Nuclei Of Radio Galaxies Research Articles

Searching for Molecular Gas in Southern Radio Galaxies

It has recently been proposed that the jets of low-luminosity radio galaxies are powered by direct accretion of the hot phase of the IGM onto the central black hole. Cold gas remains a plausible alternative fuel supply, however. The most compelling evidence that cold gas plays a role in fueling radio galaxies is that dust is detected more commonly and/or in larger quantities in (elliptical) radio galaxies compared with radio-quiet elliptical galaxies. On the other hand, only small numbers of radio galaxies have yet been detected in CO (and even fewer imaged), and whether or not all radio galaxies have enough cold gas to fuel their jets remains an open question. If so, then the dynamics of the cold gas in the nuclei of radio galaxies may provide important clues to the fuelling mechanism. The only instrument capable of imaging the molecular component on scales relevant to the accretion process is ALMA, but very little is yet known about CO in southern radio galaxies. Our aim is to measure the CO content in a complete volume-limited sample of southern radio galaxies, in order to create a well-defined list of nearby targets to be imaged in the near future with ALMA. APEX has been equipped with a receiver (APEX-1) able to observe the 230 GHz waveband. This allows us to search for CO(2-1) line emission in our target galaxies. Here we present the results of CO(2-1) APEX-1 spectroscopy taken in 2008 and 2010 for our southern sample. The experiment was successful with nearly all targets detected, and several indications for double-horned CO line profiles, consistent with ordered rotation.

VLBI imaging of M 81* at 43 GHz

The nearby spiral galaxy M81 harbors in its core a Low-Luminosity AGN (LLAGN), and appears closely related to the more distant and powerful AGNs seen in quasars and radio galaxies. The intrinsic size of this object is unknown due to scattering, and it has shown a core-jet morphology with weak extended emission rotating with wavelength. The proximity of M\,81 (D=3.63 Mpc) allows a detailed investigation of its nucleus to be made. The nucleus is four orders of magnitude more luminous than the Galactic centre, and is therefore considered a link between SgrA* and the more powerful nuclei of radio galaxies and quasars. Our main goal was to determine the size of M81* at a shorter wavelength thus directly testing whether the frequency-size dependent law Theta propto nu^(-0.8) was still valid for wavelengths shorter than 1cm. In addition, we also aimed to confirm the rotation of the source as a function of frequency. We observed the continuum 7mm radio emission of M81* using the Very Long Baseline Array on Sep 13, 2002, using nearby calibrators to apply their interferometric observables to the target source, to increase the chances of detection. The source was detected on all baselines and hybrid mapping was possible. We present the first 7mm VLBI image of the core of M81*, which represents the highest resolution image ever of this source. Modeling the interferometric visibilities with two Gaussian functions sets constraints on the angular size of its core down to 38 microarcseconds, corresponding to a maximum (projected) linear size of 138AU, and shows extended emission towards the NE with a position angle of ~50deg. A fit of one Gaussian elliptical function yields a position angle of 28+/-8 degrees for its elongated, compact structure. Combining the 7mm size with earlier measurements at other frequencies we determine a frequency-size dependence of Theta propto nu^(-0.88+/-0.04).

Molecular disks in radio galaxies

Context. It has recently been proposed that the jets of low-luminosity radio galaxies are powered by direct accretion of the hot phase of the IGM onto the central black hole. Cold gas remains a plausible alternative fuel supply, however. The most compelling evidence that cold gas plays a role in fueling radio galaxies is that dust is detected more commonly and/or in larger quantities in (elliptical) radio galaxies compared with radio-quiet elliptical galaxies. On the other hand, only small numbers of radio galaxies have yet been detected in CO (and even fewer imaged), and whether or not all radio galaxies have enough cold gas to fuel their jets remains an open question. If so, then the dynamics of the cold gas in the nuclei of radio galaxies may provide important clues to the fuelling mechanism. Aims. The only instrument capable of imaging the molecular component on scales relevant to the accretion process is ALMA, but very little is yet known about CO in southern radio galaxies. Our aim is to measure the CO content in a complete volume-limited sample of southern radio galaxies, in order to create a well-defined list of nearby targets to be imaged in the near future with ALMA. Methods. APEX has recently been equipped with a receiver (APEX-1) able to observe the 230 GHz waveband. This allows us to search for CO(2-1) line emission in our target galaxies. Results. Here we present the results for our first three southern targets, proposed for APEX-1 spectroscopy during science verification: NGC3557, IC4296 and NGC1399. The experiment was successful with two targets detected, and possible indications for a double-horned CO line profile, consistent with ordered rotation. These early results are encouraging, demonstrating that APEX can efficiently detect CO in nearby radio galaxies. We therefore plan to use APEX to obtain CO spectroscopy for all our southern targets.

