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Jet speeds in wide-angle tailed radio galaxies
We present a sample of 30 wide-angle tailed radio galaxies (WATs) thatwe use to constrain the jet speeds in these sources. We measure thedistribution of the jet-sidedness ratios for the sample, and assumingthat the jets are beamed, the jet speeds in the range (0.3-0.7)c areobtained. Whilst the core prominence of the sample, which ought to be areliable indicator of beaming, shows little correlation with the jetsidedness, we argue that due to the peculiar nature of the WATs, coreprominence is unlikely to be a good indicator of beaming in thesesources. We further show that if the jets are fast and light, then thegalaxy speeds required to bend the jets into C-shapes such as those seenin 0647+693 are reasonable for a galaxy in a merging or recently mergedcluster.
| The Warped Nuclear Disk of Radio Galaxy 3C 449
Among radio galaxies containing nuclear dust disks, the bipolar jet axisis generally observed to be perpendicular to the disk major axis. The FRI radio source 3C 449 is an outlier to this statistical majority, as itpossesses a nearly parallel jet/disk orientation on the sky. We examinethe 600 pc dusty disk in this galaxy with images from the Hubble SpaceTelescope. We find that a 1.6 μm/0.7 μm color map of the diskexhibits a twist in its isocolor contours (isochromes). We model thecolor map by integrating galactic starlight through an absorptive diskand find that the anomalous twist in the isochromes can be reproduced inthe model with a vertically thin, warped disk. The model predicts thatthe disk is nearly perpendicular to the jet axis within 100 pc of thenucleus. We discuss physical mechanisms capable of causing such a warp.We show that precessional models or a torque on the disk arising from apossible binary black hole in the AGN causes precession on a timescalethat is too long to account for the predicted disk morphology. However,we estimate that the pressure in the X-ray-emitting interstellar mediumis large enough to perturb the disk, and we argue that jet-drivenanisotropy in the excited ISM may be the cause of the warp. In this way,the warped disk in 3C 449 may be a new manifestation of feedback from anactive galactic nucleus.
| Chandra and XMM-Newton Observations of a Sample of Low-Redshift FR I and FR II Radio Galaxy Nuclei
We present spectral results from Chandra and XMM-Newton observations ofa sample of 22 low-redshift (z<0.1) radio galaxies and considerwhether the core emission originates from the base of a relativisticjet, or an accretion flow, or contains contributions from both. We findcorrelations between the unabsorbed X-ray, radio, and optical fluxes andluminosities of FR I-type radio-galaxy cores, implying a common originin the form of a jet. On the other hand, we find that the X-ray spectraof FR II-type radio galaxy cores are dominated by absorbed emission,with NH>~1023 atoms cm-2, which islikely to originate in an accretion flow. We discuss several models thatmay account for the different nuclear properties of FR I- and FR II-typecores and also demonstrate that both heavily obscured, accretion-relatedand unobscured, jet-related components may be present in all radiogalaxy nuclei. Any absorbed, accretion-related components in FR I-typegalaxies have low radiative efficiencies.
| Magnetic turbulence in cool cores of galaxy clusters
We argue that the recently reported Kolmogorov-like magnetic turbulencespectrum in the cool core of the Hydra A galaxy cluster can beunderstood by kinetic energy injection by active galaxies that drives aturbulent non-helical magnetic dynamo into its saturated state. Althoughdramatic differences exist between small-scale dynamo scenarios, theirsaturated state is expected to be similar, as we show for threescenarios: the flux rope dynamo, the fluctuation dynamo, and theexplosive dynamo. Based on those scenarios, we develop an analyticalmodel of the hydrodynamic and magnetic turbulence in cool cores. Themodel implies magnetic field strengths that fit well with Faradayrotation measurements and minimum energy estimates for the sample ofcool core clusters having such data available. Predictions for magneticfields in clusters for which the appropriate observational informationis still missing, and for yet unobserved quantities like thehydrodynamical turbulence velocity and characteristic length-scale areprovided. The underlying dynamo models suggest magnetic intermittencyand possibly a large-scale hydrodynamic viscosity. We conclude that thesuccess of the model to explain the field strength in cool core clustersindicates that in general cluster magnetic fields directly reflecthydrodynamical turbulence, also in clusters without cool cores.
| The host galaxy/AGN connection in nearby early-type galaxies. Is there a miniature radio-galaxy in every "core" galaxy?
