scholarly journals Collimated Bipolar Outflow and the Formation of Nuclear Spirals; Possible Role of Magnetic Fields

1990 ◽  
Vol 140 ◽  
pp. 447-448
Author(s):  
P. Pismis ◽  
E. Moreno ◽  
A. Garcia-Barreto

The existence of non–steady phenomena, namely activity in the form of radial motions (outflow) of matter from the nuclei of galaxies is well established at present. Active Galactic Nuclei (AGN) constitute a topic of great interest and are intensively studied by all existing observational techniques. Conventionally objects classified as AGN span a range from quasars, radio galaxies to Seyferts 1 and 2. It appears, however, that there exist galaxies which exhibit somewhat milder activity which does not qualify their inclusion in the AGN group. The designation of MAGN (M for mildly) was suggested in the past (Pismis, 1986) to cover the less energetic nuclei. It may be reasonable to consider that active nuclei form a sequence, the difference along it being due to the energetics of the nuclei, from the most active quasars and radio galaxies down to the mildest ones like M31 or our Galaxy. The phenomenon underlying the activity may thus be universal, subject to the intrinsic energetics of the nuclei (Pismis, 1987).

2019 ◽  
Vol 15 (S356) ◽  
pp. 247-251
Author(s):  
Biny Sebastian ◽  
Preeti Kharb ◽  
Christopher P. O’ Dea ◽  
Jack F. Gallimore ◽  
Stefi A. Baum ◽  
...  

AbstractThe role of starburst winds versus active galactic nuclei (AGN) jets/winds in the formation of the kiloparsec scale radio emission seen in Seyferts is not yet well understood. In order to be able to disentangle the role of various components, we have observed a sample of Seyfert galaxies exhibiting kpc-scale radio emission suggesting outflows, along with a comparison sample of starburst galaxies, with the EVLA B-array in polarimetric mode at 1.4 GHz and 5 GHz. The Seyfert galaxy NGC 2639, shows highly polarized secondary radio lobes, not observed before, which are aligned perpendicular to the known pair of radio lobes. The additional pair of lobes represent an older epoch of emission. A multi-epoch multi-frequency study of the starburst-Seyfert composite galaxy NGC 3079, reveals that the jet together with the starburst superwind and the galactic magnetic fields might be responsible for the well-known 8-shaped radio lobes observed in this galaxy. We find that many of the Seyfert galaxies in our sample show bubble-shaped lobes, which are absent in the starburst galaxies that do not host an AGN.


2016 ◽  
Vol 12 (S324) ◽  
pp. 149-156
Author(s):  
Talvikki Hovatta

AbstractAccording to the currently favored picture, relativistic jets in active galactic nuclei (AGN) are launched in the vicinity of the black hole by magnetic fields extracting energy from the spinning black hole or the accretion disk. In the past decades, various models from shocks to magnetic reconnection have been proposed as the energy dissipation mechanism in the jets. This paper presents a short review on how linear polarization observations can be used to constrain the magnetic field structure in the jets of AGN, and how the observations can be used to constrain the various emission models.


2019 ◽  
Vol 489 (1) ◽  
pp. L58-L62
Author(s):  
Andrzej A Zdziarski

Abstract We study the effect of variable jet bulk Lorentz factors, i.e. either jet acceleration or deceleration, on partially synchrotron self-absorbed radio spectra from cores of radio-loud active galactic nuclei and black hole binaries in the hard state. In about a half of quasars and radio galaxies, their core radio spectra are observed to be soft, i.e. have the spectral index of α < 0. If they are emitted by jets with constant Lorentz factors, that softness implies deposition of large amounts of energy at large distances from the centre. We show here that such soft spectra can be explained without that energetic requirement by emission of jets with the Doppler factor increasing with the distance. This can happen for either jet acceleration or deceleration, depending on the jet viewing angle. We find our model can explain the quiescent radio to X-ray spectra of the BL Lac objects Mrk 421 and Mrk 501.


2020 ◽  
Vol 497 (3) ◽  
pp. 3638-3657
Author(s):  
S Walg ◽  
A Achterberg ◽  
S Markoff ◽  
R Keppens ◽  
O Porth

ABSTRACT The class of double-double radio galaxies (DDRGs) relates to episodic jet outbursts. How various regions and components add to the total intensity in radio images is less well known. In this paper, we synthesize synchrotron images for DDRGs based on special relativistic hydrodynamic simulations, making advanced approximations for the magnetic fields. We study the synchrotron images for three different radial jet profiles; ordered, entangled, or mixed magnetic fields; spectral ageing from synchrotron cooling; the contribution from different jet components; the viewing angle and Doppler (de-)boosting; and the various epochs of the evolution of the DDRG. To link our results to observational data, we adopt to J1835+6204 as a reference source. In all cases, the synthesized synchrotron images show two clear pairs of hotspots, in the inner and outer lobes. The best resemblance is obtained for the piecewise isochoric jet model, for a viewing angle of approximately ϑ ∼ −71°, i.e. inclined with the lower jet towards the observer, with predominantly entangled (≳70 per cent of the magnetic pressure) in turbulent, rather than ordered fields. The effects of spectral ageing become significant when the ratio of observation frequencies and cut-off frequency νobs/ν∞, 0 ≳ 10−3, corresponding to ∼3 × 102 MHz. For viewing angles ϑ ≲ |−30°|, a DDRG morphology can no longer be recognized. The second jets must be injected within ≲ 4 per cent of the lifetime of the first jets for a DDRG structure to emerge, which is relevant for active galactic nuclei feedback constraints.