What Kinds of Accretion Disks Are There in the Nuclei of Radio Galaxies?

It seems to be a widely accepted opinion that the types of accretion disks (or flows) generally realized in the nuclei of radio galaxies and in further lower mass-accretion rate nuclei are inner, hot, optically thin, radiatively inefficient accretion flows (RIAFs) surrounded by outer, cool, optically thick, standard-type accretion disks. However, observational evidence for the existence of such outer cool disks in these nuclei is rather poor. Instead, recent observations sometime suggest the existence of inner cool disks of the non-standard type, which develop in a region very close to their central black holes. Taking NGC 4261 as a typical example of such light-eating nuclei, for which both flux data ranging from radio to X-ray and data for the counterjet occultation are available, we examine the plausibility of such a picture for the accretion states, as mentioned above, based on model predictions. It is shown that the explanation of the gap seen in the counterjet emission in terms of the free–free absorption by an outer standard disk is unrealistic and, moreover, the existence itself of such an outer standard disk seems to be very implausible. Instead, the model of RIAF in an ordered magnetic field (so-called resistive RIAF model) can well serve to explain the emission gap in terms of the synchrotron absorption, as well as to reproduce the observed features of the overall spectral energy distribution (SED). This model also predicts that the RIAF state starts directly from an interstellar hot gas phase at around the Bondi radius, and terminates at the inner edge, whose radius is about 100-times the Schwartzschild radii. Therefore, there is a good possibility for a cool disk to develop within this innermost region.

The active nuclei ofz< 1.0 3CRR radio sources

We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations, and the availability of more mid-infrared data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type-2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered `fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud AGN on such diagnostic diagrams.

Modeling nuclei of radio galaxies from VLBI radio observations

We present a new method to fit the variations of both coordinates of a VLBI component as a function of time, assuming that the nucleus of the radio source contains a binary black hole system (BBH system). The presence of a BBH system produces 2 perturbations of the trajectory of the ejected VLBI components. By using only the VLBI coordinates, the problem we have to solve reduces to an astrometric problem. Knowledge of the variations of the VLBI coordinates as a function of time contains the kinematical information, thus we are able to deduce the inclination angle of the source and the bulk Lorentz factor of the ejected component. Generally, there is a family of the BBH system producing the same fit to our data. To illustrate this method, we apply it to the source 1807+784. We find that the inclination of the source is i = 5.8+-1.8 degrees and the VLBI component is ejected with a bulk Lorentz factor of 3.7+-0.3. We determine the family of the BBH system which provides the best fit, assuming at first that the masses of the 2 black holes are equal and then that the masses are different. Each family of BBH systems is characterized by Tp/Tb~1.967, where Tp and Tb are the precession period of the accretion disk of the black hole ejecting the VLBI component and the orbiting period of the BBH system.

ChandraandXMM‐NewtonObservations of a Sample of Low‐Redshift FR I and FR II Radio Galaxy Nuclei

We present spectral results from Chandra and XMM-Newton observations of a sample of 22 low-redshift (z < 0.1) radio galaxies and consider whether the core emission originates from the base of a relativistic jet, or an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FR I-type radio-galaxy cores, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FR II-type radio galaxy cores are dominated by absorbed emission, with NH ≳ 1023 atoms cm-2, which is likely to originate in an accretion flow. We discuss several models that may account for the different nuclear properties of FR I- and FR II-type cores and also demonstrate that both heavily obscured, accretion-related and unobscured, jet-related components may be present in all radio galaxy nuclei. Any absorbed, accretion-related components in FR I-type galaxies have low radiative efficiencies.