This is the second of a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. We selected two sampleswith 5 GHz VLA radio flux measurements down to 1 mJy, reaching levels ofradio luminosity as low as 1036 erg s-1. In PaperI we presented a study of the surface brightness profiles for the 65objects with available archival HST images out of the 116 radio-detectedgalaxies. We classified early-type galaxies into "core" and "power-law"galaxies, discriminating on the basis of the slope of their nuclearbrightness profiles, following the Nukers scheme. Here we focus on the29 core galaxies (hereafter CoreG). We used HST and Chandra data toisolate their optical and X-ray nuclear emission. The CoreG invariablyhost radio-loud nuclei, with an average radio-loudness parameter of LogR = L5 {GHz} / LB 3.6. The optical and X-raynuclear luminosities correlate with the radio-core power, smoothlyextending the analogous correlations already found for low luminosityradio-galaxies (LLRG) toward even lower power, by a factor of 1000, covering a combined range of 6 orders of magnitude. This supportsthe interpretation of a common non-thermal origin of the nuclearemission also for CoreG. The luminosities of the nuclear sources, mostlikely dominated by jet emission, set firm upper limits, as low asL/L_Edd 10-9 in both the optical and X-ray band, on anyemission from the accretion process. The similarity of CoreG and LLRGwhen considering the distributions host galaxies luminosities and blackhole masses, as well as of the surface brightness profiles, indicatesthat they are drawn from the same population of early-type galaxies.LLRG represent only the tip of the iceberg associated with (relatively)high activity levels, with CoreG forming the bulk of the population. Wedo not find any relationship between radio-power and black hole mass. Aminimum black hole mass of M_BH = 108 Mȯ isapparently associated with the radio-loud nuclei in both CoreG and LLRG,but this effect must be tested on a sample of less luminous galaxies,likely to host smaller black holes. In the unifying model for BL Lacsand radio-galaxies, CoreG likely represent the counterparts of the largepopulation of low luminosity BL Lac now emerging from the surveys at lowradio flux limits. This suggests the presence of relativistic jets alsoin these quasi-quiescent early-type "core" galaxies.
| Gamma-ray emissions of AGN and cosmological standard candles
In this work, we compile a sample which contains 71 GeV Gamma-ray-loudActive Galactic Nuclei (AGNs) (14 BL Lacs and 57 FSRQs), 53 FR I radiogalaxies and 63 FR II radio galaxies. We make a nonlinear least-squarefit to this sample, and find that the best fit value of the Hubbleconstant is H0=71.5±3.8 kms-1Mpc-1 with a reduced χ ~= 2.46 by assumingMv = -23.0 and accepting q0 = 1.0, and thecorresponding regression line has a correlation index R ~= 0.78. Thebest fit value of H0 = 71.5±3.8 kms-1Mpc-1 is in well agreement with H0 =72±8 km s-1 obtained by the Hubble Space TelescopeKey Project. Our results show that the GeV Gamma-ray emissions of AGNscan be used as cosmological standard candles indeed.
| The cool wake around 4C 34.16 as seen by XMM-Newton
We present XMM-Newton observations of the wake-radio galaxy system 4C34.16, which shows a cool and dense wake trailing behind the host galaxyof 4C 34.16. A comparison with numerical simulations is enlightening, asthey demonstrate that the wake is produced mainly by ram pressurestripping during the galactic motion through the surrounding cluster.The mass of the wake is a substantial fraction of the mass of the X-rayhalo of an elliptical galaxy. This observational fact supports a wakeformation scenario similar to that recently demonstrated numerically byAcreman et al.: the host galaxy of 4C 34.16 has fallen into its cluster,and is currently crossing its central regions. A substantial fraction ofits X-ray halo has been stripped by ram pressure, and remains behind toform the galaxy wake.
