determinethe interaction between the intracluster medium and extended radioemission.Results.The Chandra image shows an elongation of thecluster gas along the northeast-southwest axis; aligned with the initialbending of 3C 75's jets. Additionally, the temperature profile shows nocooling core, consistent with a merging system. There is an apparentshock to the south of the core consistent with a Mach number ofM˜1.4 or speed of v˜1200 km s-1. Both activegalactic nuclei, at least in projection, are located in the low entropy,high density core just north of the shock region. We find that theprojected path of the jets does not follow the intra-cluster mediumsurface brightness gradient as expected if their path were due tobuoyancy. We also find that both central active galactic nuclei areextended and include a thermal component.Conclusions.Based on thisanalysis, we conclude that the active galactic nuclei in 3C 75 are abound system from a previous merger. They are contained in a low entropycore moving through the intracluster medium at 1200 km s-1.The bending of the jets is due to the local intracluster medium wind.
| Observations of magnetic fields in regular and irregular clusters
The existence of magnetic fields associated with the intracluster mediumin clusters of galaxies is now well established through differentmethods of analysis. The most detailed evidence for the presence ofcluster magnetic fields comes from radio observations. Magnetic fieldscan be investigated through the synchrotron emission of cluster-widediffuse sources and from studies of the rotation measure of polarizedradio galaxies. I will review efforts to measure magnetic fieldstrengths and power spectra and the main issues that have led to ourknowledge on magnetic fields in regular and irregular clusters ofgalaxies.
| 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.
| Observational evidence for binary black holes and active double nuclei
I review the observational evidence for the presence of supermassivebinary black holes in galaxies and for the presence of active pairs ofgalaxies. According to hierarchical galaxy merger models, binary blackholes should form frequently, and should be common in the cores ofgalaxies. The presence of massive black hole binaries has been invokedto explain a number of class properties of different types of galaxies,and in triggering various forms of activity. Coalescing massive blackhole binaries are powerful emitters of gravitational waves. The searchfor such binary black holes is therefore of great interest for severaltopics in astrophysics ranging from galaxy formation to activity ingalaxies.
| 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 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.
| The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.
| 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.
| Interactions of radio galaxies and the intracluster medium in Abell 160 and Abell 2462
We present Chandra and Very Large Array observations of two galaxyclusters, Abell 160 and Abell 2462, whose brightest cluster galaxies(BCGs) host wide angle tailed radio galaxies (WATs). We search forevidence of interactions between the radio emission and the hot, X-rayemitting gas, and we test various jet termination models. We find thatboth clusters have cool BCGs at the cluster centre, and that the scaleof these cores (~30-40 kpc for both sources) is of approximately thesame scale as the length of the radio jets. For both sources, the jetflaring point is coincident with a steepening in the host cluster'stemperature gradient, and similar results are found for 3C 465 and HydraA. However, none of the published models of WAT formation offers asatisfactory explanation as to why this may be the case. Therefore, itis unclear what causes the sudden transition between the jet and theplume. Without accurate modelling, we cannot ascertain whether thesteepening of the temperature gradient is the main cause of thetransition, or merely a tracer of an underlying process.
| 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 binary system of tailed radio galaxies
We present a detailed study of a binary system of tailed radio galaxieswhich, along with 3C 75, is the only such binary known to exist. Thebinary is located in a region of low galaxy density at the periphery ofa poor cluster Abell S345, but lies close to the massiveHorologium-Reticulum supercluster. The radio sources have bent-tailmorphologies and show considerable meandering and wiggling along thejets, which are collimated throughout their lengths. This work presentsobservations of the large-scale-structure environment of the binarytailed radio sources with a view to examining the influence oflarge-scale flows on the morphology and dynamics of the associated radiotails. We argue that the orbital motions of the host galaxies togetherwith tidal accelerations toward the supercluster have resulted in thecomplex structure seen in these radio tails.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| 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.