X‐ray and radio emission in the nuclei of radio galaxies and thedisk‐jet connection

AbstractIt appears that relativistic jets are produced by accreting black holes. We might therefore expect that events observed in jets, such as the appearance of bright radio knots moving at apparent superluminal speeds, originate as disturbances in the central engine. This is observed in a number of compact X‐ray binary systems in our Galaxy, manifested as changes in the X‐ray emission state followed by superluminal ejections. The author and collaborators have detected somewhat similar events in the radio galaxies 3C 120 (type Fanaroff‐Riley I) and 3C 111 (FR II). The data both confirm the disk‐jet connection in accreting supermassive black holes and provide constraints on the structure of the jet between the central engine and the millimeter‐wave core. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On the Observed Rapid Motions in Extragalactic Radio Sources

Using interferometric methods, radio astronomers have shown that very rapid motions involving radio jets are detectable in a variety of active centers: the nuclei of low-redshift radio galaxies, QSOs and BL Lac objects, and active binary systems containing a black hole or a neutron star in our own Galaxy. By comparing the positions of about 80 extragalactic sources of this type with the position of the galaxies in the de Vaucouleurs catalog and the updated QSO catalog of Veron-Cetty and Veron, it is shown that about 50% of them lie within 1° of a low-redshift spiral galaxy, and about 15% lie so close to galaxies (Δθ ≤ 10') that the associations are highly significant. Thus, the sources are probably physically associated with these galaxies. All the galaxies are classified as spiral. In addition, many of the sources have other QSOs lying very close to them. These results suggest that the motions observed in these radio sources, as well as in the nuclei of radio galaxies and in Galactic microquasars, and perhaps in many more QSOs and other objects, are all similar and are mildly relativistic, and they do not require us to invoke large Lorentz factors. Some of the problems associated with this picture are discussed.

New insights on selected radio galaxy nuclei

We are performing a survey of powerful 3CR radio galaxies and quasars in the mid-infrared. The purpose is to test for the presence of a powerful hidden quasar-like thermal nucleus by measuring the ‘waste heat’ that must be emitted by any obscuring dust. The dust is treated as a calorimeter for the central engine. Three early mid-IR detections are particularly interesting: Cygnus A, M87, and Centaurus A. These are notable both as a demonstration of our technique and as an example of the great variety of objects that are classified as radio galaxies. We confirm the presence of hidden quasar-like nuclei in Cyg A and Cen A, but M87 shows only weak mid-IR emission, indicating its AGN is non-thermal, and consistent with only synchrotron emission. We also present a new near-infrared adaptive optics image of Cygnus A which shows a secondary point source.

The Nuclei of Radio Galaxies in the Ultraviolet: The Signature of Different Emission Processes

We have studied the nuclei of 28 radio galaxies from the 3CR sample in the UV band. Unresolved nuclei (central compact cores: CCCs) are observed in 10 of the 13 FR I galaxies, and in five of the 15 FR II galaxies. All sources that do not have a CCC in the optical do not have a CCC in the UV. Two FR I galaxies (3C 270 and 3C 296) have a CCC in the optical but do not show the UV counterpart. Both of them show large dusty disks observed almost edge-on, possibly implying that they play a role in obscuring the nuclear emission. We have measured optical-UV spectral indices αo,UV between ~0.6 and ~7.0 (Fν ∝ ν-α). Broad-line radio galaxies have the flattest spectra, and their values of αo,UV are also confined to a very narrow range. This is consistent with radiation produced in a geometrically thin, optically thick accretion disk. On the other hand, FR I nuclei, which are most plausibly originated by synchrotron emission from the inner relativistic jet, show a wide range of αo,UV. There is a clear trend with orientation in that sources observed almost edge-on or with clear signs of dust absorption have the steepest spectra. These observations imply that in FR I galaxies, obscuration can be present, but the obscuring material is not in a standard geometrically thick torus. The most striking difference between these absorbing structures and the classic active galactic nucleus tori resides in the lower optical depth of the FR I galaxy obscuring material.