| A Chandra and XMM-Newton study of the wide-angle tail radio galaxy 3C465
We have observed the prototypical wide-angle tail (WAT) radio galaxy3C465 with Chandra and XMM-Newton. X-ray emission is detected from theactive nucleus and the inner radio jet, as well as a small-scale, coolcomponent of thermal emission, a number of the individual galaxies ofthe host cluster (Abell 2634), and the hotter thermal emission from thecluster itself. The X-ray detection of the jet allows us to argue thatsynchrotron emission may be an important mechanism in otherwell-collimated, fast jets, including those of classical double radiosources. The bases of the radio plumes are not detected in the X-ray,which supports the model in which these plumes are physically differentfrom the twin jets of lower-power radio galaxies. The plumes are in factspatially coincident with deficits of X-ray emission on large scales,which argues that they contain little thermal material at the clustertemperature, although the minimum pressures throughout the source arelower than the external pressures estimated from the observed thermalemission. Our observations confirm both spatially and spectrally that acomponent of dense, cool gas with a short cooling time is associatedwith the central galaxy. However, there is no evidence for the kind ofdiscontinuity in external properties that would be required in manymodels of the jet-plume transition in WATs. Although the WAT jet-plumetransition appears likely to be related to the interface between thiscentral cool component and the hotter intracluster medium, the mechanismfor WAT formation remains unclear. We revisit the question of thebending of WAT plumes, and show that the plumes can be bent by plausiblebulk motions of the intracluster medium, or by motion of the host galaxywith respect to the cluster, as long as the plumes are light.
| 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.
| High-resolution studies of radio sources in the Hubble Deep and Flanking Fields
18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have beencombined to image a 10-arcmin field centred on the Hubble Deep Field(HDF). This area also includes the Hubble Flanking Fields (HFF). Acomplete sample of 92 radio sources with S1.4 > 40 μJywas detected using the VLA data alone and then imaged with theMERLIN+VLA combination. The combined images offer (i) higher angularresolution (synthesized beams of diameter 0.2-0.5 arcsec), (ii) improvedastrometric accuracy, and (iii) improved sensitivity compared withVLA-only data. The images are amongst the most sensitive yet made at 1.4GHz, with rms noise levels of 3.3 μJy beam-1 in the0.2-arcsec images. Virtually all the sources are resolved, with angularsizes in the range 0.2 to 3 arcsec. The central 3-arcmin square wasimaged separately to search for sources down to 27 μJy. No additionalsources were detected, indicating that sources fainter than 40 μJyare heavily resolved with MERLIN and must have typical angular sizes> 0.5 arcsec. Radio sources associated with compact galaxies havebeen used to align the HDF, the HFF and a larger CFHT optical field tothe radio-based International Celestial Reference Frame. The HST opticalfields have been registered to <50 mas in the HDF itself, and to<=150 mas in the outer parts of the HFF. We find a statisticalassociation of very faint (>=2 μJy) radio sources with opticallybright HDF galaxies down to ~23 mag. Of the 92 radio sources above 40μJy, ~85 per cent are identified with galaxies brighter than I= 25mag; the remaining 15 per cent are associated with optically faintsystems close to or beyond the HFF (or even the HDF) limit. The highastrometric accuracy and the ability of radio waves to penetrateobscuring dust have led to the correct identification of several veryred, optically faint systems, including the the strongest submillimetresource in the HDF, HDF850.1. On the basis of their radio structures andspectra, 72 per cent (66 sources) can be classified as starburst oractive galactic nucleus-type systems; the remainder are unclassified.The proportion of starburst systems increases with decreasing fluxdensity; below 100 μJy > 70 per cent of the sources arestarburst-type systems associated with major disc galaxies in theredshift range 0.3-1.3. Chandra detections are associated with 55 of the92 radio sources, but their X-ray flux densities do not appear to becorrelated with the radio flux densities or morphologies. The mostrecent submillimetre results on the HDF and HFF do not provide anyunambiguous identifications with these latest radio data, except forHDF850.1, but suggest at least three strong candidates.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| Canonical Particle Acceleration in FR I Radio Galaxies