2020 ◽  
Vol 899 (1) ◽  
pp. L9
Author(s):  
Jenny E. Greene ◽  
David Setton ◽  
Rachel Bezanson ◽  
Katherine A. Suess ◽  
Mariska Kriek ◽  
...  

Author(s):  
Maxwell Deutscher

Memory is central to every way in which we deal with things. One might subsume memory under the category of intellect, since it is our capacity to retain what we sense, enjoy and suffer, and thus to become knowing in our perception and other activities. As intelligent retention, memory cannot be distinguished from our acquisition of skills, habits and customs – our capabilities both for prudence and for deliberate risk. As retention, memory is a vital condition of the formation of language. Amnesia illustrates dramatically the difference between memory as retention of language and skills, and memory as the power to recollect and to recognize specific events and situations. In amnesia we lose, not our general power of retention, but rather our recall of facts – the prior events of our life, and our power to recognize people and places. Amnesiacs recognize kinds of things. They may know it is a wristwatch they are wearing, while unable to recognize it as their own. This recall of events and facts that enables us to recognize things as our own, is more than just the ability to give correctly an account of them. One might accurately describe some part of one’s past inadvertently, or after hypnosis, or by relying on incidental information. Thus, present research on memory both as retention and as recall of specific episodes, attempts to describe the connection which persists between experience and recall. Neurological or computer models of such a connection owe something to traditional notions of a memory trace, but emphasize also the re-tracing of original memories by later experience and episodes of recall. Historically, recollection has often been thought of as a mode of perceiving the past. Such an idea lends an exaggerated status to the role of imagery, which is but one member of a family of recollective activities that includes reliving, remembering, reminiscing and mulling over what has happened. It may be not in having imagery but in miming someone’s behaviour that one relives an event. Also, like imagery, what we feel about the past may seem integral to recollection. A sense of being brought close to the past arises particularly when events that involve our feelings are concerned. Yet we may also recollect an event, vividly and accurately, while feeling clinically detached from it, devoid of imagery. How a past event or situation remains connected with subsequent recollection has become a principal theoretical question about memory. It is argued that it is because of what we did or experienced that we recollect it. Otherwise, we are only imagining it or relying upon ancillary information. Neurological or computer models of such a connection owe something to traditional notions of a memory trace, but emphasize also the re-tracing of original memories by later experience and episodes of recall. Some argue that our very idea of memory is that of the retention of a structural analogue of what we do recall of them. Such an idea is not of some perfect harmony between what we remember and our recollection of it. Rather, it is suggested, only to the extent that we retain a structural analogue of some aspect of an event or situation do we remember, rather than imagine or infer it.


1999 ◽  
Vol 194 ◽  
pp. 306-310
Author(s):  
Q. Yuan ◽  
J. Wu ◽  
K. Huang

This paper presents a test of the luminosity correlation of the X-ray selected radio-loud Active Galactic Nuclei (AGNs), based on a large sample constructed by combining our cross-identification of southern sky sources with the radio-loud sources in the northern hemisphere given by Brinkmann et al. (1995). All sources were detected both by the ROSAT All-Sky Survey and the radio surveys at 4.85 GHz. The broad band energy distribution confirms the presence of strong correlations between luminosities in the radio, optical, and X-ray bands which differ for quasars, seyferts, BL Lacs, and radio galaxies. The tight correlations between spectral indices αox and monochromatic luminosities at 5500 Å and 4.85 GHz are also shown.


1989 ◽  
Vol 134 ◽  
pp. 525-528
Author(s):  
T. J. Pearson ◽  
A. C. S. Readhead

Very Long Baseline Interferometry at radio wavelengths is the only technique available for imaging the central few parsecs of powerful radio galaxies and quasars. VLBI observations have shown that in many nuclei radio-emitting material is collimated into a jet on a scale less than a parsec and ejected at relativistic velocities. The interpretation of the observations is complicated by the relativistic motion, however: the images are dominated by those parts of the source that are moving almost directly towards the observer, and thus amplified by relativistic aberration. Nonetheless, the VLBI images are vital for understanding the nature of the central engine, the cause of the collimation, and the physics of the jets.


Sign in / Sign up

Export Citation Format

Share Document