A Stationary Core with a One‐sided Jet in the Center of M81

The nucleus of the nearby spiral galaxy M81 was observed at 8.4 GHz with a global VLBI array at 20 epochs over 4.5 yr, with a linear resolution at the source of about 2000 AU, or 0.01 pc. Phase-referenced mapping with respect to the geometric center of SN 1993J in the same galaxy enabled us to find, with a standard error of about 600 AU, a stationary point in the source southeast of the brightness peak. We identify this point as the location of the core and the putative black hole at the gravitational center of the galaxy. The 2 σ upper bound on the core's average velocity on the sky is ≤40 μas yr-1, or ≤730 km s-1 (relative to the center of SN 1993J, in excess of galactic rotation). A short, one-sided jet extends toward the northeast from the core, in projection approximately in line with the rotation axis of the galaxy, and toward the well-known extended emission 1 kpc farther out. The orientation of the jet varies smoothly, with timescales of about 1 yr and an rms of 6° around its mean position angle of 50°. Occasionally the jet appears to bend to the east. The length of the jet is only about 1 mas (3600 AU) and varies with an rms of about 20% from epoch to epoch. The inferred speeds are below 0.08c. The total flux density of the core jet varies erratically, changing on occasion by a factor of 2 over a few weeks, without any significant changes in the source size and orientation. The inferred velocity of the plasma flow is greater than 0.25c. The results are consistent with a model in which plasma condensations with short lifetimes are ejected relativistically from the core on a timescale of less than a few weeks. The condensations travel along a tube whose pattern and geometry are also variable but only on a timescale of about 1 yr. The central engine of M81 has qualitative similarities to those of powerful active galactic nuclei of radio galaxies and quasars and may also represent in power and size a scaled-up version of the largely hidden nucleus in our own Galaxy.

The compact triples 0710 + 439 and 2352 + 495 - A new morphology of radio galaxy nuclei

view Abstract Citations (48) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Compact Triples 0710+439 and 2352+495: A New Morphology of Radio Galaxy Nuclei Conway, J. E. ; Pearson, T. J. ; Readhead, A. C. S. ; Unwin, S. C. ; Xu, W. ; Mutel, R. L. Abstract The compact radio sources 0710+439 and 2352+495 have each been mapped using intercontinental VLBI arrays at frequencies of 5.0 and 10.7 GHz. The source 0710+439 has also been mapped at 1.7 GHz. Both sources consist of three principal components that lie almost on a straight line. Except for the existence of three VLBI components instead of two they both seem to have similar properties to members of the compact double class. Multi- epoch observations show that apparent velocities between the components in both sources are below c. In the source 0710+439 comparison of multi- epoch data allows us to set a limit of 0.054+/-0.14 h^-1^ c on the rate of separation of the two brightest components. In 2352+495 the multi- epoch 5 GHz VLBI data show that the flux density variations detected by total flux density monitoring are due to changes within the middle component. These fluctuations may be due to refractive interstellar scintillation or they may be intrinsic to the source; in either case the middle component must contain the most compact structure within the source. The morphology, spectra and variability of the middle components strongly suggest that the center of activity in both sources lies here rather than in either of the outer components, providing the first clear evidence of symmetric radio structure in a powerful active galaxy nucleus. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171697 Bibcode: 1992ApJ...396...62C Keywords: Active Galactic Nuclei; Active Galaxies; Galactic Structure; Radio Galaxies; Very Long Base Interferometry; Radio Sources (Astronomy); Radio Spectra; Astrophysics; GALAXIES: ACTIVE; GALAXIES: INDIVIDUAL ALPHANUMERIC: 0710; 439; GALAXIES: INDIVIDUAL ALPHANUMERIC: 2352; 495; GALAXIES: NUCLEI; RADIO CONTINUUM: GALAXIES; TECHNIQUES: INTERFEROMETRIC full text sources ADS | data products SIMBAD (10) NED (1)

Optical nuclear activity in the radio galaxy 3C 465

The presently discussed discovery of weak, high-ionization emission lines in the nuclei of radio galaxies which had been classified as quiescent absorption-line systems demonstrates that AGN-like activity does occur in the central galaxies of rich clusters. 3C 465-like objects can be considered the extreme low-luminosity end of active nuclei in the centers of rich clusters; the estimated magnitude of 3C 465's nuclear component, at -15.7, is consistent with the precipitous drop of the luminosities of quasars in clusters. 3C 465 appears to represent a new class of optically active objects. 48 refs.

RADIO CHARACTERISTICS OF GALACTIC NUCLEI.