Matched-resolution multifrequency VLA observations of four radiogalaxies are used to derive the asymptotic low-energy slope of therelativistic electron distribution. When available, low-energy slopesare also determined for other sources in the literature. They provideinformation on the acceleration physics independent of radiative andother losses, which confuse measurements of the synchrotron spectra inmost radio, optical, and X-ray studies. We find a narrow range ofinferred low-energy electron energy slopes n(E)~E-2.1 for thecurrently small sample of lower luminosity sources classified as FR I(not classical doubles). This distribution is close to, but apparentlyinconsistent with, the test particle limit of n(E)~E-2.0expected from strong diffusive shock acceleration in the nonrelativisticlimit. Relativistic shocks or those modified by the back-pressure ofefficiently accelerated cosmic rays are two alternatives to producesomewhat steeper spectra. We note for further study the possibility ofacceleration through shocks, turbulence, or shear in the flaring andbrightening regions in FR I jets as they move away from the nucleus.Jets on parsec scales and the collimated jets and hot spots of FR II(classical double) sources would be governed by different accelerationsites and mechanisms; they appear to show a much wider range of spectrathan those for FR I sources.
| 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.
| XMM-Newton and Gemini Observations of Eight RASSCALS Galaxy Groups
We study the distribution of gas pressure and entropy in eight groups ofgalaxies belonging to the ROSAT All-Sky Survey/Center for AstrophysicsLoose Systems (RASSCALS). We use archival and proprietary XMM-Newtonobservations, supplementing the X-ray data with redshifts derived fromthe literature; we also list 127 new redshifts measured with the GeminiNorth telescope. The groups are morphologically heterogeneous in boththe optical and the X-ray, and several suffer from superpositions withbackground galaxies or clusters of galaxies. Nevertheless, they showremarkable self-similarity in their azimuthally averaged entropy andtemperature profiles. The entropy increases with radius; the behavior ofthe entropy profiles is consistent with an increasing broken power lawwith inner and outer slope 0.92+0.04-0.05 and0.42+0.05-0.04 (68% confidence), respectively.There is no evidence of a central, isentropic core, and the entropydistribution in most of the groups is flatter at large radii than in theinner region, challenging earlier reports, as well as theoretical modelspredicting large isentropic cores or asymptotic slopes of 1.1 asr-->&infy;. The pressure profiles are consistent with a self-similardecreasing broken power law in radius; the inner and outer slopes are-0.78+0.04-0.03 and-1.7+0.1-0.3, respectively. The results suggestthat the larger scatter in the entropy distribution reflects the variedgasdynamical histories of the groups; the regularity and self-similarityof the pressure profiles is a sign of a similarity in the underlyingdark matter distributions.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the US (NASA). The XMM-Newton project is supported by theBundesministerium für Bildung und Forschung/Deutsches Zentrumfür Luft- und Raumfahrt (BMFT/DLR), the Max-Planck Society, and theHeidenhain-Stiftung, and also by PPARC, CEA, CNES, and ASI. Also basedon observations obtained at the Gemini Observatory, which is operated bythe Association of Universities for Research in Astronomy, Inc., under acooperative agreement with the NSF on behalf of the Gemini partnership:the National Science Foundation (US), the Particle Physics and AstronomyResearch Council (UK), the National Research Council (Canada), CONICYT(Chile), the Australian Research Council (Australia), CNPq (Brazil), andCONICET (Argentina).