| Jet termination in wide-angle tail radio sources
Wide-angle tail radio galaxies (WATs) are an uncommon class of radiosources with luminosities near the FR I/FR II break, and are usuallyassociated with central cluster galaxies. Their defining characteristicwhen imaged sensitively at high resolution is their twin,well-collimated jets, which can persist with low opening angle for tensof kiloparsecs before flaring into long, often bent, plumes. Althoughseveral models for the jet termination have been proposed, the majorityof them are unsatisfactory when confronted with observations. Here wepresent the results of a programme of radio observations made with theaims of showing that objects classified as WATs do all havewell-collimated jets and of seeing in detail how the jets disrupt asthey enter the plumes. We show that compact, `hotspot-like' features atthe ends of the jets are common but by no means universal, and discussthe constraints that this places on models of the jet-plume transition.We discuss the properties of the observed well-collimated jets, and,using relativistic beaming models, estimate their speed to be ~0.3c.Finally, we show that the distance from the galactic centre at which thebase of the plume is found is related to the temperature of the hostcluster.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| Measuring the cluster magnetic field power spectra from Faraday rotation maps of Abell 400, Abell 2634 and Hydra A
We apply a novel technique of Faraday Rotation measure (RM) map analysisto three galaxy clusters, Abell 400, Abell 2634 and Hydra A, in order toestimate cluster magnetic field strengths, length scales and powerspectra. This analysis - essentially a correlation analysis - is basedon the assumption that the magnetic fields are statisticallyisotropically distributed across the Faraday screen. We investigate thedifficulties involved in the application of the analysis toobservational data. We derive magnetic power spectra for three clusters,i.e. Abell 400, Abell 2634 and Hydra A, and discuss influences on theirshapes caused by the observational nature of the data such as limitedsource size and resolution. We successfully apply various tests tovalidate our assumptions. We show that magnetic fluctuations are probedon length scales ranging over at least one order of magnitude. Usingthis range for the determination of magnetic field strength of thecentral cluster gas yields 3 mu G in Abell 2634, 6 mu G in Abell 400 and12 mu G in Hydra A as conservative estimates. The magnetic fieldautocorrelation length lambda_B was determined to be 4.9 kpc for Abell2634, 3.6 kpc for Abell 400 and 0.9 kpc for Hydra A. We show that the RMautocorrelation length lambda RM is larger than the magneticfield autocorrelation length lambda_B - for the three clusters studied,we found lambda RM =~ 2...4lambda_B - and thus, they arenot equal as often assumed in the literature. Furthermore, weinvestigate in a response analysis if it is possible to determinespectral slopes of the power spectra. We find that integrated numberscan be reliably determined from this analysis but differentialparameters such as spectral slopes have to be treated differently.However, our response analysis results in spectral slopes of the powerspectra of spectral indices alpha = 1.6 to 2.0 suggesting thatKolmogorov spectra (alpha = 5/3) are possible but flatter spectralslopes than alpha = 1.3 can be excluded.
| Lowering Inferred Cluster Magnetic Field Strengths: The Radio Galaxy Contributions
We present a detailed examination of the relationship between themagnetic field structures and the variations in Faraday rotation acrossPKS 1246-410, a radio source in the Centaurus cluster of galaxies, usingdata from Taylor, Fabian, & Allen. We find a significantrelationship between the intrinsic position angle of the polarizationand the local amount of Faraday rotation. The most plausible explanationis that most or all of the rotation is local to the source. We suggestthat the rotations local to cluster radio galaxies may result eitherfrom thermal material mixed with the radio plasma, or from thin skins ofwarm, ionized gas in pressure balance with the observed galaxy or hotcluster atmospheres. We find that the contribution of any unrelatedcluster rotation measure (RM) variations on scales of 2"-10" are lessthan 25 rad m-2: the standard, although model-dependent,derivation of cluster fields would then lead to an upper limit of ~0.4μG on these scales. Inspection of the distributions of rotationmeasure, polarization angle, and total intensity in 3C 75, 3C 465, andCygnus A also shows source-related Faraday effects in some locations.Many effects can mask the signatures of locally dominated RMs, so thedetection of even isolated correlations can be important, althoughdifficult to quantify statistically. In order to use radio sources suchas those shown here to derive cluster-wide magnetic fields, as iscommonly done, one must first remove the local contributions. This isnot possible at present.
| Search for Groups of Extragalactic Radio Sources
We explore the possibility of searching for groups of radio sources fromthe FIRST catalog on angular scales 1-5 arcmin. We developed anefficient method of searching for such groups that takes into accountthe need for combining the components of extended sources represented inthe catalog by separate objects. We found 31 groups of radio sourceswith angular sizes <5 arcmin that contain no fewer than five sourceswith flux densities > 3 mJy. This number is at least triple theexpected number of such groups for a random Poisson distribution ofradio sources in the sky. The prospects for using groups of radiosources to detect and study distant systems of galaxies are discussed.