ABSTRACT Radio characteristics of galactic nuclei, providing such unique information as spectral data on source variability, and the long-term history of the central engine and its duration of activity and total energy, are reviewed. The compact radio source characteristics are complicated by orientation-dependent relativistic beaming and by refractive focusing in the interstellar medium. Incoherent synchrotron radiation is thought to be the emission mechanism, with the result that synchrotron self-absorption in compact sources hides the central engine from direct radio observation. However, the history revealed by the extended jets and lobes of radio galaxies and quasars favors a single massive object not supported by radiation pressure, either a spinar or a black hole, as the energy source in radio-galaxy nuclei.

Infrared photometry of the nuclei of early-type radio galaxies

On presente la photometrie J, H, K, L' a deux ouvertures et la photometrie a 10 μm (N) a une ouverture du noyau de 44 galaxies elliptiques et SO proches. Il n'y a pas de correlation entre les flux radio et les exces a 10 μm comme pour les galaxies de Seyfert

Origin of the galactic Centre lobes

Recent observations of the 10-GHz continuum1 have indicated that there is a pair of lobes having ridges of intensity extending from the galactic centre region in a direction perpendicular to the galactic plane. This observation has attracted considerable attention as it is clearly an indication of dynamic processes occurring at the centre of our Galaxy that are similar to those occurring in the nuclei of radio galaxies, though different in scale and in strength2. Here we interpret these lobes as being due to a mag-netodynamic acceleration mechanism in which the production and relaxation of the magnetic twist induced by the rotation of the contracting gas disk play a part. The plasma is accelerated in a conical cylinder with a helical velocity field, reproducing the observed feature of radio lobes.

The Galactic Nucleus

The ionized gas within 2 parsec from the centre region is distributed in spiral-like features which appear to emanate from a nucleus. This nucleus is close to the ultra-compact radio source and the infrared source IRS 16, but does not exactly coincide with either. The spiral is strongly inclined to the galactic plane. The arms are clumpy. The spiral appears to rotate, but there appear to be also large radial motions.The phenomena show some resemblance to what is observed on a very much larger scale in the nuclei of radio galaxies and quasars.An exceptionally powerful, variable source of hard X- and gamma-rays, GCX, has been found in the direction of the centre. This may be the true galactic nucleus. Strong, variable emission of the 511-keV electron-positron annihilation line has also been observed to come from the central region.The evidence for a central black hole is discussed. It is not compelling. The energy required for the strong infrared radiation emitted by the central few parsecs is probably coming from a large number of luminous stars, and due to a recent burst of star formation. The observed state of ionization indicates the burst to have occurred about one million years ago.The distribution of the dense molecular clouds within 300 pc from the centre is very asymmetric: practically all lie at positive longitudes. Their motions differ radically from circular motions. The layer of clouds is tilted relative to the galactic plane. The so-called “+40-km/s cloud” lies in front of the nuclear spiral, and is therefore moving towards the centre. Different interpretations of the systematic motions are considered. The outspoken asymmetry excludes a static condition. If the “molecular ring” at R ~ 150 parsec is actually an expanding feature, its origin might be sought in the same burst of star formation proposed to explain the state of ionization and the infrared radiation of the nuclear region.

A possible mechanism of jet formation in the nuclei of radio galaxies

A hypothesis is being put forward that the formation of jets in the nuclei of radio galaxies is due to a high-speed energy excretion (explosion) in the accretion disk around a massive black hole. The explosion can be induced, for example, by a fall of the star into the black hole. For the accretion disk featuring an exponential high-density distribution, an asymmetrical explosion can be obtained: the shock front moves in the direction of decreasing the density accelerately and achieves the relativistic velocity swiftly, carrying away the most fraction of the explosion energy. Radio emission of the jet involves synchrotron radiation of relativistic electrons which are accelerated by such shock wave in the magnetic field driven up by the shock front.

A possible interpretation of ?superluminal? motions in radio galaxies

It was shown in an earlier paper (Gorbatskii, 1981) that jet-like ejections from the active nuclei of radio galaxies can be created by ejectors of relativistic electrons moving away from the nucleus. The observations indicate that the magnetic field in the neighborhood of an active nucleus is tangled and very inhomogeneous. If the gradient of the magnetic field is sufficiently large and its direction differs little from the direction of the velocity of the bunches of ejectors, it is possible for there to be superluminal (from the point of view of the observer) motion of bright knots in the jet-like structure near the nucleus.