| The Bologna Complete Sample of Nearby Radio Sources
We present a new, complete sample of 95 radio sources selected from theB2 Catolog of Radio Sources and the Third Cambridge Revised Catalog(3CR), with z<0.1. Since no selection effect on the core radio power,jet velocity, or source orientation is present, this sample is wellsuited for statistical studies. In this first paper we present theobservational status of all sources on the parsec (milliarcsecond) andkiloparsec (arcsecond) scale; we give new parsec-scale data for 28sources and discuss their parsec-scale properties. By combining thesedata with those in the literature, information on the parsec-scalemorphology is available for a total of 53 radio sources with differentradio power and kiloparsec-scale morphologies. We investigate theirproperties. We find a dramatically higher fraction of two-sided sourcesin comparison with that of previous flux-limited VLBI surveys.
| The HST view of the nuclear emission line region in low luminosity radio-galaxies
We study the properties of the emission line regions in two samples oflow luminosity radio-galaxies, while focusing on the Compact EmissionLine Region (CELR) revealed to be a characteristic feature of theseobjects by HST narrow-band imaging. We find a strong correlation betweenline and optical continuum nuclear emission, which suggests that theoptical cores (most likely of non-thermal origin) can be directlyassociated to the source of ionizing photons, i.e. that we are seeing ajet-ionized narrow line region. A photon budget argument indicates thatthe optical nuclear sources produce sufficient photon flux provided thatthe covering factor of the circum-nuclear gas is rather large, onaverage 0.3. Analysis of HST images and spectra suggests that the CELRmay take the form of a pc-scale, high filling factor structure, possiblyan optically thin torus. Estimates of the CELR mass lead to values assmall as 10{-}10^3 Mȯ, and photon counting sets a limitto the Broad Line Region mass of M_BLR < 10-2Mȯ. When considered together with the low accretion rateand the tenuous torus structure, a general paucity of gas in theinnermost regions of low luminosity radio-galaxies emerges as the maincharacterizing difference from more powerful Active Galactic Nuclei.
| Multi-frequency study of the B3 VLA sample. IV. 74-MHz flux densities from VLA A-array data
We present 74-MHz data of 365 B3 VLA sources, which were obtainedas a by-product of the observations of Tschager et al. (2003, A&A,402, 171) who observed a large area of sky with the VLA in A-array.Apart from the lowest-frequency observations of B3 VLA sourcesperformed so far contributing to our on-going multi-frequency study ofthis survey, these A-array data also provide the first morphologicalinformation at this low frequency. The most intriguing result is thediscovery of two new Giant Radio Galaxy candidates, B3 1232+397Bat a redshift of 3.22 and B3 1419+419 at a redshift of 0.367.This means that B3 1232+397B would be the most distant GRG knownto date. At the same time, these two objects are two new examples ofsources with recurrent activity. Our results give a foretaste of theimpact which extremely low-frequency radio continuum observations willhave for the study of source evolution.
| 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.
| 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.
| 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.
| 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.
| Implications for unified schemes from the quasar fraction and emission-line luminosities in radio-selected samples
We use a principal components analysis of radio-selected (3CRR, 6CE and7CRS) active galactic nuclei (AGN) data sets to define two parametersrelated to low-frequency (151-MHz) radio luminosity L151 and[OIII] luminosity L[OIII]: a parameter α encoding theL151-L[OIII] correlation and a parameter βencoding scatter about this correlation. We describe methods forconstructing generalized luminosity functions (GLFs) based on α,β, redshift and schemes for unifying quasars and radio galaxies.These GLFs can be used to generate radio luminosity functions (RLFs)which improve on those of Willott et al. (2001a), mostly because theyincorporate scatter and are therefore much smoother.Luminosity-dependent unified schemes (e.g. a receding-torus scheme) havebeen invoked to explain the low quasar-to-radio galaxy fraction at lowα and the differences in emission-line luminosities of radioquasars and radio galaxies. With the constraints of the 3CRR, 6CE and7CRS data sets and radio source counts, our GLF approach was used todetermine whether a receding-torus-like scheme is required if there aretwo populations of radio sources: one at low α, consisting of`starved AGN' the other at high α, consisting of `Eddington-tunedAGN'. Because of the overlap between these two populations and theeffects of the β parameter, schemes with or without a recedingtorus can produce a low quasar fraction at low α and differencesin [OIII] luminosity between radio galaxies and quasars. The recedingtorus may be a physical process important in one or more populations ofradio sources, but this is not yet proved either by the quasar fractionor the emission-line properties of radio-selected samples.