| The Millennium Arecibo 21 Centimeter Absorption-Line Survey. I. Techniques and Gaussian Fits
We review the theory of measuring spectral lines in emission/absorptionobservations and apply it to a new survey of the 21 cm line against 79continuum sources. We develop an observing technique and least-squaresprocedure to determine the opacity profile, the expected emissionprofile, and their uncertainty profiles. We discuss the radiativetransfer for the two-component interstellar H I gas and use Gaussiancomponents, separate ones for the warm and cold neutral media (WNM andCNM), as a practical implementation of a simple but physically correctmodel that successfully treats both simple and complicated profiles. OurGaussians provide CNM spin temperatures, upper limits on kinetictemperatures for both CNM and WNM from the line widths, columndensities, and velocities; we discuss these astrophysical aspects inPaper II.
| 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.
| 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.
| The radio galaxy 3C 356 and clues to the trigger mechanisms for powerful radio sources
We present deep near-infrared images, taken with the Subaru Telescope,of the region around the z=1.08 radio source 3C 356 which show it to beassociated with a poor cluster of galaxies. We discuss evidence thatthis cluster comprises two subclusters traced by the two galaxiespreviously proposed as identifications for 3C 356, which both seem toharbour active galactic nuclei, and which have the disturbedmorphologies expected if they underwent an interpenetrating collision atthe time the radio jets were triggered. We explain the high luminosityand temperature of the diffuse X-ray emission from this system as theresult of shock heating of intracluster gas by the merger of two galaxygroups. Taken together with the results on other well-studied powerfulradio sources, we suggest that the key ingredient for triggering apowerful radio source, at least at epochs corresponding to z~1, is agalaxy-galaxy interaction which can be orchestrated by the merger oftheir parent subclusters. This provides an explanation for the rapiddecline in the number density of powerful radio sources since z~1. Weargue that attempts to use distant radio-selected clusters to trace theformation and evolution of the general cluster population must addressways in which X-ray properties can be influenced by the radio source,both directly, by mechanisms such as inverse Compton scattering, andindirectly, by the fact that the radio source may be preferentiallytriggered at a specific time during the formation of the cluster.
| Magnetic Fields in Cluster Cores: Faraday Rotation in A400 and A2634
We present Faraday rotation data for radio sources in the centers of theAbell clusters A400 and A2634. These clusters contain large (>~100kpc), tailed radio sources, each attached to the central cD galaxy.These clusters do not have strong cooling cores. Our data extendprevious work on rotation measure in cluster centers to larger scalesand noncooling clusters. The rotation measure, and thus the magneticfield, is ordered on scales ~10-20 kpc in both clusters. The geometry ofthe rotation measure appears to be determined by the distribution of theX-ray-emitting gas, rather than by the radio tails themselves. Wecombine our data with previously published X-ray and radio data in orderto analyze the magnetic fields in all 12 clusters whose central radiosources have been imaged in rotation measure. We find that the fieldsare dynamically significant in most clusters. We argue that the Faradaydata measure fields in the intracluster medium, rather than in a skin ofthe radio source. Finally, we consider the nature and maintenance of themagnetic fields in these clusters and conclude that either thecluster-wide field exists at similar levels or a weaker cluster-widefield is amplified by effects in the core.
| Formation of Galactic Nuclei
We investigate a model in which galactic nuclei form via the coalescenceof preexisting stellar systems containing supermassive black holes.Merger simulations are carried out using N-body algorithms that canfollow the formation and decay of a black hole binary and its effect onthe surrounding stars down to subparsec scales. Our initial stellarsystems have steep central density cusps similar to those inlow-luminosity elliptical galaxies. Immediately following the merger,the density profile of the remnant is homologous with the initialdensity profile and the steep nuclear cusp is preserved. However, theformation of a black hole binary transfers energy to the stars andlowers the central density; continued decay of the binary creates aρ~r-1 density cusp similar to those observed in brightelliptical galaxies, with a break radius that extends well beyond thesphere of gravitational influence of the black holes. Our simulationsare the first to successfully produce shallow power-law cusps frommergers of galaxies with steep cusps, and our results support a picturein which the observed dependence of nuclear cusp slope on galaxyluminosity is a consequence of galaxy interactions. We follow the decayof the black hole binary over a factor of ~20 in separation afterformation of a hard binary, considerably farther than in previoussimulations. We see almost no dependence of the binary's decay rate onnumber of particles in the simulation, contrary to earlier studies inwhich a lower initial density of stars led to a more rapid depletion ofthe binary's loss cone. We nevertheless argue that the decay of a blackhole binary in a real galaxy would be expected to stall at separationsof 0.01-1 pc unless some additional mechanism is able to extract energyfrom the binary. We discuss the implications of our results for thesurvivability of dark matter cusps.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
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