| Construction of a Celestial Coordinate Reference Frame from VLBI Data
A large number (2 million) of VLBI observations have been reducedin order to refine the measured coordinates of the observed radiosources. The data reduction was carried out in the OCCAM package usingthe least squares colocation method. Corrections to the coordinates of642 objects were derived. The accuracy of the catalog is no worse than0.2 milliseconds of arc for stable sources.
| Obscuration and Origin of Nuclear X-Ray Emission in FR I Radio Galaxies
We present X-ray observations of the nuclear region of 25 Fanaroff-Rileytype I (FR I) radio galaxies from the 3CRR and B2 catalogs, using datafrom the Chandra and XMM-Newton archives. We find the presence of aX-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources thedetection of a CCCX is uncertain, while in the remaining 9/25 sources noCCCX is found. All the sources are embedded in a diffuse soft X-raycomponent, generally on kiloparsec scales, which is in agreement withthe halo of the host galaxy and/or with the intracluster medium. TheX-ray spectra of the cores are described by a power law with photonindices Γ=1.1-2.6. In eight sources excess absorption over theGalactic value is detected, with rest-frame column densitiesNzH~1020-1021cm-2 thus, we confirm the previous claim, based on opticaldata, that most FR I radio galaxies lack a standard optically thicktorus. We find significant correlations between the X-ray coreluminosity and the radio and optical luminosities, suggesting that atleast a fraction of the X-ray emission originates in a jet; however, theorigin of the X-rays remains ambiguous. If the X-ray emission isentirely attributed to an isotropic, accretion-related component, wefind very small Eddington ratios,Lbol/LEdd~10-3to10-8, and wecalculate the radiative efficiency to beη~10-2to10-6 on the basis of the Bondiaccretion rates from the spatial analysis. This suggests thatradiatively inefficient accretion flows are present in the cores oflow-power radio galaxies.
| K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light
We investigate the near-infrared K-band properties of the brightestcluster galaxies (BCGs) in a sample of 93 X-ray galaxy clusters andgroups, using data from the Two Micron All Sky Survey. Our clustersample spans a factor of 70 in mass, making it sensitive to any clustermass-related trends. We derive the cumulative radial distribution forthe BCGs in the ensemble and find that 70% of the BCGs are centered inthe cluster to within 5% of the virial radius r200; thisquantifies earlier findings that BCG position coincides with the clustercenter as defined by the X-ray emission peak. We study the correlationsbetween the luminosity of the BCGs (Lb) and the mass and theluminosity of the host clusters, finding that BCGs in more massiveclusters are more luminous than their counterparts in less massivesystems and that the BCGs become less important in the overall clusterlight (L200) as cluster mass increases. By examining a largesample of optically selected groups, we find that these correlationshold for galactic systems less massive than our clusters(<3×1013 Msolar). From the differencesbetween luminosity functions in high- and low-mass clusters, we arguethat BCGs grow in luminosity mainly by merging with other luminousgalaxies as the host clusters grow hierarchically; the decreasing BCGluminosity fraction (Lb/L200) with cluster massindicates that the rate of luminosity growth in BCGs is slow compared tothe rate at which clusters acquire galaxy light from the field or othermerging clusters. Utilizing the observed correlation between the clusterluminosity and mass and a merger tree model for cluster formation, weestimate that the amount of intracluster light (ICL) increases withcluster mass; our calculations suggest that in 1015Msolar clusters more than 50% of total stellar mass is inICL, making the role of ICL very important in the evolution andthermodynamic history of clusters. The cluster baryon fractionaccounting for the ICL is in good agreement with the value derived fromcosmic microwave background observations. The inclusion of ICL reducesthe discrepancy between the observed cluster cold baryon fraction andthat found in hydrodynamical simulations. Based on the observed ironabundance in the intracluster medium, we find that the ICL predicted byour model, together with the observed galaxy light, match the ironmass-to-light ratio expected from simple stellar population models,provided that the Salpeter initial mass function is adopted. The ICLalso makes it easier to produce the ``iron excess'' found in the centralregions of cool-core clusters.
| Distribution of Faraday Rotation Measure in Jets from Active Galactic Nuclei. II. Prediction from Our Sweeping Magnetic Twist Model for the Wiggled Parts of Active Galactic Nucleus Jets and Tails
Distributions of Faraday rotation measure (FRM) and the projectedmagnetic field derived by a three-dimensional simulation of MHD jets areinvestigated based on our ``sweeping magnetic twist model.'' FRM andStokes parameters were calculated to be compared with radio observationsof large-scale wiggled AGN jets on kiloparsec scales. We propose thatthe FRM distribution can be used to discuss the three-dimensionalstructure of the magnetic field around jets and the validity of existingtheoretical models, together with the projected magnetic field derivedfrom Stokes parameters. In a previous paper we investigated the basicstraight part of AGN jets by using the result of a two-dimensionalaxisymmetric simulation. The derived FRM distribution has a generaltendency to have a gradient across the jet axis, which is due to thetoroidal component of the magnetic field generated by the rotation ofthe accretion disk. In this paper we consider the wiggled structure ofthe AGN jets by using the result of a three-dimensional simulation. Ournumerical results show that the distributions of FRM and the projectedmagnetic field have a clear correlation with the large-scale structureof the jet itself, namely, three-dimensional helix. Distributions,seeing the jet from a certain direction, show a good matching with thosein a part of the 3C 449 jet. This suggests that the jet has a helicalstructure and that the magnetic field (especially the toroidalcomponent) plays an important role in the dynamics of the wiggleformation because it is due to a current-driven helical kink instabilityin our model.
| 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.
| Distribution of Faraday Rotation Measure in Jets from Active Galactic Nuclei. I. Predictions from our Sweeping Magnetic Twist Model
Using the numerical data of MHD simulation for active galactic nucleus(AGN) jets based on our ``sweeping magnetic twist model,'' we calculatedthe Faraday rotation measure (FRM) and the Stokes parameters to comparewith observations. We propose that the FRM distribution can be used todiscuss the three-dimensional structure of magnetic field around jets,together with the projected magnetic field derived from the Stokesparameters. In the present paper, we assumed the basic straight part ofthe AGN jet and used the data of axisymmetric simulation. The FRMdistribution that we derived has a general tendency to have gradientacross the jet axis, which is due to the toroidal component of thehelical magnetic field generated by the rotation of the accretion disk.This kind of gradient in the FRM distribution is actually observed insome AGN jets, which suggests a helical magnetic field around the jetsand thus supports our MHD model. Following this success, we are nowextending our numerical observation to the wiggled part of the jets,using the data of three-dimensional simulation based on our model, in anupcoming paper.
| Dust and Ionized Gas in Nine Nearby Early-Type Galaxies Imaged with the Hubble Space Telescope Advanced Camera for Surveys
We present V and I continuum images and Hα+[N II] maps of nineearly-type galaxies observed with the Wide Field Channel of the AdvancedCamera for Surveys on the Hubble Space Telescope. Dust and ionized gasare detected in all galaxies. The optical nebulae are primarilyconcentrated on the nuclei and extend out to radii of a few hundredparsecs in compact clumps, filaments, or disks. Two galaxies, NGC 6166and NGC 6338, also possess diffuse, ionized filaments on kiloparsecscales. The ionized gas is entirely contained within the nuclear disksof ESO 208-G021, NGC 3078, and NGC 7720. In the radio-loud galaxy NGC6166, emission-line filaments are detected along the radio lobes,possibly as a result of shock ionization. A wide range of ionized gasmasses, Mg~7×102-3×106Msolar, are calculated from the observed fluxes. Even in thissmall sample, the orientation of the ionized material correlates wellwith the major or minor axis of the galaxies, consistent with anexternal origin for the dust and gas.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
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